HYDROLOGICAL CONNECTIVITY BETWEEN HEADWATER STREAMS … · HYDROLOGICAL CONNECTIVITY BETWEEN...

HYDROLOGICAL CONNECTIVITY BETWEEN HEADWATER STREAMS ANDDOWNSTREAM WATERS: HOW SCIENCE CAN INFORM POLICY1

Tracie-Lynn Nadeau and Mark Cable Rains2

ABSTRACT: In January 2001, the U.S. Supreme Court ruled that the U.S. Army Corps of Engineers exceededits statutory authority by asserting Clean Water Act (CWA) jurisdiction over non-navigable, isolated, intrastatewaters based solely on their use by migratory birds. The Supreme Court’s majority opinion addressed broaderissues of CWA jurisdiction by implying that the CWA intended some ‘‘connection’’ to navigability and that isola-ted waters need a ‘‘significant nexus’’ to navigable waters to be jurisdictional. Subsequent to this decision(SWANCC), there have been many lawsuits challenging CWA jurisdiction, many of which are focused on head-water, intermittent, and ephemeral streams. To inform the legal and policy debate surrounding this issue, wepresent information on the geographic distribution of headwater streams and intermittent and ephemeralstreams throughout the U.S., summarize major findings from the scientific literature in considering hydrologicalconnectivity between headwater streams and downstream waters, and relate the scientific information presentedto policy issues surrounding the scope of waters protected under the CWA. Headwater streams compriseapproximately 53% (2,900,000 km) of the total stream length in the U.S., excluding Alaska, and intermittentand ephemeral streams comprise approximately 59% (3,200,000 km) of the total stream length and approxi-mately 50% (1,460,000 km) of the headwater stream length in the U.S., excluding Alaska. Hillslopes, headwaterstreams, and downstream waters are best described as individual elements of integrated hydrological systems.Hydrological connectivity allows for the exchange of mass, momentum, energy, and organisms longitudinally,laterally, vertically, and temporally between headwater streams and downstream waters. Via hydrological con-nectivity, headwater, intermittent and ephemeral streams cumulatively contribute to the functional integrity ofdownstream waters; hydrologically and ecologically, they are a part of the tributary system. As this debate con-tinues, scientific input from multiple fields will be important for policymaking at the federal, state, and local lev-els and to inform water resource management regardless of the level at which those decisions are being made.Strengthening the interface between science, policy, and public participation is critical if we are going to achieveeffective water resource management.

(KEY TERMS: Clean Water Act (CWA); waters of the U.S.; hydrological connectivity; headwater streams; inter-mittent and ephemeral streams; navigable waters; SWANCC; Rapanos.)

Nadeau, Tracie-Lynn, and Mark Cable Rains, 2007. Hydrological Connectivity Between Headwater Streams andDownstream Waters: How Science Can Inform Policy. Journal of the American Water Resources Association(JAWRA) 43(1):118-133. DOI: 10.1111/j.1752-1688.2007.00010.x

1Paper No. J06077 of the Journal of the American Water Resources Association (JAWRA). Received June 5, 2006; accepted October 23,2006. ª 2007 American Water Resources Association.

2Respectively, Lead Environmental Scientist, U.S. Environmental Protection Agency, Office of Wetlands, Oceans and Watersheds, 1200Pennsylvania Avenue NW, Washington, DC 20460; and Assistant Professor, Department of Geology, University of South Florida, 4202 E.Fowler Avenue, SCA 528, Tampa, Florida, 33620 (E-mail ⁄ Nadeau: [email protected]).

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Vol. 43, No. 1 AMERICAN WATER RESOURCES ASSOCIATION February 2007

INTRODUCTION

Debate over the scope of waters protected underthe Federal Clean Water Act (CWA) has been on-going nearly since it was enacted, in 1972, as a com-prehensive effort to control water pollution. The juris-dictional scope of the CWA is ‘‘navigable waters,’’which are defined in the CWA as ‘‘waters of the Uni-ted States, including the territorial seas.’’ The debatehas intensified over recent years (e.g., Kusler, 2001;Albrecht and Nickelsburg, 2002; Downing et al.,2003; Wood, 2004; Kusler, 2006; Sapp et al., 2006),primarily because of the U.S. Supreme Court’s decis-ion in Solid Waste Agency of Northern Cook County v.U.S. Army Corps of Engineers, 531 U.S. 159 (2001)(SWANCC). In SWANCC, the Supreme Court wasasked to address whether a specific isolated, intra-state, non-navigable water could be considered partof the waters of the U.S. for purposes of CWA juris-diction based solely on its use as habitat by migratorybirds. Also before the Supreme Court was the issue ofwhether Congress had the authority under the Com-merce Clause of the U.S. Constitution to include iso-lated, intrastate, non-navigable waters as waters ofthe U.S. In a 5-4 decision, the Supreme Court heldthat the CWA is not intended to protect isolated, in-trastate, non-navigable waters based solely on theiruse by migratory birds. However, the Supreme Courtdid not decide whether Congress had the authority toregulate such waters (Downing et al., 2003).

The Supreme Court’s discussion in the majorityopinion also touched upon broader issues of CWAjurisdiction, and the reasoning implies that the CWAintended some ‘‘connection’’ to navigability and thatisolated waters need a ‘‘significant nexus’’ to navig-able waters to be jurisdictional (Downing et al.,2003). Subsequent to SWANCC, there have beenmany lawsuits challenging CWA jurisdiction, many ofwhich are focused on headwater streams and inter-mittent and ephemeral streams. The concepts of ‘‘tri-butary,’’ ‘‘adjacency,’’ and ‘‘significant nexus’’ are themain jurisdictional issues in the post-SWANCCdebate.

