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Scientists Brief Council on Flow Studies Discussion on flows will help guide Delta Plan development

February 2011

How in-stream flow requirements should be addressed in the Delta Plan was the topic of in-depth discussion at the Delta Stewardship Council’s January meeting. The Delta Reform Act requires the Council to consider flow criteria developed last year by the State Water Resources Control Board and the Department of Fish and Game in drafting the Delta Plan.

Having heard earlier presentations from the two state agencies, the Council learned how flow requirements have been considered for other rivers and streams in the United States and elsewhere in the world.

Council Chair Phil Isenberg set the tone for the discussion – how to address flow requirements in the Delta while still achieving the coequal goals of water supply reliability and a healthy ecosystem. “I’m drawn to an analysis that tries to balance the use of human demand for absolute guaranteed levels of water most of the time, with the fact that you have to adjust it depending on the hydrology and all the other conditions. But I want to know how you build the ecosystem into that kind of equation to see how we could meet the coequal goals.”



The Council heard presentations from its Lead Scientist Dr. Cliff Dahm and Dr. Lucas Paz, a consultant with Arcadis.

Dr. Dahm led the discussion by giving an overview on scientific methodologies for setting flow criteria for rivers and estuaries. He gave examples of processes for setting flow criteria in Florida and Texas and in South Africa and Australia. Dahm also presented an overview of some practical approaches and methodologies used to set flow criteria.

He used an example of a river in Florida where the annual hydrograph was divided into three blocks: low flow, base flow, and high flow periods. Flow criteria was established for each period using hydrologic models and models of the needs of specific fish species throughout their life cycles. This approach leads to a percentage of allowable flow reduction in each segment of the hydrograph to protect the ecosystem and provide water supply.

He then presented, with the assistance of Senior Water Resources Engineer Chris Enright, some preliminary analyses for the Delta using this methodology. One analysis showed that a 25-33 percent flow reduction linked to the annual hydrographs for rivers entering the Delta yielded exports comparable to those actually permitted from 1990-1999 and from 2000-2009.

Dr. Paz later presented an overview of performance measures designed to evaluate the effectiveness of flow standards on large-scale restoration programs. He gave examples from Puget Sound, the Lower Columbia River and the Lower Colorado River.

In his presentation, Dr. Paz explained that a single minimum flow level at all times of the year does not provide adequate protection to the Delta ecosystem. Paz suggested going beyond a minimal flow standard by creating functional flows; in essence, mimicking the natural hydrograph that is needed to support ecosystem processes.

Council Vice Chair Randy Fiorini said he is troubled by scientific reports that focus on flow as the main problem in the Delta and the theory that simply fixing flow will solve the Delta’s problems. “There’s a lot more going on that needs to be solved that can be solved with other measures than simply adjusting flows,” Fiorini said.

Dr. Paz explained that a useful tool could be the establishment of an export/inflow ratio set for each week or month. “It would allow exports to occur when there is significant inflow to the system,” Paz said. “In order to maintain the natural hydrograph, which is an ultimate goal from an ecological perspective, you can still maintain that pattern and still draw a significant amount of water to support water supply needs if a set of rules are established in order to do Quote

Joe Grindstaff, Council executive officer, pointed out the challenge with that type of prescription. “When you get to a very dry year and you have to mimic the natural hydrology,” he said, “you’ll end up changing the salinity of the Delta and causing problems for farmers in the Delta who rely on water for agricultural purposes.” Grindstaff explained it is not the role of the Council to set flow standards in the Delta. The State Water Board is the regulatory entity required to set flows. “We will have to say something in the Delta Plan in respect to flows, in essence giving some thought and passing it on to the State Water Board,” Grindstaff told the Council.

Chair Isenberg recognized the difficulty of addressing Delta flows as he closed discussion on the topic. “The decisions we will have to make on flow will be a mixture of scientific advice and political judgments,” he said. “How to construct a Delta Plan that assists in leading to what the law currently requires, which is periodically setting Delta flow requirements, without allowing politics to intervene will be critical.”

Coequal goals

The Delta Stewardship Council was created in legislation to achieve the state mandated coequal goals for the Delta. "'Coequal goals' means the two goals of providing a more reliable water supply for California and protecting, restoring, and enhancing the Delta ecosystem. The coequal goals shall be achieved in a manner that protects and enhances the unique cultural, recreational, natural resource, and agricultural values of the Delta as an evolving place." (CA Water Code §85054)