We investigated whether hydrologic restoration at Sturgeon Lake, Oregon, USA has sufficiently increased water flux and reduced stagnation, improving environmental conditions for juvenile salmon and waterfowl. This 19.2km² lake is a pivotal environmental feature in the area, providing a haven for salmon on the Columbia River before reaching the Pacific Ocean and winter habitat for hundreds of thousands of waterfowl and migratory birds on the Pacific Flyway. The Oregon Conservation Strategy names restoring natural hydrology to Sturgeon Lake as a key step toward conservation in this area. We use stable isotopes of water from the lake, surrounding water bodies, and precipitation to understand the restoration work’s efficacy and whether further efforts are necessary to restore healthy habitats. Because of its importance to bird migration and salmon spawning, we focus on seasonal patterns in lake hydrology. We determined that approximately 36.5% and 9.5% of water input was lost to evaporation during the summer and winter, respectively, after restoration. We estimate the residence time of water in the lake to average ~43.2 days during the study period. Based on these results, we determined that the lake habitat is being adequately maintained in the winter, when it is most valuable to local fauna, but that some stagnation and potential ecosystem degradation occurs in the summer. Neither juvenile salmonids nor migratory birds utilize the lake during the summer, therefore the restoration work is effective at maintaining habitat for these species, but further summer-focused work could be beneficial.