There have been over 35 post-SWANCC decisionsat the Federal court level. The Federal government isconsistently arguing that the SWANCC decision wasnarrowly focused on isolated waters. In the vastmajority of cases the Federal government has pre-vailed, and most Federal courts have construed CWAjurisdiction as not significantly reduced by theSWANCC decision, with the exception of the FifthCircuit, covering Texas, Louisiana and Mississippi(Sapp et al., 2006). In October 2005, the SupremeCourt granted certiorari in two Sixth Circuit deci-sions, John A. Rapanos et al. v. United States, U.S.,

No. 04-1034 (2005) (Rapanos) and June Carabellet al. v. United States Army Corps of Engineers andthe United States Environmental Protection Agency,U.S., No. 04-1384 (2005) (Carabell). On February 21,2006, the Supreme Court heard oral arguments inboth the Rapanos and Carabell cases. In both cases,petitioners argued that CWA jurisdiction extendsonly to wetlands that actually abut navigable-in-factwaters. In addition, at oral argument petitionersargued that non-navigable tributaries were similarlyunprotected by the CWA. They also argued that ifthe CWA extends to any other wetlands, Congresshas exceeded its Commerce Clause authority. It islikely that the Supreme Court will reach a decisionon both cases by July 2006 (see Postscript). The out-come of these cases may determine whether a waterbody needs to be navigable-in-fact to be jurisdictionalunder the CWA.

While CWA Section 404 – which requires a permitfor the discharge of dredged or fill material intowaters of the U.S. – triggered the SWANCC, Rapan-os, and Carabell cases, critically important in thispost-SWANCC debate is that the definition of ‘‘watersof the U.S.’’ affects all CWA programs, not just Sec-tion 404 (Downing et al., 2003). This includes Section402 National Pollutant Discharge Elimination System(NPDES) permits, the Section 401 water quality cer-tification program, the Section 303 water qualitystandards, and oil and hazardous spill preventionand clean-up under Section 311. Thus, legal and pol-icy deliberations surrounding this issue have broadimplications for water quality and water resourcesacross the nation.

This featured collection of the Journal of theAmerican Water Resources Association was developedto help provide scientific input, from multiple fields,to inform these on-going legal and policy debates, inparticular, the debates surrounding the extent of thetributary system. It is not meant to be an exhaustivereview of the extensive literature on headwaterstreams, but rather it aims to review the state of ourunderstanding of the contributions of headwaterstreams to the integrity of downstream waters and tohighlight the processes involved in providing thosefunctions. Contributors were asked to address severalkey topics, including the hydrological connectivitybetween headwater streams and downstream waters;the roles played by headwater streams in maintain-ing the physical, chemical, and biological integrity ofdownstream waters; the roles played by headwaterstreams in maintaining the physical, chemical, andbiological integrity of the larger stream network; andthe spatial and temporal scales over which thishydrological connectivity is relevant. Another line ofquestioning addressed the relative roles of surfacewater and ground-water flow paths, and the possible

HYDROLOGICAL CONNECTIVITY BETWEEN HEADWATER STREAMS AND DOWNSTREAM WATERS: HOW SCIENCE CAN INFORM POLICY

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consequences to downstream systems of eliminatingor otherwise impacting headwater stream resources.

This paper proceeds in three sections. First, wepresent information on the geographic distribution ofheadwater streams and intermittent and ephemeralstreams throughout the U.S. Second, we summarizethe major findings from the contributed papers andthe existing scientific literature in considering hydro-logical connectivity between headwater streams anddownstream waters. Finally, we relate the scientificinformation presented to policy issues surroundingthe scope of waters protected under the CWA.

GEOGRAPHIC DISTRIBUTION

Headwater streams are generally defined as theuppermost streams in a watershed. For the purposesof this paper, unless otherwise indicated, we specific-ally define headwater streams as those streams thatare first order when viewed at the 1:100,000 scale(Strahler, 1952). Headwater streams may be peren-nial, intermittent or ephemeral. Perennial streamsflow year-round during a typical year. They typicallyreceive appreciable quantities of water from numeroussources including snowmelt and ground water.Because their channels are typically below the watertable, ground-water discharge may be the primarysource of the annual streamflow (e.g., Stringer et al.,in review). Intermittent streams cease to flow duringdry periods. They may receive appreciable quantitiesof water from numerous sources including snowmeltand ground water (e.g., Rains and Mount, 2002; Rainset al., 2006). Ephemeral streams flow only in directresponse to precipitation. They do not receive appreci-able quantities of water from any other source, andtheir channels are, at all times, above the water table.

Headwater streams are the most abundantstreams in both number and length in a stream net-work (Horton, 1945; Leopold et al., 1964). The geo-graphic extent of headwater streams in the U.S. ispoorly documented due to mapping limitations. (Werefer the reader to Leopold [1994] for a discussion ofthe assumptions and caveats associated with blue-line mapping.) However, oft-cited estimates indicatethat first- and second-order streams comprise approxi-mately 70% of the total stream length in the U.S.(Leopold et al., 1964).

The National Hydrography Dataset (NHD) mediumresolution data at the 1:100,000 scale provides themost comprehensive national overview of streammiles in the U.S., excluding Alaska (Simley, 2006).The NHD does not identify headwater or first-orderstream segments per se, but it does identify stream

segments that have no other streams flowing intothem as start reaches. Neither does the NHD differ-entiate between intermittent and ephemeral streamsegments per se, but it does identify stream segmentswhich contain water only part of the year. Therefore,we used start reaches as a surrogate for headwaterstreams and reaches which contain water for onlypart of the year as a surrogate for combined intermit-tent and ephemeral streams. We accessed the NHDReach Address Database (RAD) v2.0 in February2004. The NHD has since been undergoing a revision,including updated flow data; we expect that the dif-ferences between our calculations and calculationsthat could be made as of the publication date wouldnot be greater than 5-10%. Based on studies usinghigher resolution data, data at the 1:100,000 scaleunderestimate headwater, intermittent, and ephem-eral stream channel length (e.g., Leopold, 1994;Meyer and Wallace, 2001).

