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

Abstract

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 bedload 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 timescales ranging from within individual floods to decades. Consequently, although our continuous‐measurement approach yields only a ~20–30% 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 timescales, over which the largest discharge‐independent changes in sand transport occur.