San Francisco Estuary and Watershed Science

Climate change and resulting changes in hydrology are already altering—and are expected in the future to continue to alter—the timing and amount of water flowing through rivers and streams. As these changes occur, the historical reliability of existing water rights will change. This study evaluates future water rights reliability in the Sacramento–Feather–American river watersheds. Because adequate data are not available to conduct a comprehensive analysis of water rights reliability, a condition placed into certain water rights, known as Term 91, is used to model projected water rights curtailment actions. Comparing the frequency and length of the historical and simulated future water diversion curtailments provides a useful projection of water rights reliability and water scarcity in the Sacramento–San Joaquin Delta (Delta) watershed.

Projections of future water rights curtailments show that water rights holders are likely to be curtailed much more frequently, and for significantly longer durations, as we move through the 21st century. Further, many more water rights holders will be affected by curtailment actions in the future. As curtailments last longer and become more common, more water users will have to access other supplies, such as groundwater or water transfers, or will have to fallow land or conserve water in other ways to meet their demands. These activities will likely ratchet up the potential for additional conflicts over water in the Delta watershed.

The Central Valley fall-run Chinook salmon (Oncorhynchus tshawytscha) is the dominant population complex supporting the California and Southern Oregon commercial salmon fishery. The stock is largely dominated by hatchery productionand has shown high variability in adult returns, suggesting that hatchery practices are critical to thelong-term sustainability of the fishery. We compiled information from numerous sources to synthesizetrends in the number, location, size, and timingof fall-run Chinook salmon released from the five Central Valley hatcheries between 1946 and 2012. Approximately 2 billion fish were released duringthis period, nearly half of which were released from the single federally operated hatchery. Juveniles have been planted off-site in the estuary with increasing frequency since the early 1980s, particularly by state-operated hatcheries. Approximately 78% of all releases occurred between January and June, including ~25% in April and ~20% in May. Release timing and size trends differed among hatcheries,and were correlated. For example, the Coleman and Nimbus hatcheries tended to release small fish (<5 g, on average) early in the year, while the Feather, Mokelumne, and Merced hatcheries tendedto release larger fish (>10 g, on average) later in the year. Moreover, sizes-at-release (by month) haveincreased since the 1980s, leading to the emergence of a new life-history type that now comprises nearly all of the estuary releases: springtime releases of large ocean-ready “advanced smolts.” We collapsed release timing and size data into a single index of life-history diversity and our results indicate a reduction in juvenile life-history diversity, with decreased variability in release number, timing, and size through time. Together, these results indicate a reduction in the diversity of life-history types represented in the fall-run Chinook salmon hatchery releases, which may be a factor that contributes to the decreased stability of the Central Valley fall-run Chinook salmon stock complex.

We used available data to estimate changes in land use and wet, non-farmable, and marginally farmable (WNMF) areas in the Delta from 1984 to 2012, and developed a conceptual model for processes that affect the changes observed. We analyzed aerial photography, groundwater levels, land–surface elevation data, well and boring logs, and surface water elevations. We used estimates for sea level rise and future subsidence to assess future vulnerability for the development of WNMF areas. The cumulative WNMF area increased linearly about 10-fold, from about 274 hectares (ha) in 1984 to about 2,800 ha in 2012. Moreover, several islands have experienced land use changes associated with reduced ability to drain the land. These have occurred primarily in the western and central Delta where organic soils have thinned; there are thin underlying mud deposits, and drainage ditches have not been maintained. Subsidence is the key process that will contribute to future increased likelihood of WNMF areas by reducing the thickness of organic soils and increasing hydraulic gradients onto the islands. To a lesser extent, sea level rise will also contribute to increased seepage onto islands by increasing groundwater levels in the aquifer under the organic soil and tidal mud, and increasing the hydraulic gradient onto islands from adjacent channels. WNMF develop from increased seepage under levees, which is caused by changing flow paths as organic soil thickness has decreased. This process is exacerbated by thin tidal mud deposits. Based primarily on projected reduced organic soil thickness and land–surface elevations, we delineated an additional area of about 3,450 ha that will be vulnerable to reduced arability and increased wetness by 2050.