Estimating river-sediment discharge to the ocean: application to the Eel margin, northern California

HYDROTREND, a synthetic river-discharge numerical model, can simulate the natural variability (daily, annually and longer) in the flux of water and sediment delivered to the coastal ocean. The numerical model is designed to make discharge predictions based on climate, even though field measurements of river flow are not available. Provided that appropriate assumptions are made regarding past climate, the model can predict how a river may have behaved in the geological past. Sediment load is determined from a river's rating coefficients. When applied to the flood-dominated Eel River, northern California, HYDROTREND captures the intermonthly and interannual variability of discharge and loads over a century or more. HYDROTREND is applied to the ice-age paleo-Eel River when sea level was lower and climate much cooler and wetter. The paleo-Eel River at 18,000 year BP would see its hinterland runoff increase, but not the magnitude of the peak discharges. The post-dicted sediment load of the paleo-Eel River was 25% greater than today's, although basin precipitation was 45% greater, and the hinterland area 4% greater (through sea-level lowering). A method is provided to obtain sediment-discharge rating coefficients for unmonitored modern rivers and paleo-rivers. The relationship Q s= αρg 1/2 H 3/2 A 1/2 provides for an estimate of a river's sediment load Q s, to within a factor of 3 over 7 orders of magnitude, from knowledge of basin relief H, and basin area A. Rating coefficients are determined from this estimate of Q s and the power-law rating curve, Q s= aQ b+1 where Q is discharge.