A query of the NHD at the 1:100,000 scale indi-cates that headwater streams comprise approxi-mately 53% (2,900,000 km) of the total stream lengthin the U.S., excluding Alaska (Figure 1). These datasuggest that headwater streams are not uniformlydistributed across the U.S. Rather, these data suggestthat headwater streams are more highly concentratedin the eastern U.S. in hydrologic landscape regions(HLRs) that are humid to sub-humid plains or pla-teaus and humid to semi-arid mountains (Wolocket al., 2004). However, it is likely that this distribu-tion pattern is, at least in part, an artifact of theNHD dataset. Headwater streams are more likely tobe perennial and therefore show up as blue lines inthe humid to sub-humid eastern U.S. than in the aridto semi-arid western U.S. Therefore, headwaterstreams are more likely to be mapped in the NHD da-taset in the eastern U.S. than in the western U.S.Channels form when flow strength exceeds a criticalthreshold above which surface erosion occurs. Thecritical threshold above which surface erosion occursvaries greatly with topography, lithology, soils, andvegetation which, in turn, vary greatly throughoutthe eastern and western U.S. Thus, high drainagedensities can be found in both the eastern and west-ern U.S. (e.g., in the badlands near Perth Amboy,New Jersey and in the badlands in Petrified ForestNational Park, Arizona), and low drainage densitiescan be found in both the eastern and western U.S.(e.g., in the carboniferous sandstones of the Appala-chian Plateau, Pennsylvania and in the igneous-metamorphic complexes of the Coast Ranges,southern California) (Strahler, 1957). Therefore,though headwater streams are certainly not uni-formly distributed across the U.S., the NHD datasetat the 1:100,000 scale – while currently providing themost comprehensive overview of stream miles in the

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FIGURE 1. Headwater Stream Length, as a Proportion of Total Stream Length Within Each 8 Digit HUC Watershed,in the U.S., Excluding Alaska, as Computed Querying the NHD RAD v2.0 for Reaches That Have No Other Inflowing Streams

at the 1:100,000 Scale. The NHD RAD v2.0 Does not Capture Streams Under 1 mile (i.e., 1.61 km) in Length.

FIGURE 2. Combined Intermittent and Ephemeral Stream Length in the U.S., Excluding Alaska, as a Proportion ofTotal Stream Length Within Each 8 Digit HUC Watershed, as Computed by Querying the NHD RAD v2.0 for Reaches That Contain Water

Only Part of the Year at the 1:100,000 Scale. The NHD RAD v2.0 does not capture streams under 1 mile (i.e., 1.61 km) in length.



U.S. – may not be the best way to determine the pre-cise distribution patterns.

A query of the NHD at the 1:100,000 scale indi-cates that intermittent and ephemeral streams com-prise approximately 59% (3,200,000 km) of the totalstream length in the U.S., excluding Alaska (Fig-ure 2). Intermittent and ephemeral streams accountfor approximately 50% (1,460,000 km) of the head-water stream length, i.e., start reaches, in the NHD(data not shown). Figure 2 suggests that intermittentand ephemeral streams also are not uniformly distri-buted across the U.S. Rather, these data suggest thatintermittent and ephemeral streams are more highlyconcentrated in the western U.S., excluding mostmountainous and coastal regions, in HLRs that arearid to semi-arid plains or plateaus. For example,intermittent and ephemeral streams comprise 11%(11,900 km) of the total stream length in New Yorkand 94% of the total stream length in Arizona(Figures 3a and b). This distribution pattern is lar-gely a function of climate, with average annual preci-pitation east of the 100th meridian ranging from�20-80 inches, and average annual precipitation westof the 100th meridian, excluding most mountainousand coastal regions, ranging from �10-20 inches(National Climatic Data Center).

These data also suggest that intermittent and eph-emeral streams are not uniformly distributed acrossindividual states. In North Carolina, for instance,intermittent and ephemeral streams are generallyconcentrated on the upper coastal plain where preci-pitation is lower, soils are thick, and ground-waterlevels are well-below the land surface, while in Wash-ington, intermittent and ephemeral streams are gen-erally concentrated east of the Cascade Range whereprecipitation is lower and particularly in the south-east where soils are thick and ground water is well-below the ground surface (Figures 4a and b).

HYDROLOGICAL CONNECTIVITY

Hydrological Connectivity as a Continuum

Hydrological connectivity is the hydrologicallymediated transfer of mass, momentum, energy, ororganisms within or between compartments of thehydrologic cycle (e.g., Pringle, 2001; Freeman et al.,this issue). Hydrological connectivity is implicit in theglobal water cycle, in which the hydrosphere is seen asa single hydrological system comprised of hydrological-ly connected components (Winter et al., 1998). Thisglobal-scale hydrological connectivity is recognizedin general circulation models, which model fluid

dynamics at the continental scale but include under-lying hydrological processes such as precipitation andevapotranspiration at the regional or smaller scales(e.g., Rind et al., 1992). However, hydrological connec-tivity exists on a continuum. At one end of the con-tinuum are the tight linkages, such as rainfall andrunoff in small headwater stream basins with no con-nection to regional ground-water flow systems. At theother end of the continuum are the weak linkages,such as rainfall and connate waters, i.e., deep groundwaters that were emplaced with geologic deposits andthat are largely separated from the modern hydrologiccycle. Somewhere in between, both spatially and tem-porally, lies the hydrological connectivity betweenheadwater streams and downstream waters.

In the majority opinion in SWANCC, the SupremeCourt implied that non-navigable, isolated, intrastatewaters need a ‘‘significant nexus’’ to navigable watersto be jurisdictional under the CWA. In stream sys-tems, this implies that a ‘‘significant nexus’’ mustexist between a headwater stream and a navigablewater for the headwater stream to be a ‘‘tributary.’’To date, neither the responsible regulatory agenciesnor the courts have defined ‘‘significant nexus.’’ Theobjective of the CWA is to ‘‘restore and maintainthe chemical, physical, and biological integrity of theNation’s waters’’ (33 U.S.C. §1251). For our purposes,we consider a ‘‘significant nexus’’ to exist if a head-water stream contributes to the physical, chemical, orbiological integrity of a navigable water.

Hydrological Connectivity Between HeadwaterStreams and Downstream Waters

It is tempting to think of hydrological connectivityin stream networks as operating only in the down-stream direction. However, stream networks arecharacterized by a high degree of spatial andtemporal heterogeneity with mass, momentum,energy, and organisms flowing in four dimensions:longitudinally (i.e., channel-channel), laterally (i.e.,channel-floodplain), vertically (i.e., channel-aquifer),and temporally (i.e., time-time) (Ward, 1989). Thisfour-dimensional model provides a useful frameworkfor the discussion of the roles of hydrological connec-tivity in maintaining the physical, chemical, andbiological integrity of the nation’s waters.

Streams cover a small proportion of the land sur-face. Therefore, most precipitation falls on uplandsand takes one of four pathways downgradient:ground-water flow (i.e., subsurface flow below thewater table), throughflow (i.e., subsurface flow abovethe water table), infiltration excess overland flow(i.e., overland flow that occurs when the rainfall rateexceeds the infiltration rate), and saturated overland

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flow (i.e., overland flow that occurs when the watertable rises to the surface) (Knighton, 1998).

