Floodplain Inundation and Salinization From a Recently Restored First‐Order Tidal Stream

Abstract

The systematic response of coastal ecosystems to inundation and salinity exposure is fundamental to their ecology and biogeochemical function. Here we observe and model freshwater‐seawater interactions in a first‐order stream—floodplain system where tidal access was recently restored. Subsurface flow and transport modeling were used to quantify and better understand the interplay of processes, properties, and conditions that control water level and salinity in the floodplain to the tidal stream. Water levels in the stream were highly correlated with tidal forcing, which resulted in episodic inundation of the floodplain at quasi‐monthly frequency. The tidal stream is the only source of salinity to the floodplain, yet shallow groundwater salinity was considerably higher than average stream salinity. The low‐permeability clay floodplain soils limit lateral groundwater flow and transport, resulting in floodplain groundwater and salinity dynamics driven almost exclusively by infiltration during inundation events. As inundation occurs during high tide, estuarine waters reach the floodplain with minor attenuation in salinity from the stream's freshwater discharge. Infiltration and salinity exposure are topography controlled and regulated by ponding depth and duration, seasonal ground saturation, and depth to water table. The model suggests that floodplain salinity is currently in an early stage of transition from pre‐restoration freshwater conditions and will not reach equilibrium for ~20 years. These findings have broad relevance for understanding how and over what time scales coastal ecosystems will respond to increasing seawater exposure from sea level rise, ocean‐originating storms, and changes in natural and man‐made barriers.

Impact of river flow modification on wetland hydrological and morphological characters

“…Flood water, flow of river water tie channel, groundwater flow connects the wetland with the river (Tootchi et al, 2019;Yabusaki et al, 2020). Disconnection of some wetlands from lateral flood spread limit deteriorates the water supply to the wetland.…”

Biogeochemical control points of connectivity between a tidal creek and its floodplain

“…We established a network of aquatic sensors at Beaver Creek (BC), a first-order stream in Washington State. BC drains a catchment of 382 ha, and is characterized by forested hillslopes that bound lower elevation floodplains, which account for 13% of the catchment area (Yabusaki et al 2020). BC is a tributary to the Johns River near its confluence with Grays Harbor, which flows into the Pacific Ocean (Fig.…”

“…Differences between pre-and post-cleaning readings were used to correct offsets associated with fouling/drift. Hydraulic head, which equalizes water levels between sites to a common vertical datum, and salinity were calculated from sensor data following methods in Yabusaki et al (2020).…”

“…Together, these changes form a compounding disturbance to coastal physical, chemical, and biological processes (Ward et al 2020;Yabusaki et al 2020). A strong, mechanistic understanding of the patterns and drivers of these processes is required to accurately predict how such shifts in biogeochemistry will affect coastal ecosystems, the services they provide, and the populations that depend on them.…”

The Influence of Forest Management and Changed Hydrology on Soil Biochemical Properties in a Central-European Floodplain Forest

“…Floodplain forest salinization is a phenomenon typical especially for semi-arid and arid [5,19] or brackish areas [20][21][22], where the predominant capillary rise and groundwater evaporation are the main processes of salt accumulation in soil. Under certain conditions, the process of salinization can be also expected in some inland temperate floodplains, particularly as a result anthropogenic modifications to the hydrologic cycle [23].…”