Sediment deposition patterns in a tropical floodplain, Tana River, Kenya

•Sediment deposition and storage along 380 km tropical floodplain reach investigated.•Sedimentation rates recorded ranged between 1.01 - 1.52g cm-2 yr-1.•Variable sediment source, including deeply mobilized, radionuclide-poor sediments.•Upto 30% of river sediment load is deposited during a flood within the study reach.•Floodplain sediment stocks controlled by flooding frequency at ca.1.05 flood yr-1.

Floodplains exert strong controls on downstream sediment transport and are as such important in material budgets of river systems. To understand sediment budgets in the Tana River, we investigated sediment storage along a 380 km floodplain reach (Garissa-Garsen) in the lower Tana River (Kenya), using a combination of approaches: (i) measurements of sediment deposition after an important flood event and, (ii) quantification of sediment storage using fallout radionuclide activities (137Cs and 210Pbex). Event-based sediment deposition ranged between 2-15 mm vertical accretion, corresponding to an average of 0.58 ± 0.42 g cm-2 (dry weight). Average annual sediment storage based on fallout radionuclide activities were at a 50 yr mean of 1.15-1.21 g cm-2 yr-1 and a 100 yr mean of 1.01 g cm-2 yr-1 (using 137Cs and 210Pbex, respectively). Sediment deposition rates were mainly dependent on distance relative to the main river, flood height and microtopography. The deposited sediments originated from various sources including deeply mobilized, radionuclide-poor sediments. Shallow overbank deposits (< 2 m depth), dominated by silty-clay sediment fractions were observed in most depositional areas. Floodplain sediment stocks are controlled by annual overbank flooding, with a 73 yr flooding frequency of 1.05 flood yr-1. On average ca. 1-2 Mt (15-30%) of the river sediment load is deposited in the Garissa-Garsen floodplain reach. However, the overall channel dynamics including channel sediment storage and reworking are important and may have a very high impact on short-term sediment storage and release.