Post on 17-Dec-2015
transcript
Mark A. Gonzalez
National Riparian Service Team
HISTORICAL EVOLUTION OF THE SAN PEDRO RIVER: 8000 YRS BP TO AD 1950S
INTRODUCTION
Straw Dog
Project Area
San Pedro Riparian National Conservation Area
International border to St. David
SPRNCA boundary
Pre-entrenchment River Conditions (Holocene history)
Entrenchment (1890s-1900s)WhenCausesFeaturesEcological/Environmental Consequences
Channel Evolution (1910s -1950s)Channel wideningFloodplain formationChannel narrowing
OUTLINE
HOLOCENE SETTING
Piedmont/Fan
Inner Valley
Basin Fill
Basement Rock
HOLOCENE SETTING
Inner ValleyPre-entrenchment Landforms
EntrenchmentLandforms
HOLOCENE SETTING
Weik Ranch Mbr. 6500-4300 yrs BP (Qwk)
Hargis Ranch Mbr. 3500-2000 yrs BP (Qha)
McCool Ranch Mbr. 2000 BP to AD 1880 (Qmc)
Little Ice Age (AD 1450-1850) paleosol
QwkQhaQmc-B
Qmc-AQmc-A
Qmc-B
Teviston alluvium (Qtv)
Sources: Haynes 1987; Hereford 1993; Waters and Haynes 2001
HOLOCENE SETTING
Weik Ranch Mbr. 6500-4300 yrs BP (Qwk)
Hargis Ranch Mbr. 3500-2000 yrs BP (Qha)
McCool Ranch Mbr. 2000 BP to AD 1880 (Qmc)
Little Ice Age (AD 1450-1850) paleosol
QwkQhaQmc-B
Qmc-AQmc-A
Qmc-B
Teviston alluvium (Qtv)
7500 yrs BP4000 yrs BP
2600 yrs BP
1900 yrs BP
Historic
Sources: Haynes 1987; Hereford 1993; Waters and Haynes 2001
HOLOCENE SETTING
QwkQhaQmc-B
Qmc-AQmc-A
Qmc-B
Important points:• Wide-scale valley erosion and entrenchment in early Holocene (8000 –
6500 yrs BP)• Periods of aggradation (valley filling) and degradation (channel incision
and erosion) repeated throughout the Holocene• McCool Ranch paleosol likely formed during the Little Ice Age (AD 1450-
1850), a globally cooler and wetter period• Channel incision natural process throughout Holocene
HOLOCENE SETTING
QwkQhaQmc-B
Qmc-AQmc-A
Qmc-B
Important points:• 15,000 to 8000 yrs BP: Cool/wet climate
• Inner valley stable, gradually accumulating sediment• Desert basin floor covered in woodlands• Water tables high
• ~8000 yrs BP (start of Altithermal period): Warm/dry climate• Water tables dropped; channel entrenchment• Desert basin floors covered in desert scrub
HOLOCENE SETTING
QwkQhaQmc-B
Qmc-AQmc-A
Qmc-B
Important points:• Aggradation associated with overall wetter
climates• Entrenchment associated with dry
periods/droughts punctuated by isolated or a few powerful runoff events.
HOLOCENE SETTING: MCCOOL RANCH PALEOSOL
1 mi SSummers
WellsGarden Wash
¼ mi N of Casa de
San PedroBoquillas
Wash
¼ mi S of
Boquillas Wash
Qtv
Qmc
Qtv
Qmc
Qtv
Qmc
Qtv
Qmc
Qtv
Qmc
Physical ConditionsLittle Ice Age (AD 1450-1880)Low-energy environmentShallow depth to water tableHigh organic-matter content (esp. from paleo-cienega)
Water storage and release patterns
HOLOCENE SETTING: MCCOOL RANCH PALEOSOL
Qtv
Qmc
Biological ConditionsWidespread cienega formationHerbaceous dominated communities
PRE-ENTRENCHMENT CONDITIONS
Biological ConditionsSacaton/mesquite bosques on periphery of cienegas
PRE-ENTRENCHMENT CONDITIONS
Timing (1880s-1910s +/-, Hereford 1993)Downstream initiation (1882, Contention area)Upstream migration (1908 – Hereford Bridge)Spread into and up tributary drainages
PERIOD OF ENTRENCHMENT
Causes: Ultimately – Big Floods 1. Changes in climate
• Rainfall intensity and frequency• Drought with a few severe storms
2. Changes in land uses• Mining/Deforestation• Grazing
3. Combination of 1 and 2
4. Tectonic shifts in groundwater levels
PERIOD OF ENTRENCHMENT
Immediate cause: Series of large floods in 1880s and 1890s1881–Flood destroys dam upstream of Charleston1887–Local newspapers reported damaging floods Jul. thru Sep.1890–Damaging flood in August1891–Floods caused extensive damage to farms and rr. in Aug.1893–Large flood threatened Fairbank and stalled rr. traffic
south of Benson1894–Large flood washed out dam at St. David and damaged
ranches below in August1896—Extensive flood damage reported July thru October1900—Flood-weakened bridges delay trains1901—troublesome floods lo the lower San Pedro area in Aug.1904/05—Flood damaged structures and shifted the channel
locally(Source: Hereford 1993)
PERIOD OF ENTRENCHMENT
Question remains: Why were floods particularly damaging during 1880s
and 1890s?
PERIOD OF ENTRENCHMENT
Causes: Change in land use/changes in basin/upland hydrology
PERIOD OF ENTRENCHMENT
Low Runoff / High Infiltration High Runoff / Low Infiltration
Causes: Deforestation/mining?Rapid development of mining
claims in watershed during the 1870s
Extensive tree cutting in uplands at this time for mining and fuel
Changes in upland hydrology?
