The use of continuous sediment-transport measurements to improve sand-load estimates in a large sand-bedded river: The lower Chippewa River, Wisconsin

Accurately determining sediment loads is necessary for managing river environments but is difficult because multiple processes can lead to large discharge-independent changes in sediment transport. Thus, estimations of sediment load using discharge-sediment rating curves fit to sparse or historical sediment-transport measurements can be inaccurate, necessitating alternative approaches to reduce uncertainty. Continuous sediment-transport measurements reduce uncertainty because they can be used to detect discharge-independent changes in transport and are therefore unaffected by hysteresis. We used largely continuous approaches to measure sand transport in the lower Chippewa River, a large sand-supplying tributary to the Mississippi River. We used side-looking acoustic-Doppler profilers to continuously measure suspended-sand concentration, and bedform-tracking techniques to episodically measure bedload transport. Bedload transport was then continuously estimated using a discharge-dependent ratio of bedload to suspended-sand transport. This approach allowed determination of sand loads that were not estimated based only on water discharge. Our continuous suspended-sand measurements show that hysteresis between discharge and suspended-sand concentration occurs during most floods. Quasi-continuous bed-elevation measurements using a scour monitor show that lags between discharge and dune geometric adjustment is also common, causing hysteresis between discharge and bed-load transport during floods. Furthermore, comparisons of our measurements with historical sediment-transport measurements indicate large discharge-independent declines in both suspended-sand and bedload transport since the 1980s. These findings indicate that sand transport is a non-stationary function of water discharge over time-scales ranging from within individual floods to decades. Consequently, although our continuous-measurement approach yields only a 2030% improvement over rating-curve estimates of sand load over multi-year periods, our approach yields up to a factor-of-five improvement in sand-load estimates over the shorter, within a flood time-scales, over which the largest discharge-independent changes in sand transport occur.