Collaborative Research: Processes Controlling Depositional Signals ofEnvironmental Change in the Fly River Sediment Dispersal system: Ratesand Mechanisms of Floodplain Deposition

William Dietrich, Univ. of California, Berkeley; Parker

S2S

7/1/02 – 6/30/05 OCE 02-03351

Source-to-Sink (S2S) accomplishments by the University of California, Berkeley andUniversity of Minnesota Research Groups. The Berkeley group is focused on fieldstudies while the Minnesota group is developing floodplain sedimentation theory.

• We are asking two fundamental questions: 1) How does flow and sediment routingthrough a lowland floodplain system moderate short and longer-term variations insediment delivery towards offshore depositional environments?; and 2) Whatcontrols the proportion of a river’s sediment load that is deposited on itsfloodplain? We hypothesize that net sediment loss to the floodplains in largelowland rivers was highest during Holocene sea-level rise and that, after nearstabilization of sea level, the proportion of the sediment load deposited in thefloodplain has progressively declined.

• The Middle Fly River is currently losing about 40% of its sediment load to itsfloodplain environment, primarily through overbank losses on the main stem andthrough outflows of Fly river water up tributary and tie channels that then spill ontothe floodplain. This forms a depositional web of elevated sedimentation about 1 kmof either side of this network of floodplain channels. We hypothesize that theStrickland River, which where it joins the Fly has twice the drainage area anddischarge, 7 times the sediment load and about 8 times the channel gradient asdoes the mainstem Fly, has more fully responded to sea level rise and its relativerate of sediment loss to the floodplain is much lower (Figs. 1, 2, and 3).

• In June 2003, a team of 5 Americans, two Australians, and 4 Papua NewGuineans collected over 500 core samples from the Strickland floodplain, obtained43 velocity and suspended profiles of the Strickland River, and made a kinematicGPS survey of the lower Strickland.

• The core samples were split, with half being analyzed for elevated Pb and Agassociated with mining activity that began in 1990 in the headwaters (providing atime tracer to calculate deposition rate), and the other half being analyzed by RolfAalto (University of Washington) for 210Pb. Preliminary analyses of Pb and Agshow a clear signal and suggest that deposition at least one channel width eitherside of the Strickland River may approach 1cm/year.

• Theoretical work to date has focused on developing a numerical model to describethe co-evolution of a river channel and its floodplain. The model assumes there isa feedback between deposition on a floodplain and erosion from it. When thefloodplain is in some sense low, we expect that it should flood frequently andconsequently aggrade at a relatively high rate. However, as it aggrades, floodingshould become less and less frequent, resulting in lower and lower aggradationrates. If we can assume that there is always some removal of material from thefloodplain due to bank migration, then at some point the net rate of overbankdeposition should equal the net removal rate. At this point, the floodplain should bein a sort of equilibrium. The same story could be told if upstream flow andsediment loads were reduced, thus resulting in a floodplain that was in somesense too high. In this case, erosion would tend to reduce the floodplain elevationuntil the increased frequency of flooding associated with this reduction causedfloodplain deposition to come into balance with the migration-associated removalrate. (Figure 4)

sense too high. In this case, erosion would tend to reduce the floodplain elevationuntil the increased frequency of flooding associated with this reduction causedfloodplain deposition to come into balance with the migration-associated removalrate. (Figure 4)

• A more complicated model has also been developed that predicts channel bed andfloodplain response to changes in sediment load, in either fine (washload) sizes orbed material sizes. To our knowledge, this model represents the first of its kind inthat it uses separate sub-models for floodplain deposition and erosion to predict ariver’s evolution toward an equilibrium depth.

• The Papuan “Connection” scientific workshop was held at Berkeley, 10 December,2003, with over 25 participants. Bill Dietrich and Chuck Nittrouer organized themeeting. Gary Parker (Minnesota), Wes Lauer (Minnesota), Rolf Aalto(Washington), Kathleen Swanson (Berkeley), Joel Rowland (Berkeley) and SimonApte (CSIRO, Australia- collaborator with Berkeley group) were in attendance fromthe Berkeley- Minnesota group. Gary Parker presented preliminary modeling of theFly/Strickland delta response to late Pleistocene sea level rise. The model showedpotential for rapid delta retreat and possible abandonment during high sea levelrise rates.

• Maria Bera (undergraduate student at University of Papua New Guinea)participated in the Washington and Berkeley field programs. In addition, she spentone month in Berkeley in November-December, 2003, analyzing samples andparticipating in the American Geophysical Union meetings.

• Collaborations were established with OK Tedi Mining, LTD , Placer Dome(Porgera) Mining, Lihir Mining, and CSIRO (Australia).

Figures and Captions

Figure 1: NASA image of the junction of the Fly and Strickland Rivers. Numerousblocked valley lakes were formed along the low gradient Fly River during Holocene sealevel rise and main channel aggradation. The largest lake, draining into the Strickland, isLake Murray. It appears that aggradation of the Strickland has led to backwater, swampyconditions on the mainstem Fly.

Figure 2: SRTM derived topographic map of the Middle Fly River area. The well-definedFly and Strickland floodplains are bordered by highly dissected Quaternary deposits.The elevated scroll bar complex area on the Fly and Strickland are evident in theupstream portions of the reaches shown.

Figure 3: SRTM derived longitudinal profile of the Middle Fly and Strickland Rivers. Themean profile matches field observations well, although the high spatial variation inelevation is an artifact of the SRTM survey.

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Frequent flooding causes net deposition,raising the floodplainsurface

Lack of deposition due to infrequent floodingleads to net floodplain lowering

Erosion and depositionnearly in equilibrium

Figure 4: Evolution of the floodplain at a cross section where deposition and floodplainerosion have been set out of equilibrium by either increasing or decreasing channeldepth. The model predicts evolution back toward the state where erosion and depositionare in equilibrium.

Publications and Presentations

Presentations at the Fall 2003 American Geophysical Union:

Lauer, J.W. and Parker, G. “Morphodynamic Modeling of the Co-evolution ofChannel and Floodplain n Large, Sand-bed Rivers.” EOS Trans. AGU, 84(46), FallMeet. Suppl., Abstract OS11A-02, 2003.

Apte, S.C., Dietrich, W.E., Day, G.M., Reibe, C., Aalto, R., Sanders, J., Lauer, W.“Mapping the Extent and Rate of Overbank Deposition Using Mine-Derived SedimentTracers Along the Strickland River, Papua New Guinea.” EOS Trans. AGU, 84(46),Fall Meet. Suppl., Abstract OS12A-0193, 2003.

Paper submitted:

Lauer, J.W. and Parker, G. “Modeling Channel-Floodplain Co - evolution in Sand-Bed Streams.” ASCE World Water and Environmental Resources Congress, SaltLake City, June 28- July 1, 2004.