PERIOD OF ENTRENCHMENT
Qtv
Qmc
Causes: Livestock grazing?
“…the malady of overcrowding is with us in an aggravated form….” Southwestern Stockman, 1890
PERIOD OF ENTRENCHMENT
San Pedro River, mid-1980s, BLM
CausesClimate change/pattern?
Intensity, frequency, and amount of rainfallENSO activity strong at end of Little Ice Age
PERIOD OF ENTRENCHMENT
Time
Pre
cipi
tatio
n
Features: Formation of arroyos
PERIOD OF ENTRENCHMENT
Pre-entrenchment channel: Shallow
Post-entrenchment channel: Deep
San Pedro River
Ecological ConsequencesConversion of low-energy cienega to high-energy stream environment
Conversion of fine to coarse sedimentIncrease in overall sediment load
PERIOD OF ENTRENCHMENT
Ecological Consequences: Drop in water table
PERIOD OF ENTRENCHMENT
Pre-entrenchment water table: Shallow
Post-entrenchment water table: Deep
San Pedro River channel
Ecological Consequences: Loss of water storage
PERIOD OF ENTRENCHMENT
Assume: • Available water in silt loam = 1.7”/ft; in sand = 0.9”/ft;• Pre-entrenchment alluvium is 20’ thick; averages silt loam;• Pre-entrenchment alluvium averages 0.7 miles (3700’) wide;• Pre-entrenchment paleo-cienega soils average 3’ thick;• Soil organic matter holds 16,000 gallons for each percent of
organic matter; • Post-entrenchment alluvium is 10’ thick; averages sand; and• Post-entrenchment alluvium averages 0.2 miles (1050’) wide.
Ecological Consequences: Loss of water storage
PERIOD OF ENTRENCHMENT
kmMiles
0 0.5
0.5
1.51
1
4200 ft
4100 ft
Inner ValleyPre-entrenchment alluvium
WestEast
V.E. = 100X
Available water volume in SPRNCA pre-entrenchment alluvial aquifer (WVa) composed of silt loam:WVa = (448 acres/mile X 40 miles) X 20 ft thickness X 1.7”/ft ÷ (12”/ft)WVa = 50,000 acre-feet
Pre-entrenchment alluvial aquifer (with 3’ thick cienega soil on 3600 acres (1/5 of riparian area) and MODEST 5% organic matter):
WVa = 50,000 acre-ft + (16,000 gallons X 5 X 3’ X 3600 acres) ÷ (325,851 gals./acre-ft)
WVa = 50,000 acre-ft + 2650 acre-ft = 52,650 acre-ft
PERIOD OF ENTRENCHMENT
kmMiles
0 0.5
0.5
1.51
1
4200 ft
4100 ft
Inner ValleyWest
East
V.E. = 100X
Available water volume in SPRNCA for post-entrenchment alluvial aquifer (WVa) composed of sand:
WVa = (128 acres/mile X 40 miles) X 10 ft thickness X 0.9”/ft ÷ (12”/ft)
WVa = 3840 acre-feet
Post-entrenchment alluvium
Ecological Consequences: Loss of water storage
Channel widening: Areal extent calculated from sequential aerial photography for channel 3.2 km N of Hereford bridge
CHANNEL EVOLUTION
(modified from Hereford, 1993)
Pre-entrenchmentChannel
Post-entrenchment channels
Year 1890-1908
Acres 17
Increase ----
1955
80
50%
0 1000 m
2000 ft
1937
40
58%
Later on the channel narrows and meander rates declined
CHANNEL EVOLUTION
t2
t3
t2 downcutting
t3 widening
t4 aggradation
t2 t3 t4
Consequences of channel widening and floodplain formationGreater opportunities to dissipate
stream energy during peak flowsSlower water discharge and greater
water subtractionIncreased volume of floodplain aquifer
for water storagePotential to increase base flow
CHANNEL EVOLUTION
Pre-incision conditions: Holocene periodAlternating periods of aggradation (filling) and
degradation (channel entrenchment)Aggradation: cooler/wetter periods; high water
tableEntrenchment: warmer/dryer periods
SUMMARY
Pre-incision conditions: AD 1450-1850Little Ice AgePeriod of cooler/wetter climateHigh water tableLow-energy riparian environmentLow sediment flux / Soil developmentCienegas widespreadAmple supply of water and lots of water storage
SUMMARY
Entrenchment period: AD 1890-1908Numerous large floodsENSO strongCauses?
ClimateLand useCombination
SUMMARY
Post-entrenchment period: AD 1908-1955 Initial deepening and then widening of
channelHigh sediment fluxLost water table and dewatering of
alluvium
SUMMARY
Haynes, CV Jr 1987. Curry Draw, Cochise County, Arizona: A late Quaternary strat igraphic record of Pleistocene extenction and paleo-Indian activi t ies, in ML Hi l l (ed.), Cordi l leran Section of the Geological Society of America, Geological Society of America Centennial Field Guide Vol. 1,pp. 23-28
Henrickson, DA and Minckley, WL 1984. Cienegas—Vanishing cl imax communit ies of the American Southwest. Desert Plants 6:131-175.
Hereford, R 1993. Entrenchment and widening of the upper San Pedro River, Arizona. Geological Society of America Special Paper 282, 46 p.
Waters, MR and Haynes, CV 2001. Late Quaternary arroyo formation and cl imate change in the American Southwest. Geology 29:399-402.
REFERENCES