The relative contribution of ground-water dischargeto annual streamflow varies as a function of geologyand climate (Winter et al., 1998; Winter, this issue). Ina study of 54 streams throughout the U.S., ground-water discharge contributed approximately half of theannual streamflow, though precise contributions ran-ged from 14% to 90% of the annual streamflow (Winteret al., 1998). Where channels of perennial streams arebelow the water table, ground-water dischargetypically is the primary source of annual streamflow(Stringer et al., in review). However, even where chan-nels of intermittent streams are above regional watertables, ground-water discharge from perched aquifers(i.e., aquifers in which free ground water is perched onlow-permeability deposits above regional water tables)may still be the primary source of annual streamflowin intermittent streams (Rains et al., 2006).

Base flow, by definition, is supported entirely byground-water discharge (Wilson and Moore, 1998).However, perhaps the most important finding in thehydrological sciences in the past two decades is thatstormflow also is largely supported by soilwater andground-water discharge, with soilwater and ground-water discharge often contributing approximatelyhalf of the total stormflow and more than 80% of theinstantaneous stormflow (Eshleman et al., 1994; Hin-ton et al., 1994; Cey et al., 1998; Iqbal, 1998; Burnset al., 2001; Stringer et al., in review). This is becauseevent water passing through the subsurface mixeswith and mobilizes pre-event soilwater and groundwater, and it is this mix of event and pre-event waterthat discharges to the stream channel. Water in thesubsurface typically moves slowly and may be storedas soilwater or ground water for many years, whilewater in channels typically moves rapidly, though itmay take short-term excursions over the floodplain

(a) (b)

FIGURE 3. Combined Intermittent and Ephemeral Stream Length as a Proportion of Total Stream Length within Each 8 Digit HUCWatershed as Computed by Querying the NHD RAD v2.0 for Reaches that Contain Water Only Part of the Year at the 1:100,000 Scale. Toassign NHD features on a state-by-state basis, a spatial intersect was run using Oracle Spatial 9i. The boundary of the states’ layer and theboundary of the NHD data are not an exact match, which may have introduced some small amount of (non-correctable) error into theanalysis. The NHD RAD v2.0 does not capture streams under 1 mile (i.e., 1.61 km) in length. (a) New York. (b) Arizona.



(Brunet et al., 2003) or through the hyporheic zone(i.e., the portion of the aquifer where stream waterand ground water freely mix) (Morrice et al., 1997;Hill and Lymburner, 1998; Hill et al., 1998). There-fore, much of the temporal storage is in upland soils,so weighted mean ages of water in headwaterstreams and downstream waters are often similar(Burns et al., 2003; McGlynn et al., 2003; Vitvaret al., 2004; McGuire et al., 2005).

Taken together, the characteristic baseflows andstormflows comprise the natural flow regime (Poffet al., 1997). The natural flow regime is characterizedby the magnitude (i.e., how much water is movingpast a given location per unit time), the frequency(i.e., how frequently a flow of given magnitudeoccurs), the duration (i.e., how long a flow of a givenmagnitude occurs), the timing (i.e., the time of yearthat a flow of a given magnitude occurs), and the rateof change (i.e., how rapidly a flow of a given magni-tude changes to a flow of a different given magni-tude). The diverse array of flows characteristic to thenatural flow regime transfer mass, momentum,energy, and organisms at various spatial andtemporal scales. Therefore, the natural flow regimecreates and maintains a diversity of local- and basin-scale habitats ranging from constantly inundated,in-channel habitats to never-inundated, terrace habi-tats. Individual floral and faunal species have evolvedin response to these kinds of habitats, and local- and

basin-scale floral and faunal assemblages have organ-ized in response to these kinds of habitat mosaics(Fonda, 1974; Walker et al., 1986; Van Splunderet al., 1995; Hupp and Osterkamp, 1996; Scott et al.,1996). The juxtaposition of these diverse habitats isparticularly important for those species that requiremore than one habitat to complete their life-historyrequirements (Copp, 1989; Copp et al., 1994; Sempes-ki and Gaudin, 1995a,b,c).

The biogeochemical processing of dissolved constit-uents is controlled by complex interactions betweenthe rate at which water flows through surface andsubsurface flowpaths and the rate at which dissolvedconstituents are processed by such processes asadsorption to sediments or uptake by microorganismsand vegetation (Phillips et al., 1993; Hamilton andHelsel, 1995). Microorganisms and vegetation aretypically abundant in upland soils and sediments thatcomprise the boundaries of streams. Therefore,upland soils and sediments that comprise the bound-aries of streams are areas in which a large proportionof the biogeochemical processing of dissolved constitu-ents occurs (Peterjohn and Correll, 1984; Munn andMeyer, 1990; Vervier et al., 1993; Dahm et al., 1998;Hill et al., 1998; Hill and Lymburner, 1998; Alexan-der et al., 2000; Triska et al., this issue). This is par-ticularly true for soils adjacent to streams andsediments that comprise the boundaries of streamsbecause these are areas that support low oxidation-

(a) (b)

FIGURE 4. Combined Intermittent and Ephemeral Stream Length as a Proportion of Total Stream Length within Each 8 Digit HUCWatershed as Computed by Querying the NHD RAD v2.0 for Reaches that Contain Water Only Part of the Year at the 1:100,000 Scale. Toassign NHD features on a state-by-state basis, a spatial intersect was run using Oracle Spatial 9i. The boundary of the states’ layer and theboundary of the NHD data are not an exact match, which may have introduced some small amount of (non-correctable) error into theanalysis. The NHD RAD v2.0 does not capture streams under 1 mile (i.e., 1.61 km) in length. (a) North Carolina. (b) Washington.

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reduction environments and ⁄ or steep oxidation-reduc-tion gradients essential for numerous biogeochemicalprocesses (Ponnamperuma, 1972).

