Sediment load and floodplain deposition rates: Comparison of the Fly and Strickland rivers, Papua New Guinea

Abstract

Rates of aggradation and infilling of accommodation space along lowland channels in response to postglacial sea level rise should depend on sediment supply. The Strickland and Fly rivers join at just 6 m above sea level and have experienced the same Holocene sea level rise. Historically, the Strickland has carried about 7 times the sediment load and 1.4 times the water discharge as the Fly. Therefore we hypothesize that the lowland Strickland floodplain should be more developed and consequently should presently be capturing proportionately less sediment than the floodplain of the lowland Fly River. We use mine‐derived elevated Pb and Ag concentrations in 111 shallow (<1 m) floodplain cores collected in 2003 to determine deposition rates across the lower Strickland floodplain. Sediment deposition rates decrease across the floodplain with distance from the channel bank, and the average rate of deposition is 1.4 cm/a over the first 1 km. Overbank deposition along the lowland sandbedded Strickland results in ∼13% loss of the total load, ∼0.05%/km of channel length of the main stem. Deposition rates over the first 1 km from the channel bank on the Strickland are about 10 times those on the Fly (for estimated natural sediment loads); however, the proportional loss per channel length on the Strickland is less than that on the Fly (0.09%/km of main stem channel length) because of an extensive network of tributary and tie channels that convey sediment to the floodplain on the Fly. Furthermore, the lateral migration of the Strickland channel is ∼5 times that on the Fly, such that most overbank deposits on the Strickland are returned to the channel, causing the net loss of sediment to the floodplain to be small. We conclude that the Strickland River, which has a much higher overbank deposition rate than the Fly River as a function of distance from channel bank, nonetheless has significantly less net accumulation than the Fly because (1) a large proportion of the sediment load is conveyed up tributaries and tie channels on the Fly, and (2) lateral migration (which presumably results from the higher load) on the Strickland sweeps sediment back into the channel. Hence our field observations support our initial hypothesis, though the primary reason for this is because of active lateral migration rather than low overbank deposition rates.