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