Extended Data Fig. 5: Sand deposit characteristics.

a, Grain size distribution metrics for proximal zone floodplain deposits (mixed samples, see Methods) with moving average values. D₅₀ values for samples from one scour hole (□) and near-river samples (o, “D50 VU” from Extended Data Fig. 9) are added for comparison. In a 20 km section downstream of the scour holes, deposits contain only fine sediment. b, Average mixing percentages and average D₅₀ grainsize of floodplain samples, including the standard deviations, for sections of 50 km downstream of the scour holes, showing no significant trend in composition for downstream reaches. c-f, Concentrations of Pb, Zn, Cu and Ni in sediment samples from XRF analyses. Green bars indicate the scour hole section in the river; scour hole samples were taken at km 39, in the most downstream scour hole. Low concentrations in the scour holes show eroded Neogene sediments (23-5 million B.P.) to be uncontaminated, upstream deposits to be of more recent origin and the Neogene sediments to be deposited on downstream floodplains. Panels b-f are based on Van der Niet⁷⁹.