San Joaquin River Pulse Flows

Abstract

The San Joaquin River (SJR) was once home to the largest spring runs of Chinook salmon on the west coast, but due to diversions and impoundments, salmon were nearly eradicated. After an 18 year-long lawsuit, a settlement created the San Joaquin River Restoration Program to achieve two goals: (1) restoring and maintaining self-sustaining salmon populations from California’s Friant Dam to the confluence of the Merced River, while (2) avoiding adverse water supply impacts to all Friant water-users. Part of the settlement states that “flushing flows,” also known as planned pulses, will be implemented to reach these goals. These planned pulses are typically deployed to mitigate adverse impacts of regulated rivers and restore or maintain the benefits of a naturally flowing river. Dams alter natural flow by reducing sediment transport, fragmenting habitat, and can eliminate native species. My study examines how planned pulses in 2018 and 2022 behaved as they moved 52 miles (84 km) downstream of Friant Dam. I found that the behavior of both planned pulses was similar despite different dam release operations. Both pulse events attenuated, broadened and decreased in height, by 76 to 84% from the first to the last river station. Travel times were observed between planned pulses, the pulses’ initial arrival travel rate ranged from 1.5 to 2 miles per hour (mph) (2.4 to 3.2 km/h) which was quicker than the pulses’ peak at 0.9 to 1 mph (1.5 to 1.6 km/h). In addition, I applied my methods to a natural pulse event that had comparable results to the planned pulses. Difficulty in determining pulse end times resulted in uncertainty and inaccuracy in estimates of pulse total volumes at downstream gauging stations making it impractical to estimate water losses or gains. To improve the accuracy of pulse timing and volume measurements, future work should refine methods for identifying pulse start and end time, separating baseflows from flood flows, and calculating total event volume and peak discharge. Utilizing hydrologic and hydraulic models to examine past and future planned and natural pulses would help to generalize findings across a wider range of conditions. My analysis of planned pulse behavior is a step towards understanding all types of pulses on the SJR and can, in turn, assist water managers with important decisions on river releases downstream of Friant Dam.