Evaluation of restoration alternatives using water-budget tools for the Wapato Lake National Wildlife Refuge, northwestern Oregon

Abstract

The lakebed in Wapato Lake National Wildlife Refuge (NWR) in northwestern Oregon was farmed for decades prior to the establishment of the refuge in 2013. Planning for restoration of these lands required extensive data collection and construction of a water budget and tools to design and evaluate potential restoration strategies. The U.S. Geological Survey (USGS) and U.S. Fish and Wildlife Service worked together to monitor streamflow and water levels in and around Wapato Lake NWR, apply the USGS Shoreline Management Tool (SMT), then construct and apply a water-budget-based Water Management Scenario Tool (WMST). The SMT was used to determine the spatial availability of different water depths (as potential habitat for different species) as a function of water level and other factors, based on topographic data. The WMST uses a water-budget approach to predict daily water levels, inflows, outflows, and areas of specific categories of water depth in the refuge over the course of a water year in response to a range of hydrologic and meteorological conditions and potential water-management strategies. In this study, two hypothetical water-management strategies were simulated to predict their effect on water levels and areas with specific water depths as an indicator of potential habitat. In the first scenario, several tributaries that had been diverted around the lakebed since the 1930s were reconnected to the lake, and an outflow weir was used to control lake level and to create a lake and seasonal wetlands of specific depths. In the second scenario, an outflow weir was combined with pumps to help meet target lake levels. Results showed that reconnecting the largest three tributaries to Wapato Lake would provide sufficient water to create a range of aquatic conditions in most years. For a median water year, rainfall and tributary flows in these scenarios provided 99 percent of total inputs to the lake, whereas pumping, weir outflows, and open-water evaporation accounted for 95–97 percent of losses. Management of lake levels could be accomplished with a variable-elevation outflow weir or a combination of a weir and pumps. The lake would take longer to fill to a higher seasonal target level during a dry year. Without an outflow weir or other means of allowing water to flow out of the lake, the largest of two existing pumps would need to be used during late spring or early summer to attain a lower seasonal target water level in summer. High-water conditions downstream of Wapato Lake may prevent the use of a simple outflow weir, as historical downstream water levels in winter and spring sometimes were higher than the target water levels used in these scenarios. Water-budget-based methods applied in this study have proven to be valuable for the design and evaluation of potential restoration strategies at Wapato Lake NWR.