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The Rate and Pattern of Deposition on Lowland River Floodplains

  • Author(s): Swanson, Kathleen
  • Advisor(s): Dietrich, William E;
  • Stacey, Mark
  • et al.

On large lowland rivers, overbank flows can deposit as much as 40% of the river's sediment load across adjacent floodplain environments on an annual basis. The spatial pattern of this deposition and the corresponding grain size distributions are not well known, despite the importance of understanding floodplain evolution, the generation of alluvial architecture, and the spread of contaminants. The lack of field data of floodplain deposition also limits our ability to evaluate models of floodplain sedimentation. Here I report floodplain overbank depositional patterns and grain sizes on the Strickland River in Papua New Guinea, compare these data with similar data from the Fly River, to which the Strickland drains, and explore distinct differences between the two rivers using a simple 1-D diffusion-advection model to predict the field observations.

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 seven times as much sediment load and twice as much water discharge as the Fly. As a result of this higher sediment flux, the shorter lowland Strickland River floodplain should be more developed in response to Holocene sea-level rise and, consequently, should presently be storing proportionately less sediment than the floodplain of the middle Fly River. A field campaign to collect samples to document floodplain sedimentation rates was conducted in 2003. Mine-derived elevated Pb and Ag concentrations in 111 shallow (<1 m) floodplain cores collected in 2003 were used to determine sediment deposition rates across the lower Strickland floodplain. Observed sediment deposition rates decrease across the floodplain with distance from the channel bank, and the average rate of deposition was 1.4 cm/yr over the first 1 km.

The magnitude of overbank deposition along the lowland sand-bedded Strickland resulted in a ~13% loss of the total sediment load, corresponding to overbank deposition of about ~0.05% of the total load per km of channel length of the mainstem. Deposition rates over the first 1 km from the channel bank on the Strickland were about ten times higher than those estimated for natural sediment loads on the Fly. However, the proportion of the sediment load deposited per channel length on the Strickland was less than that of the Fly (0.09%/km of mainstem channel length) due to 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 was ~5 times that on the Fly, such that most overbank deposits on the Strickland were returned to the channel, causing the net loss of sediment to the floodplain to be small.

In addition to the differences in the magnitude of deposition, the pattern of declining floodplain deposition with increasing distance from the channel differed between the two river systems: deposition decreased exponentially with distance from the channel on the Fly River floodplain and non-exponentially on the Strickland River floodplain. In order to assess the processes controlling this difference in the pattern of overbank deposition, sediment grain size distribution needed to be assessed. Particle size distributions for floodplain samples from the same cores used to determine deposition rates were measured using a Coulter LS laser particle sizer. Sediments from the Fly River floodplain did not vary much in size with distance from the channel and sand comprises about 5% of samples at all distances from the channel. This contrasted significantly with sediment size distributions across the Strickland floodplain, where sediments rapidly fined with distance from the channel. There was little sand or coarse silt present beyond 400 m from the channel and clay and fine silt comprised the majority of sediment beyond this distance.

Extensive studies of the Fly and Strickland River floodplain deposits motivated further study of the mechanisms of deposition. The differences in both the rate of deposition and the variation of grain size as a function of distance from the channel were hypothesized to be due to a difference in the relative importance of diffusive and advective transport of sediment from the channel to the floodplain. A 1-D numerical model was developed to investigate this hypothesis. The model indicated that lateral advection was the primary transport mechanism of sediment across both floodplains and that the key difference in depositional pattern may be explained by flocculation of all sediment particle sizes transported on to the swampy, highly-organic waters of Fly River floodplain and flocculation of primarily fine sediment transported across the less-swampy Strickland River floodplain. Lateral variability in accretion from overbank deposition can be explained by a mechanistically-derived exponential function that is solved by balancing a simple advection-deposition transport formula, but the parameters needed to evaluate the relationship are difficult to measure directly. Model parameters were estimated from the limited data available. A depositional length-scale was used to calculate the possible duration of active deposition, the effective settling velocity of flocculated particles, and floodplain flow velocity. The non-exponential deposition pattern observed on the Strickland River floodplain was likely the result of deposition of a distribution of particles sizes and a distribution of effective settling velocities. The observed non-exponential accretion on the Strickland River floodplain arose from the sum of the exponential functions for each particle size. The flocculated particles on the Fly River floodplain likely had a single effective settling velocity resulting in the observed exponential deposition pattern and lack of fining in overbank deposits.

The Fly and Strickland Rivers serve as a unique natural laboratory where relatively undeveloped, large lowland river systems with similar climate, geology, ecology and history can be studied to examine the fundamental processes controlling the evolution of alluvial valleys. The extensive analysis of floodplain sediment cores demonstrated that the rate, pattern and proportional loss of sediment overbank was quite different on the two systems as a result of differing sediment loads, subsequent adjustment to Holocene sea level rise, and, probably, the relative importance of flocculation of sediments transported overbank. Despite these differences, the overbank deposition pattern on both river systems can be describe by a simple advection-settling model. The pattern of deposition can be used to evaluate the model parameters and quantify key variables such as particle and flow characteristics which are logistically difficult to obtain on large floodplains. This understanding enhances our ability to evaluate floodplain sediment transport dynamics based upon depositional records and identifies key measurements that should be incorporated into future studies.

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