INSIGHTS ON INUNDATION AND HYDROBIOGEOCHEMISTRY IN A FIRST-ORDER TIDAL STREAM FLOODPLAIN (Invited Presentation)

The systematic response of coastal terrestrial – aquatic ecosystems to inundation and salinity exposure is fundamental to the biogeochemical functioning of these systems under changing climate and land use. Fish passage and seawater access to Beaver Creek, a first-order tidal stream in western Washington State, were recently restored in 2014. This makes the site a potential analog for studying the hydrobiogeochemical effects of seawater encroachment in established freshwater ecosystems. In this study, freshwater-seawater interactions in a Beaver Creek floodplain system are characterized and modeled. Water levels in the stream are highly correlated with tidal forcing, which results in periodic inundation of the floodplain at monthly frequency. Low permeability floodplain materials limit lateral groundwater flow and transport, increasing residence times. With limited lateral exchange with the stream, the monthly inundation events, which select for the highest seawater fraction in a tidal cycle, are a significant perturbation to the floodplain hydrobiogeochemistry. Infiltration during inundation events accounts for the bulk of the mass input to the floodplain. The magnitude and duration of inundation as well as the chemical composition reflect the seasonal conditions in the Grays Harbor estuary. Modeling is used to better understand the interplay of processes, properties, and conditions that mechanistically link floodplain biogeochemistry to the inundation events. The current focus is on the flow of carbon through the floodplain in the context of interactions with the upland forest and tidal stream.