FUNCTIONS OF FLOODPLAINS IN A CHANGING CLIMATE: DIFFERENCES IN SEDIMENT AND PHOSPHORUS CAPTURE ON FLOODPLAINS AND WETLANDS FROM EXTREME AND MODERATE FLOODS

Floodplain and riparian wetland capture of sediment and nutrients provides significant water quality benefits. These benefits can be highly variable depending on the complex interactions of geomorphic, geologic, and land use factors that operate at multiple scales, but also because of differences in the timing, magnitude, and frequency of flood events. Increasing precipitation intensities in the Northeastern United States are making extreme flood events more common, while warmer temperatures also lead to drier years with small floods. In this work, we evaluate the hydrologic controls on sediment and phosphorus deposition patterns across floodplains and riparian wetlands in the Lake Champlain Basin, Vermont, to test the hypothesis that a shifting flow regime will alter the storage function of these systems. For five years we collected event-scale deposits at 190 monitoring plots distributed across a network of 25 floodplain and wetland sites, located on a wide range of river settings, draining between 5 to 2750 km² and having channel slopes less than 0.01% to 0.6%. Our monitoring period captured both large flood events, with annual exceedance probabilities between 1 and 0.2%, and more moderate floods, with annual exceedance probabilities greater than 20%. Results indicate that flood magnitude is important for understanding sediment and phosphorus deposition rates, and rates are much greater during large floods, in part because a greater proportion of the floodplain is activated. Our work highlights the increasing importance of well-connected floodplains for mitigating the effects of a changing climate on water quality.