Deltas and Environmental PerspectiveJames P.M. Syvitski
Environmental Computation and Imaging Facility, INSTAAR, CU-Boulder
Dynamics and Vulnerability of River Delta Systems, Boulder CO, 2007
With the great help of A. Kettner, I. Overeem, E. Hutton, Y. Saito
Defining a delta’s area:
1) the seawardprograding landarea that hasaccumulated since 6kyr, when global sealevel stabilizedwithin a few m ofthe present level(Amorosi and Miller,2001).
Problem:i) Relative sea level
height is globallyvariable (i.e. SL didnot stabilizeeverywhere at thesame time);
ii) shifting riverchannels maydisconnect deltaiclobes.
4300-2000 yr BP
2000-1000 yr BP
1000-150 yr BP
Since 1855
Dynamics and Vulnerability of River Delta Systems, Boulder CO, 2007
Defining a delta’s area:2) the seaward area of a river valley
after the main stem of a riversplits into distributary channels(Syvitski and Saito, 2007);
Problem: The split (or hinge) point canchange over time
Dynamics and Vulnerability of River Delta Systems, Boulder CO, 2007
Defining a delta’s area:3) the area of a river valley underlain
by Holocene marine sediments (Kuboet al., 2006);
Problem: While likely the most exactdefinition, this is a very difficultassessment, involving the collectionand dating of boreholes (cores), andgeophysical profiling over anexpansive area; &/or time consumingnumerical modeling.
Dynamics and Vulnerability of River Delta Systems, Boulder CO, 2007
Defining a delta’s area:4) accumulated river sediment that has
variably been subjected to fluvial, waveand tidal influences (Overeem et al.,2005);
Dynamics and Vulnerability of River Delta Systems, Boulder CO, 2007
Problem:No time is specified. Is it Holocene
or Pleistocene, or is it in the lastfew hundred years?
Defining a delta’s area:
5) the area drained by riverdistributary channels that are underthe influence of tides.
Problem: Is it saltwater intrusion? Or isit tidal damming?
Dynamics and Vulnerability of River Delta Systems, Boulder CO, 2007
1200
km
Dynamics and Vulnerability of River Delta Systems, Boulder CO, 2007
Sediment delivery to a delta is a function ofdrainage basin characteristics:1) Basin area2) Precipitation/runoff/water use: river discharge3) Basin relief4) Basin temperature5) Basin lithology6) Extent of glaciers7) Human landuse: de- / re-forestation, mining,agricultural practices, urbanization, channelhardening, use of stop-banks8) Level of impoundments: number, placement andsize of reservoirs
Dynamics and Vulnerability of River Delta Systems, Boulder CO, 2007
Relative Sea level for Deltas
Natural RSL rates = Eustatic Rate + Isostatic Rate + Compaction = (3.0 mm/y) + (0.3 to 3.8 mm/y) + (0.7 to 2.2 mm/y) = 4.0 to 7.8 mm/y
Accelerated compaction = petroleum & groundwater mininge.g. Po in the 1950’s: >60 mm/y — down to 20 mm/y after controls
Yangtze: 28 mm/y — down to 3 mm/y after controlsNiger (today): 25 to 125 mm/yChao Phraya (today): 50 - 100 mm/y
CAPUTO et al.,
1970
1958-1962
60 mm/y
40 mm/y
BONDESAN &
SIMEONI, 19831967-1974
20 mm/y
10 mm/y
End-Member Deltas
Dynamics and Vulnerability of River Delta Systems, Boulder CO, 2007
Low Marine Energy: Pm:Pr 0.2
Volga: channel feathering
Mississippi: channel splitting
End-Member Deltas
Dynamics and Vulnerability of River Delta Systems, Boulder CO, 2007
Polar
Lena: sea ice, permafrost,thermokarst, short seasonaldischarge (Qmx:Qav > 3)
Tropical
Niger: intense convective rainfall
Orinoco: intense rainfall leads to runoff channelsthat influence the pathway and connectivity ofthe distributary channels
End-Member Deltas
Dynamics and Vulnerability of River Delta Systems, Boulder CO, 2007
Desert
Nile: Low Runoff (Q:A 2.6; widemouth to river width TCw:Rw = 36
High WaveEel: high Pm:Pr ; few river mouths