Headwater streams also can be important sourcesof dissolved constituents such as nitrogen (Alexanderet al., 2000; Alexander et al., this issue) and dis-solved organic carbon (DOC) (Moore, 2003). The rel-atively high iron oxide contents of many upland soilsstrongly sorb and, therefore, immobilize DOC in soil-water and ground water (Hobson and Dahlgren,1998). Therefore, DOC concentrations tend to belower in soilwater and ground water than in head-water streams (Rains et al., 2006). Furthermore,though DOC includes recalcitrant forms of organicmatter, microbial decomposition in the shallowsubsurface may nevertheless consume some of thesimple forms of DOC in soilwater and ground water.DOC accumulates in stream water through desorp-tion from mineral surfaces under anaerobic condi-tions (Jardin et al., 1989) and from allochthonous(i.e., off-channel) and autochthonous (i.e., in-channel)production (Vannote et al., 1980; Wipfli et al., thisissue) and subsequent leaching of the particulateorganic matter (Orem et al., 1986). DOC is thenreadily transported to downstream waters (Moore,2003).

There is a delicate balance, however, between thebiogeochemical processing and downstream transportof dissolved constituents. This balance is easily dis-rupted by off-channel activities (e.g., mining, logging,agriculture, and urban development) and on-channelactivities (e.g., damming), with the attendant conse-quences typically being an increase in the down-stream transport of dissolved constituents andsubsequent shifts in ecosystem structure and functionin the downstream waters (Freeman et al., this issue;Hauer and Stanford, this issue).

Similarly, headwater streams also can be import-ant sources of coarse particulate organic matter (Van-note et al., 1980; Wipfli and Musslewhite, 2004;Wipfli et al., this issue). Overhanging upland andriparian vegetation contribute large quantities of allo-chthonous material to headwater streams, includingboth coarse particulate organic matter and terrestrialinsects (Wipfli and Musslewhite, 2004; Wipfli et al.,this issue). The fate of this allochthonous material isunclear, though much of it is transported to down-stream waters, particularly as pulses during periodsof high flows (Naiman, 1982; Wallace et al., 1995;Kiffney et al., 2000). Wind throw, bank failure, andespecially mass movements such as debris flows alsomay contribute large quantities of allochthonousmaterial to headwater streams, including sedimentsand large woody debris (Keller and Swanson, 1979;Gomi et al., 2004; May and Gresswell, 2004). Once inthe headwater streams, larger pieces of large woody

debris may be relatively immobile, but smaller piecesmay be relatively mobile and therefore be transpor-ted to downstream waters (Keller and Swanson,1979). Those larger pieces of large woody debris thatmay be relatively immobile trap and retain sedi-ments by forming organic dams that physically blocksediment transport and ⁄ or by creating local flowseparation and consequent sediment deposition(Abbe and Montgomery, 1996). Though these sedi-ments are ultimately released, they are neverthelessdelayed which has the effect of slowing the rate ofdenudation in the headwater streams and the rate ofaggradation in the downstream waters (Montgomeryet al., 2003).

The structure and function of headwater streamsare strongly controlled by local physical, chemical,and biological conditions. Local-scale differences inphysical, chemical, and biological conditions, e.g., par-ent material, substrate, climate, water chemistry,and riparian vegetation, create varied types of head-water streams which support a diverse array ofmicrobial, plant, and animal habitats. These diversehabitats may be taxa rich and may provide for all orsome of the life history requirements for a large pro-portion of the total taxa in a given stream network(Meyer et al., this issue).

Some taxa move into headwater streams fromdownstream waters opportunistically, for example, toseek refuge from predators or from physical or chem-ical hydrological extremes, while some taxa move intoheadwater streams from downstream waters to com-plete critical parts of their life history, for example, tospawn and ⁄ or rear. For these latter taxa, in partic-ular, the hydrological connectivity between headwaterstreams and downstream waters is critical. However,hydrological connectivity between headwater streamsand downstream waters also may be critical for taxathat are unique to headwater stream systems. Forthese taxa, headwater streams are habitable featuresin otherwise uninhabitable landscapes. Therefore,these taxa may exist in metapopulations, i.e., groupsof populations that interact via dispersal (Hanski,1999; Bohonak and Jenkins, 2003). Spatial dispersalis critical for many reasons, including gene flowbetween populations which maintains genetic popula-tions that are resilient to minor perturbations andupstream and downstream colonization which aids inrecovery from major perturbations (Meyer et al., thisissue). Though spatial dispersal of some of these taxamay occur via flight or passive attachment to migra-ting animals, spatial dispersal of most of these taxaoccurs via active or passive transport in flowing water(Bohonak and Jenkins, 2003). Therefore, the spatialdispersal of most of these taxa requires hydrologicalconnections, even if these hydrological connections areephemeral or intermittent.



Taxa moving from downstream waters to head-water streams can represent an important pathway bywhich nutrients are transported from downstreamwaters to headwater systems (Gresh et al., 2000;Naiman et al., 2002; Wilkinson et al., 2005). Anadrom-ous fish, such as salmonids, move from headwaterstreams to the ocean as small fingerlings. In theocean, these salmonids accumulate large quantities ofnutrients in their bodies as they grow to adulthood.Once they reach adulthood, these salmonids return toheadwater streams to spawn. Some of these salmonidsdie and decompose in the headwater stream, whileothers of these salmonids are transferred to nearbyriparian and upland ecosystems by predators (e.g.,bear) or scavengers (e.g., bald eagles). In both cases,nutrients uptaken in the ocean are ultimately releasedin the headwater systems. These contributions maynot be trivial. Spawning salmonids historically mayhave delivered �7,000 million tons of N and �800 mil-lion tons of P to watersheds in California, Oregon,Washington, and Idaho (Gresh et al., 2000). As aresult of population declines, spawning salmonids maycurrently only deliver �400 million tons of N and �50million tons of P to watersheds in California, Oregon,Washington, and Idaho (Gresh et al., 2000). Neverthe-less, these current contributions can be measurable inriparian zone plants (Wilkinson et al., 2005).

Hydrological Connectivity in Intermittent andEphemeral Headwater Streams

The physical, chemical, and biological integrity offloodplains depends upon episodic connectivitybetween main channels and floodplains during floodpulses (Junk et al., 1989; Heiler et al., 1995; Wardand Stanford, 1995; Tockner et al., 2000). These epi-sodic connections provide for the critical exchange ofmass, momentum, energy, and organisms betweenthe channel and floodplain. For example, flood pulsestransfer dissolved nutrients from main channels tofloodplains and particulate organic matter from flood-plains to main channels (Heiler et al., 1995). Simi-larly, the physical, chemical, and biological integrityof some downstream waters can depend upon episodicconnectivity between intermittent or ephemeralstreams and downstream waters during flow pulses.These flow pulses may only connect intermittent andephemeral streams to downstream waters for shortperiods of time (Izbicki, this issue). Nevertheless,these episodic connections may, in the absence ofother substantial inputs, provide a large proportion ofthe mass, momentum, energy, and organisms deliv-ered annually to the downstream waters.

