The Role of Floodplains through River Corridors: How will increasing reconnection alter downstream export?

Abstract:

Extreme floods deliver high loads of sediment and nutrients through river corridors to downstream. As floods increase in magnitude and frequency, their impacts to downstream infrastructure and excess nutrient delivery to coastal environments requires rethinking the role of the river corridor. Throughout river networks, many streams and rivers are disconnected to their adjacent floodplain or there is hydrologic “short-circuiting”. A functioning river corridor system allows for transport during flood events into and out of the floodplain. Active exchange between a river and its adjacent floodplain not only reduces downstream flood peaks by temporarily storing water, but can also result in nutrient removal. Thus, floodplain reconnection is one approach to mitigate effects of flood events. Here, we will highlight the role of floodplains throughout the river corridor, from headwaters to large rivers. Within smaller streams, water residence times generally do not support high removal of dissolved nutrients, but do impact solute mobilization, sedimentation, and downstream river stage. In larger floodplain systems, flood events occur over longer time periods resulting in the potential for significant solute removal and decreased flood peaks. To illustrate this, we will present results from the second largest flood on record in the Lower Mississippi River and from multiple field studies in smaller floodplain systems. Collectively, our findings suggest that smaller floodplains may be a material source during extreme events, while still serving a role in buffering downstream peak river stages. In contrast, larger floodplains are likely a sink and reduce excess material transport during large events. Adaption to future events will require rethinking the importance of river corridors, not only as conduits for transport, but as places to promote exchange of water, sediment, and nutrients for enhancing ecosystem services.