I. Delta Inflows Declined
II. Old and Middle River Flow Upstream
III. The Delta Estuary Shrank
IV. Fish Populations Declined and Crashed

Sturgeon Catch; Early 1960s

Sturgeon Catch; Early 1960s

When the Central Valley Project and State Water Project started operating together in the late 1960s, what happened to river flows and to fish?

The State Water Resources Control Board issued and approved its Delta Flow Criteria Report in August 2010.

An analysis by scientists William Fleenor, William Bennett, Peter Moyle, and Jay Lund of the University of California at Davis helps to illustrate the Delta’s flow and fish problems. In a nutshell, upstream diversions to storage reservoirs have reduced inflows to the Delta year-round (especially in the San Joaquin River basin). The Delta essential experiences more low flow conditions than it would under unimpaired or historical conditions prior to operation of the major water projects.

Delta Smelt

Delta Smelt


Delta In-Flows Declined

Hydrographs of the Sacramento and San Joaquin Rivers before and after State Water Project and Central Valley Project operations began (courtesy of Fleenor, Bennett, Moyle, and Lund, 2010). Inflows to the Delta have decreased from both these rivers.




Old and Middle River Flow Upstream

“Reverse flows” occur in these rivers because of the suction from the Delta export pumps. This probability curve shows that under unimpaired flow conditions, net downstream flows on Old and Middle Rivers would occur nearly 85 percent of the time, but only 8 percent of the time under water project operations (courtesy of Fleenor, Bennett, Moyle, and Lund, 2010)



The Delta Estuary Shrank

“X2” is a measure of the position of the low salinity zone. When it is closer to the Golden Gate, the Delta estuary grows in size, which is better for fish and other aquatic creatures. This chart is saying when the Delta is under less intense or no water project operations, the Delta estuarine habitat is larger as more fresh water flows through it, and it shrinks when water project operations are more intense (courtesy of Fleenor, Bennett, Moyle, and Lund, 2010)

View the full report by William Fleenor and his University of California at Davis colleagues from which these charts are excerpted. Their paper on “prescriptions for freshwater flows” to the Delta is also available.


Courtesy of Bay Institute of San Francisco

Courtesy of Bay Institute of San Francisco


Graph showing relative abundance of pelagic fish (that is, fish that live in open water, rather than at the bottom or along shore), dating back to late 1960s. Species included are: DS=Delta smelt; LFS=longfin smelt; ST=sturgeon;  SB=striped bass; AS=American shad; TFS=threadfin shad.


Courtesy of the Bay Institute of San Francisco

Courtesy of the Bay Institute of San Francisco

Charts showing declines of winter- and spring-run Chinook salmon population declines since the late 1960s.

View the Bay Institute’s presentation about Delta fisheries decline to the National Research Council, from which these two charts are drawn.

It seems logical to ask: if fish populations are declining, how much water flows would they need in order to recover?

Of course, the answer involves not just how much, but also what quality of water, and in what time of year do fish and other public trust resources benefit from water flows?

While it has some complex answers, this simple question had not been taken up in formal policy deliberations since 1992 by the State Water Board. This is important because the Water Board has the duty and responsibility to regulate flows through their authority to allocate rights to water.

Among the flurry of new water laws passed last November 2009, the California Legislature assigned to the State Water Resources Control Board the task of studying and reporting back on what river flows and Delta outflows fish need to recover their populations.

What Flows Do Delta Fish Need to Recover? 


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