Changes in lateral floodplain connectivity accompanying stream channel evolution: Implications for sediment and nutrient budgets

Authors: BECK, WILLIAM - Iowa State University; MOORE, PETER - Iowa State University; SCHILLING, KEITH - University Of Iowa; WOLTER, CALVIN - Iowa Department Of Natural Resources; ISENHART, THOMAS - Iowa State University; Cole, Kevin; Tomer, Mark

Submitted to: Science of the Total Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/5/2019
Publication Date: 4/10/2019
Citation: Beck, W.J., Moore, P.L., Schilling, K.E., Wolter, C.F., Isenhart, T.M., Cole, K.J., Tomer, M.D. 2019. Changes in lateral floodplain connectivity accompanying stream channel evolution: Implications for sediment and nutrient budgets. Science of the Total Environment. 660:1015-1028. https://doi.org/10.1016/j.scitotenv.2019.01.038.
DOI: https://doi.org/10.1016/j.scitotenv.2019.01.038

Interpretive Summary: Floodplains can act as both a source of sediment in streams and a sink that can accumulate and store sediment delivered from the stream during flooding events. This dynamic role can change in time because of disturbance impacts that increase sediment delivery from uplands, and because of changes in channel geometry. This study investigated how channel geometry changed within the Walnut Creek watershed (Iowa, USA) from 1998 to 2014, and consequences for floodplain inundation frequency and delivery of sediment and phosphorus to floodplain storage. A combination of field observations and hydraulic modeling were used in the study. Channel cross-sectional area increased by 17% over the 16 year period through both down-cutting and widening of the stream. The stream discharge required to generate flow onto the floodplain increased 15%, decreasing floodplain inundation volumes by 37% over the study due to these changes in channel geometry. This decreased sediment and phosphorus delivery to the floodplain by about 25%, and carried potential increases in watershed export of these contaminants. This suggests that water quality degradation may be driven solely by changes in stream channel geometry, which is a natural and long-term response to disturbance associated with past land use conversion to agriculture. This information will be of interest to the conservation community and emphasizes the importance of stream restoration in water quality improvement efforts.

Technical Abstract: Floodplain storage commonly represents one of the largest sediment fluxes within sediment budgets. In watersheds responding to large scale disturbance, floodplain-channel lateral connectivity may change over time with progression of channel evolution and associated changes in channel geometry. In this study we investigated the effects of channel geometry change on floodplain inundation frequency and flux of SS and TP to floodplain storage within the 5218 ha Walnut Creek watershed (Iowa, USA) through a combination of 25 in-field channel cross section transects, hydraulic modeling (HEC-RAS), and stream gauging station-derived water quality and quantity data. Channel cross sectional area increased by 17% over the 16 year study period (1998 – 2014), and field data indicate a general trend of degradation and widening (stage IV channel evolution) to be present along Walnut Creek’s main stem. Estimated stream discharge required to generate lateral overbank flow increased 15%, and floodplain inundation volume decreased by 37% over study duration. Flux of SS and TP to floodplain storage decreased by 24 and 26% over study duration, respectively. The estimated reductions in flux to floodplain storage have potential to increase watershed export of SS and TP by 8 and16%, respectively. Increased contributions to SS and TP export may continue as channel evolution progresses. Thus, it is critical that stage and progression of channel evolution be taken into consideration when addressing sediment and phosphorus loading at the watershed scale.