PREDICTED EFFECTS OF FUTURE LAND-USE CHANGE ON FLOODPLAIN AND STREAMBANK LOADS OF SEDIMENT AND NUTRIENTS IN THE MID-ATLANTIC US

Floodplain wetlands and streambanks play a key role in watershed nutrient and sediment transport and stream water quality. However, current tools have been insufficient to make scalable, spatially-explicit, quantitative predictions of sediment and nutrient fluxes. We measured bank erosion and floodplain deposition over decadal time scales using sediment characteristics, dendrogeomorphology, and reach geomorphology along 68 stream reaches that represent the diversity of geology and land-use of the U.S.mid-Atlantic region. We then related upstream watershed characteristics, including land use, to measured reach-scale sediment and associated N, P, or C fluxes for both streambanks and floodplains using Random Forest regression modeling. This model was then used to predict current fluxes at each unmeasured NHDPlusV2 reaches throughout the Chesapeake and Delaware river watersheds. Further, the model was used to predict the effects of future land use change scenarios of population growth and planning strategies on fluxes in central Virginia through 2061.

The magnitude of measured fluxes was similar across Valley and Ridge, Piedmont, and Coastal Plain physiographic provinces, while smaller fluxes were measured in the higher relief Appalachian Plateau and Blue Ridge provinces. The average sediment flux of floodplain deposition for all sites within a reach was twice that of streambank erosion. Random Forest regression models explain between 4 to 32% of the spatial variation in measured fluxes. When predicted for each of the 99,662 NHDplusV2 stream reaches of the combined Chesapeake and Delaware River watersheds, bank erosion was offset by floodplain deposition.

The results suggest a slightly larger magnitude of upland and gully erosion delivery to streams that was similar to catchment incremental increases in downstream suspended sediment load. Possible future changes in land use in central Virginia were estimated to increase streambank erosion by 2-3% compared to the current downstream sediment load, but to increase floodplain deposition by 3-6%. In conclusion, the fluvial exchange of sediment between streams and floodplains is critically important for understanding watershed transport processes and the impacts of land use change on stream water quality.