*2.4. Contemporary Conditions*

Extensive salinity intrusion in the Delta in the early 20th century, caused by a combination of hydrologic variation and upstream land use and hydrologic change, motivated a series of Delta field investigations that led to a better understanding of the relationship between sources of water flows and salinity patterns in the Delta [65]. These findings supported the development of reservoirs in the upstream watershed to store winter and spring flows and supply irrigation water needs in the summer months. Among the various reservoirs built in the Central Valley, the federal government completed construction of the 4.5 million acre-feet (5.6 BCM) Lake Shasta in 1944 as part of the Central Valley Project (CVP) and the state government completed construction of the 3.5 million acre-feet (4.3 BCM) Lake Oroville in 1968 as part of the State Water Project (SWP) (see Figure 1). Over

a period of roughly three decades, a complex network of reservoirs, aqueducts, pumps and gates was constructed to facilitate transport of water to other parts of the state for agricultural and municipal use.

Today, regulatory activity related to the management of estuarine flow and salinity is led by the California State Water Resources Control Board (CSWRCB), an agency concerned with both the water quality and water rights adjudication in California. In August 1978, the CSWRCB adopted its Delta Plan and Decision 1485 which set objectives for Delta outflow [69]. CSWRCB updated its Delta Plan in 1995 and adopted Decision 1641 in 2000 [70], which is still in force. The position of the X2 isohaline is a particular focus of salinity regulation in the estuary, and target ranges are defined by season and water year type. The position of the X2 isohaline is managed through control of out-of-basin exports from the Delta and reservoir outflows from major CVP and SWP reservoirs in the watershed.

Based on continued risk to certain endangered aquatic species, additional restrictions were imposed on the system through biological opinions rendered by the U.S. Fish and Wildlife Service in 2008 [71] and the National Marine Fisheries Service [72] under the U.S. Endangered Species Act. Both biological opinions were recently updated [73,74]. Additional flow regulations are being considered as part of the CSWRCB's Delta Plan periodic review [18,75].

A variety of models have been developed to interpret flow and salinity intrusion in the contemporary Delta; these models are used for research, regulatory planning and for CVP and SWP operations support. Jassby et al. [12] is an example of a commonly used empirical X2-outflow model; Rath et al. [76] provides a comprehensive review of this and other published empirical X2-outflow models. Mechanistically based hydrodynamic models of the estuary include the one-dimensional Delta Simulation Model 2 or DSM2 [77] and more complex three-dimensional models such as SCHISM [78], UnTRIM [61,79], and Delft3D [80].
