The importance of groundwater to the upper Columbia River floodplain wetlands

The Columbia Wetland complex is a rare example of a North American river system with relatively little disturbance from human infrastructure and is the only undammed portion of the main 2000 km stretch of the Columbia River. Declining river flows in western North America, including the upper Columbia River, have reduced the area of open water wetlands in the floodplain and raised concern that the Columbia Wetlands will not remain viable under increasing climate change. In this study we use water isotopes (& delta;O-18 and & delta;H-2) and electrical conductivity to quantify the proportion of groundwater, river water and precipitation contributing to wetland water balance, as well as the role of evaporation, in the Columbia Wetlands through the spring, summer and fall of 2019. We found strong seasonality of water input sources. Groundwater and precipitation were important in spring and fall, while river water was dominant during the summer. An individual wetlands' location in the floodplain as well as relative connectivity to the river channels influenced its seasonal pattern of input sources. Quantifying the relative contributions of the main input water sources to wetlands provides important new understanding of hydrologic connectivity in the Columbia Wetlands.