In arid to semi-arid climates, flow pulses in inter-mittent and ephemeral streams may provide a sub-

stantial amount of the annual nutrient subsidies todownstream waters as a result of the asynchronybetween hydrological and biological processes. Annualplants senesce in the dry season. However, microbialactivity continues, nitrogen is mineralized, andnitrate accumulates in the upland soils. Annualplants germinate early in the wet season but do notdevelop substantial biomass until the middle to lategrowing season. Therefore, during early wet-seasonstorm events, there is little biological demand fornitrate, hence nitrate is leached from upland soilsand discharged to intermittent and ephemeralstreams and transported as a pulse to downstreamwaters (Holloway and Dahlgren, 2001; Rains et al.,2006).

In arid to semi-arid climates, flow pulses in inter-mittent and ephemeral streams may provide a sub-stantial amount of the annual ground-water rechargeto the underlying regional aquifers (Izbicki et al.,1995; Girard et al., 1997; Williams, 1997; Izbickiet al., 2002; Izbicki, this issue). In these environ-ments, the regional water table may be many tens tohundreds of meters below the ground surface so infil-trating water must traverse a deep vadose (i.e.,unsaturated) zone prior to reaching the water table.There is limited precipitation, hence vegetation isadapted to uptake and conserve much of the availablesoil moisture. Still, diffuse ground-water rechargefrom precipitation can occur when water infiltrates tobelow the rooting zone. This is most likely to occurwhen a series of winter storms provides abundantprecipitation while suppressing evapotranspiration.This being an infrequent condition, diffuse ground-water recharge from precipitation tends to be low. Ina survey of 17 studies, diffuse recharge from precipi-tation was found to be <10% of annual precipitationunder most circumstances and less than 1% of annualprecipitation in many circumstances (Stephens,1994). Conversely, intermittent and ephemeralstreams may flow only for short periods of time inresponse to infrequent winter frontal storms or sum-mer convective storms, but much of the water mayleak through the streambed and form a moist butunsaturated curtain directly below and adjacent tothe channel that penetrates to well-below the rootingzone and facilitates regional ground-water recharge(Riesenauer, 1963; Constantz et al., 2001, 2003).Recharging water moves slowly and may take manyhundreds of years to reach the regional aquifers(Izbicki, 2002), though some recharging water maymove substantially faster through preferential path-ways (Izbicki et al., 2002). Once in the regional aqui-fer, this water may move long distances over thecourse of hundreds to thousands of years beforedischarging to the surface to support baseflows indownstream waters (Izbicki et al., 1995).

NADEAU AND RAINS


INFORMING POLICY AND WATERRESOURCE MANAGEMENT

Legal Arena

Following the SWANCC decision, there has beena great deal of focus on the legal status of isolatedwetlands (e.g., Kusler, 2001; Albrecht and Nickels-burg, 2002; Christie and Hausmann, 2003; Downinget al., 2003) and on the need to synthesize currentscientific knowledge on the geographic distribution ofisolated wetlands and the contributions of isolatedwetlands to broader aquatic ecosystems (e.g., Tineret al., 2002; Bedford and Godwin, 2003; Gibbons,2003; Haukos and Smith, 2003; Leibowitz, 2003;Leibowitz and Nadeau, 2003; Nadeau and Leibowitz,2003; Richardson, 2003; Sharitz, 2003; Tiner,2003a,b; Whigham and Jordan, 2003; Winter andLaBaugh, 2003; van der Valk and Pederson, 2003;Zedler, 2003; Comer et al., 2005a,b). Meanwhile, alsofollowing the SWANCC decision, numerous lawsuitshave arisen challenging CWA jurisdiction over isola-ted wetlands and other similar waters (Downinget al., 2003; Wood, 2004; Kusler, 2006).

Given this national attention and recognizing theneed for public input, the U.S. Army Corps of Engin-eers (Corps) and the U.S. Environmental ProtectionAgency (EPA) issued an Advance Notice of ProposedRulemaking (ANPRM) on January 15, 2003 request-ing input on issues associated with the definition of‘‘waters of the U.S.’’ for purposes of CWA jurisdiction.In the ANPRM, the Corps and EPA requested infor-mation and data in several areas including: (1) whe-ther regulation should define ‘‘isolated waters’’ and, ifso, using what factors, (2) the effectiveness of otherfederal and state programs in protecting isolatedwaters, (3) potential impacts of SWANCC to aquaticresources, and (4) whether any other changes areneeded to jurisdictional regulations.

In response to the ANPRM, the Corps and theEPA received approximately 130,000 comments from43 states, 4 tribes, and numerous local governments,scientific associations, nonprofit organizations, regu-lated industries and laypersons, and general layper-sons. These comments remain available for publicreview under Docket ID# EPA-HQ-OW-2002-0050 athttp://www.regulations.gov/. Greater than 90% ofthese comments supported limited or no jurisdictionalchanges to the current regulations.

The state comments best highlight the primaryunderlying areas where science can inform the ongo-ing policy debate. Although the ANPRM requestedcomments on isolated waters specifically, many statesprovided information on other waters that could poss-ibly be impacted with a change in jurisdiction. In

particular, arid states in the West and Midwestexpressed concern regarding CWA jurisdiction overheadwater, intermittent, and ephemeral streams.Many states provided GIS-based estimates of thesewaters, as well as estimates regarding existing CWAprotections on those waters and ⁄ or the ecosystem ser-vices provided by those waters. For instance, Arizonaestimated that 97% of their existing NPDES permitsat the time of the study discharged to headwater,intermittent, or ephemeral streams, while Pennsylva-nia reported that 317 drinking water surface intakes,servicing over 1.5 million people, were in headwater,intermittent, or ephemeral streams. Many statesexpressed concern that, given the inherent hydrologi-cal connections, the overall objectives of the CWAcould not be met if these waters were no longer juris-dictional.

