In-Delta Flow

Flows through the interior of the Delta, represented by the ratio of outflow from the Delta to the inflow into the Delta, are an important component of functional flows and have wide-ranging effects on the health of the Delta and the lower San Francisco Bay Estuary. The ratio of Delta outflows to inflows encompasses a vast array of flow and ecological complexity, and is relevant to in-Delta flow patterns, including the overall quantity, timing, and variability of flow regimes. Variability of flows between years is very high in California, therefore a ratio is used to account for different water year types. A higher outflow to inflow ratio contributes to restoring more natural functional flow patterns in the Delta.

Over the past century, land use change, construction of large-scale water management infrastructure, export of water, in-Delta diversions, and consumptive water use have greatly changed flow dynamics in the Delta. Several rivers, creeks, and other water bodies contribute water to the Delta, the largest of which are the Sacramento and San Joaquin Rivers. Freshwater inflow into and through the Delta greatly influences the ecological health of the Delta, as well as water quality and species abundances. Studies continue to evaluate ecosystem responses to Delta flows and how flow, non-flow, and other factors interact to impact specific species. 

As of late 2018, the State Water Resources Control Board (SWRCB) proposed an update to the Bay-Delta Water Control Plan to improve flows to support a connected and functioning ecosystem. This proposal impacts the watersheds of the Sacramento and San Joaquin Rivers. The update called for linking flow measures such as inflow, cold water habitat, Delta outflow, and interior Delta with non-flow measures such as improving habitat restoration, fish passage, and predator control. In December 2018, SWRCB adopted a plan designed to increase water flows through the Lower San Joaquin River and its tributaries—the Stanislaus, Tuolumne and Merced Rivers. In September 2019, the Water Board released a Draft of the initial biological goals for the Lower San Joaquin River.

In February 2020, the California Natural Resources Agency and the California Environmental Protection Agency released a framework for potential voluntary agreements to improve river flows and habitat to help recover salmon and other native fishes in the Sacramento-San Joaquin Delta. The framework is a milestone with ongoing discussions related to the Bay-Delta Water Quality Control Plan, which includes updates to the flow objectives in the Delta and its tributaries (PM ER R01-01).