Last week, two scientists writing for the U.S. Geological Survey published a paper that adds to the growing body of scientific concern about the effects of exports on the Bay-Delta Estuary. The paper, by James E. Cloern and Alan D. Jassby, is Drivers of Change in Estuarine-Coastal Ecosystems: Discoveries from Four Decades of Study in San Francisco Bay.
It is worth remembering how much of the Bay-Delta Estuary lies outside the Delta (which has a specified boundary at Chipps Island just west of Antioch): Cities and habitat in Suisun Bay, San Pablo Bay, Central Bay, and South San Francisco Bay all depend to varying degrees on outflows from the Delta, as do ocean-specific species beyond the Golden Gate. The map on page 4 of the paper illustrates why outflow from the Delta into the rest of the estuary cannot be considered “wasted.”
Cloern and Jassby explore the impacts of reduced amounts and altered timing of freshwater flow beginning in 1956. Flows in the San Francisco Bay-Delta watershed are heavily managed for reservoir operations, interbasin diversion, and irrigation consumption. The authors note that “Flow management of the . . . watershed is so pronounced that a median 39% of its unimpaired runoff is consumed upstream or diverted from the estuary . . . .” One result is that annual exports sometimes exceed 50% of inflow.
The authors distinguish between upstream effects and Delta effects – uses and diversions that occur once flows reach the Delta. Looking at the period from 1956-2003, they note that “The Delta effect . . . increased over time, at the expense of outflow to San Francisco Bay. The trend in Delta effect is due to a trend in water exports from the Delta . . . as opposed to within-Delta depletion that contributes a median of only 19% of the Delta effect and has no long-term time trend.” Exports increased from approximately 5% to 30% of Delta inflow, leveling off in the 1990s following the Bay-Delta Accord to set monthly quotas for export.
The effects of high exports are varied and dramatic.
“[M]odifications of inflow and salinity are contributing factors to population declines of native species in low-salinity habitats of the San Francisco Bay system . . . and to the remarkably successful establishment of nonnative species. . . including species that have restructured food webs and their productivity. . . . Water export from the Sacramento-San Joaquin Delta is a direct source of mortality to fish, including imperiled species such as delta smelt and longfin smelt . . . and export plus within-Delta depletion alters system energetics of an already low-productivity ecosystem by removing phytoplankton biomass equivalent to 30% of Delta primary production. . . . Reduced autumn inflows and associated salinity increases . . . have lowered habitat quantity and quality for species endemic to the upper estuary, such as the endangered delta smelt. . . .”
This echoes the findings of other environmental NGOs that the drop in phytoplankton due to water exports has had a negative impact on herring production and on the number starry flounder. We are seeing more and more evidence of the link between Delta water quality and the health of Bay fisheries.
Cloern and Jassby explore other drivers of ecosystem change as well: human modification of sediment supply; introduced species; sewage input; environmental policy; and shifts in the ocean-atmosphere system, including those that cause salmon stocks to fluctuate. They appear cautious in comparing multi-decade climate patterns derived from fisheries observations to the much shorter-term observations of estuarine-coastal systems.
“Given the intense human modification of estuarine-coastal systems through changes in freshwater and sediment input, introductions of alien species, and nutrient enrichment, there is uncertainty that ecological responses to oscillating climate patterns can be detected in observations having the large and varied signals of human disturbance shown above.”
We can’t do much about multi-decade climate patterns. What we can do is look at the ways human activities modify the system. Cloern and Jassby cite the 2010 findings by the State Water Resources Control Board that current flows to the San Francisco Bay-Delta “are insufficient to protect public trust resources” and that “[f]low modification is one of the few immediate actions available to improve conditions to benefit native species.”
Clearly, decreasing exports and increasing outflows is one major driver of change that we can manage.