States’ concerns are echoed in the debate that hascontinued in the legal arena. As the number of law-suits challenging CWA jurisdiction continues toincrease, it is clear that the primary focus of this liti-gation is not isolated waters including wetlands, i.e.,those waters including wetlands lacking a surfacewater connection to other waters but, rather, whethera water needs to be navigable-in-fact, or directly adja-cent to a water that is navigable-in-fact, to be jurisdic-tional under the CWA (Wood, 2004; Sapp et al., 2006).Of the more than 35 Federal court decisions afterSWANCC, none have involved isolated waters butinstead have explored the jurisdictional status of tri-butaries or adjacent wetlands. Further public com-ment on these issues can be found in the amici curiaebriefs filed with the Supreme Court in response to theRapanos and Carabell cases. An amicus curiae brieffiled by the Attorney’s General of Michigan (the statein which both the Rapanos and Carabell cases began)and New York, with 34 states and the District ofColumbia as signatories, stated that the protection ofnon-navigable tributaries and wetlands is essential toprotecting downstream navigable waters because non-navigable tributaries and wetlands compose the vastmajority of a watershed and have the best ability toreduce pollution near the sources through natural pro-cesses. State amici curiae briefs also raised concernsthat comprehensive CWA coverage is necessary tomaintain the federal-state framework established bythe CWA. The Environmental Law Institute (ELI) andthe Association of State and Interstate Water Pol-lution Control Administrators (ASIWPCA) took theunprecedented steps of filing amici curiae briefs toemphasize the importance of non-navigable tributariesfunctional contributions to downstream waters, andthe State-Federal partnership created by the CWA, toState protections for natural and economic resources.

If, in deciding the Rapanos and Carabell cases, theSupreme Court significantly reinterprets the scope of



waters protected under the CWA, it is likely to resultin a shift in how we achieve water resource protec-tion. Protecting the physical, chemical, and biologicalintegrity of the nation’s waters will rely to a greaterextent on states, tribes, and even local governments(Christie and Hausmann, 2003; Downing et al., 2003;Sapp et al., 2006).

Broader Management Approaches

The need for broader, and more innovative, man-agement approaches that consider how hydrologicaland ecological systems function at various temporaland spatial scales is increasingly emphasized inaquatic resource management and protection. Severalof the authors contributing to this featured collectionrecommend that broader approaches to waterresources management be employed. Hauer and Stan-ford (this issue), Meyer et al. (this issue) and Wipfliet al. (this issue) recommend that land-use activitiesbe considered in the development of effective manage-ment strategies aimed at water-related environmen-tal objectives, such as water quality protection,fisheries conservation, and the maintenance of bio-logical diversity. Triska et al. (this issue) point to theneed for a better understanding of temporal-spatialcontrols on processes such as nutrient cycling, andFreeman et al. (this issue) recommend that cumula-tive and interacting impacts be considered on regio-nal and global scales. Wipfli et al. (this issue) stressthat public perception is a key factor in any success-ful management strategy.

Managing water resources at the watershed scaleoffers the potential of balancing the often competingdemands placed on water resources (NRC, 1999).Furthermore, watershed management is an integra-tive way of considering the various land-based activit-ies that occur within a watershed that have effectson, or are affected by, water (NRC, 1999). Many fed-eral and state agencies promote taking a watershedmanagement approach as an effective method forintegrating environmental, economic, and socialaspects of water-related problem solving. Forinstance, the EPA has been instrumental in develop-ing the principles of a watershed approach as anintegrated, holistic problem-solving strategy forrestoring and maintaining the physical, chemical,and biological integrity of aquatic ecosystems and forprotecting human health (USEPA, 1993). A water-shed management approach uses an integrated set oftools (federal, state, tribal, local) and programs (vol-untary and regulatory), includes all stakeholders,and applies an iterative planning or adaptive man-agement process to strategically address prioritywater resource goals.

An ongoing challenge for effective watershed plan-ning and management is to relate science to decision-making and public participation. There are manyways in which science can inform that process, as wemove away from short-term, single-focus or site-by-site approaches towards management plans that arebased on the longer and larger scales of the complexsystems being managed. Many questions remainunresolved, so continued research is needed toprovide the data, knowledge, and technology to sup-port effective watershed management. In particular,long-term, large-scale monitoring and research thatare integrated across spatial and temporal scales willhelp provide the theoretical and empirical founda-tions necessary to identify problems and problemsources. Monitoring data are also necessary to ascer-tain whether programs or practices are working, andto thus inform adaptive management strategies foraddressing problems and achieving water-relatedgoals. Furthermore, there is a need for more predic-tive models that can address the effects of alternativeactions or policies on hydrological, ecological, and eco-nomic and social outcomes. The combined contribu-tions of Triska et al. (this issue) and Alexander et al.(this issue) to this featured collection are good exam-ples of research, at different temporal and spatialscales, that is focused on informing effective manage-ment and remediation strategies for a timely issue ofnational importance: hypoxia in the Gulf of Mexico.

CONCLUSION

On-going debate in the legal and policy arenas con-tinues to focus national attention on the status ofheadwater, intermittent, and ephemeral streams forpurposes of CWA jurisdiction. Headwater streams,generally defined as the uppermost streams in awatershed, are the most abundant streams in bothnumber and length in a stream network (Horton,1945; Leopold et al., 1964). The physical, chemical,and biological integrity of any ecosystem is the syner-gistic product of many physical, chemical, and biologi-cal processes operating at many spatial and temporalscales. The strength of these interactions and the dis-tances and times over which these interactions occurvary greatly. Regardless, hydrological connectivityallows for the exchange of mass, momentum, energy,and organisms longitudinally, laterally, vertically,and temporally throughout stream networks and theunderlying aquifers. Therefore, hillslopes, headwaterstreams, and downstream waters are best describedas individual elements of integrated hydrological sys-tems. Based on the papers contributed to this fea-

NADEAU AND RAINS


tured collection and the existing literature, scientificevidence does not support the existence of a brightline separating headwater streams from downstreamwaters within these integrated hydrological systems.Via hydrological connectivity, headwater, intermit-tent and ephemeral streams cumulatively contributeto the functional integrity of downstream waters;hydrologically and ecologically, they are a part of thetributary system.

Notwithstanding, how such terms as ‘‘tributary,’’‘‘adjacency’’, and ‘‘significant nexus’’ are defined forregulatory purposes will ultimately be decided in thelegal and policy arenas. Social and economic out-comes, as well as hydrological and ecological out-comes, impact decision-making. Furthermore, giventhe tremendous climatic and geologic diversity acrossthe nation, which results in a diverse geographic dis-tribution of water resources such as intermittent andephemeral streams, policies are likely to have differ-ent reverberations in different regions of the countryor within a state if that diversity is not considered. Ifthe Supreme Court significantly reinterprets thescope of waters protected under the CWA as a resultof Rapanos and Carabell, it could effectively create agap in water resource protection and shift greaterresponsibility to states, tribes, and local governmentsfor achieving water quality goals. Regardless of theSupreme Court’s decision in those cases, scientificinput in several areas will be important for policy-making at the federal, state, and local levels and toinform water resource management regardless of thelevel at which those decisions are being made. Wehave good science, but to make science usefulrequires an effective interface between science, policy,and public participation. Strengthening that interfaceis critical if we are going to achieve effective waterresource management.

POSTSCRIPT

On June 19, 2006 the U.S. Supreme Court handeddown its decision addressing the jurisdictional scopeof the CWA in the consolidated cases of Rapanos andCarabell, 126 S. Ct. 2008 (2006). The 4-1-4 decisionhas three substantive opinions; Justice Scalia’s plu-rality opinion joined by three justices concluding thatthe cases should be remanded to the lower courts, aconcurring opinion by Justice Kennedy that the casesshould be remanded, and Justice Stevens’ dissentingopinion joined by three justices which would haveaffirmed the decisions by the lower courts that theCWA term ‘‘waters of the U.S.’’ included wetlandsadjacent to tributaries of navigable-in-fact waters.

However, Justice Kennedy did not agree with Jus-tice Scalia’s plurality analysis on what constituted‘‘waters of the U.S.’’ for purposes of the CWA. Theplurality opinion concluded that ‘‘waters of the U.S.’’includes ‘‘relatively permanent, standing or flowingbodies of water’’ and does not include ‘‘ordinarily drychannels through which water occasionally or inter-mittently flows.’’ Justice Scalia elaborated that thephrase ‘‘relatively permanent’’ does not necessarilyexclude seasonal rivers. Justice Scalia further conclu-ded that ‘‘only those wetlands with a continuous sur-face connection to bodies that are ‘waters of theUnited States’ in their own right, so that there is noclear demarcation between ‘waters’ and wetlands’’ arecovered by the CWA. Justice Kennedy, however, con-cluded that wetlands are ‘‘waters of the U.S.’’ where‘‘the wetlands, either alone or in combination withsimilarly situated lands in the region, significantlyaffect the chemical, physical, and biological integrityof other covered waters more readily understood as‘navigable.’’’ Justice Kennedy looks to not just hydro-logical relationships, but to ecological relationshipsbetween water bodies, and observes that waters incombination can have important functions thatimpact downstream waters (Murphy, 2006). The con-currence by Justice Kennedy further stated that:‘‘[a]bsent more specific regulations,…the Corps mustestablish a significant nexus on a case-by-casebasis[.]’’

The Federal government’s position in litigation isthat Rapanos has established that waters are juris-dictional under the CWA where they meet either theScalia standard or the Kennedy standard for ‘‘signifi-cant nexus’’ (Cruden, 2006). For the Scalia standard,it seems two criteria must be met: the channel towhich the wetland is adjacent is a ‘‘relatively perma-nent’’ body of water, and secondly that the wetlandhas a ‘‘continuous surface connection’’ with such achannel which makes it difficult to demarcate a clearboundary between the wetland and the channel.Kennedy’s standard calls for a case-by-case test of‘‘significant nexus,’’ described as waters affecting thephysical, chemical, or biological integrity of navigablewaters, not necessarily requiring a hydrological con-nection in all instances, and allowing for categoricalclassification of waters and their cumulative func-tion. At the time of this writing, the Agencies havenot released guidance in response to the Rapanosdecision.

The Rapanos decision is extremely complex; therehas been much legal analysis in the few months sincethe decision was handed down (e.g., Cruden, 2006;Kusler and Christie, 2006; Moller, 2006; Murphy,2006; Thomas, 2006; Urban, 2006; Ward, 2006), andwill likely continue to be much more as the lowercourts seek for legal standards to apply when faced



with jurisdictional questions. Clearly many questions,some highly technical in nature, remain regardingCWA jurisdiction over headwater, intermittent, andephemeral streams following this decision. There is aspecific and immediate role for science in addressingthese questions, including: clearly establishing andquantifying the contributions of headwater, intermit-tent, and ephemeral streams and wetlands to thechemical, physical, and biological integrity of navig-able waters; establishing categories of these watersand addressing their functional role in aggregate;assessing and quantifying the frequency and durationof non-flowing headwater streams and wetlands lack-ing a continuous surface water connection; developingpredictive models to address questions of flow; devel-oping practical tools that can be applied in the field.Finally, given that jurisdictional determinations willbe made even while this debate continues and thatJustice Kennedy’s ‘‘significant nexus’’ test is amongthe possible bases for jurisdiction, watershed assess-ments, plans, and monitoring data are potentiallypivotal sources of information for jurisdictional deter-minations to aid the understanding of the relation-ship between a particular water body and adownstream navigable water.

ACKNOWLEDGMENTS

We especially thank each of the authors for contributing to thisfeatured collection. We gratefully acknowledge Jim Wigington formoving forward our proposal for the AWRA invited session andresulting JAWRA featured collection, Ken Reid and the organizersof the AWRA 2005 Annual Meeting for working with us on theinvited session, and JAWRA’s John Warwick, Ken Lanfear, LauraHelsel, and Susan Scalia for facilitating this featured collection.Indus Corp. provided assistance with the GIS analyses and earlierdrafts of Figures 1-4. Thanks to Kai Rains and Donna Downing forcritical reviews that improved the quality of the manuscript.Finally, Tracie Nadeau thanks EPA’s Geographic JurisdictionWorkgroup, members past and present, for their sustained effortson this important issue. The content of this paper represents thepersonal views of the authors, and does not necessarily reflect offi-cial policy of the U.S. Environmental Protection Agency or anyother agency.

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