Ecological Flows Help Maintain Healthy Aquatic Ecosystems

Some say “environmental,” some say “ecological,” while others say “instream.” No matter which word you use, in general, these terms, coupled with the word “flow” are just different ways of describing the same thing: the amount of water in a river or stream needed to maintain healthy aquatic ecosystems. As Californians manage their state’s natural river systems for flood control, hydropower and water supply, flows are increasingly a part of policy discussions. Today very few rivers in California flow to the ocean without barriers or interruptions. Instead, they are managed and regulated by a variety of state, federal and private agencies, with some of them mandated to have at least a minimum flow for environmental purposes.

Flow, however, is more than just the amount of water. Timing, duration, rate of change, and frequency of flows also play an important part. Resource managers spend a lot of time trying to determine the parameters for flows in managed streams and rivers to achieve or maintain ecosystem processes that support habitat for sensitive species. In a 2002 Environmental Management journal article, authors Stuart Bunn and Angela Arthington suggest that four key principles should guide development of ecological flow regimes for managed river systems:

  1. Flow determines the extent and type of physical habitat, which in turn determines the types of living organisms in that habitat;
  2. Aquatic species have evolved in such a way as to be well adapted to the natural flow regime to which they have been historically exposed;
  3. Maintenance of natural patterns of high flows, low flows and flow variation is essential to the viability of native riverine species; and
  4. The alteration of flow regimes contributes to the invasion and success of exotic (non-native) species in rivers.

Determining environmental flows is a major challenge worldwide, and there are many ways to approach an assessment for developing environmental flow recommendations or requirements for river ecosystems.

“Many methods examine the annual hydrograph, which is a plot of long-term average river flow at a gauge station throughout the year,” said Cliff Dahm, CALFED Lead Scientist. If there is a period of time when flows were measured before major human modifications occurred, that time period is used to set the baseline or natural, unmanaged flow conditions. If no such data exists, similar streams or rivers in comparable climatic and geologic settings that have not been heavily impacted are used to establish historic conditions. Early efforts to develop flow requirements often simply stipulated that a low flow not go below a set minimum based on historic conditions. Now most hydrologic analyses set recommended flows throughout the year, providing flow recommendations for each hydrologic season (e.g. low flow, snowmelt, rainy season).

More comprehensive assessments include hydrodynamic habitat modeling, which involves determining the aquatic habitat that exists under differing flow regimes. “These holistic approaches, pioneered in South Africa and Australia, link hydrodynamics, spatial habitat analyses, and the behavior and ecology of key aquatic organisms (e.g. fish and aquatic invertebrates),” Dahm said. In the Delta, flow stipulations are part of two recent biological opinions (BOs) for Chinook salmon and delta smelt.

In December 2008, the U.S. Fish and Wildlife Service (FWS) released a biological opinion on the delta smelt, limiting the amount of water to be pumped out of the Delta during certain times of the year so as not to jeopardize the endangered smelt. To survive and recover, the opinion said, smelt need improved habitat conditions that are determined by flow. To that end, increased fall flows are required in years with above normal precipitation. The fall flows are designed to move the location of low salinity water westward from September to December in years with above normal precipitation.

In June, 2009, the National Marine Fisheries Service (NMFS) released a biological opinion stating that water pumping operations from the Delta jeopardized the continued existence of several threatened and endangered species including Chinook salmon, green sturgeon and steelhead. Modified flow rates and setting flow management standards and year-round flow regimes in the Sacramento River system (including the San Joaquin, American and Stanislaus rivers) are required by the NMFS opinion to improve spawning habitat and downstream migration of juvenile fish.


“A holistic, multi-species approach to ecological flow requirements in the Delta is needed.”

-- Cliff Dahm,
CALFED Science Program Lead Scientist


Both the smelt and salmon biological opinions are being challenged in court. “A holistic, multi-species approach to ecological flow requirements in the Delta is needed,” Dahm said. A group of water and environmental interests are working with the regulatory agencies to develop such an approach through the Bay Delta Conservation Plan (BDCP).

The Nature Conservancy (TNC), a global leader in development and implementation of environmental flow science, has generated tools and methods for assessing environmental flow needs.

Specifically in California, the Conservancy has recently completed the initial phase of an environmental flow study for the Sacramento River, upstream of the Delta, which includes the Sacramento River Ecological Flows Tool (SacEFT) developed to evaluate the ecological consequences of management-related changes in flow regime.

“SacEFT is used to compare ecological trade-offs between different flow regimes, and has been used to investigate some target flows for the Sacramento River (primarily upstream of Colusa) that have been brought into the BDCP discussions,” said Maurice Hall, a hydrologist with the California Water Program at TNC.

In 2008, TNC was awarded a grant from the CALFED Ecosystem Restoration Program to expand SacEFT to the Delta, but that program is presently on hold due to state funding postponements, Hall said. “The objective of that effort is to develop a flow tool to allow comparison of the ecological benefits of different flow regimes in the Delta. It is not expected that this tool would be complete in time to support development of a BDCP project, but would be a useful tool for long-term adaptive management of the Delta,” he said.

“The Conservancy’s work to date, both globally and in California, has emphasized the importance of flow timing, frequency of different key flow events, etc. - much more than just flow magnitude,” Hall said.

“Flow requirements have a dramatic effect on restoration and on exports from the Delta,” he added. “There is no clear consensus on how much flow is needed at any point in the Delta to meet a specific environmental requirement, much less how much flow is needed in total. One very important complicating factor is that most of the information we have about flow requirements in the Delta is based on the Delta of today (or of the past few decades), while the BDCP is likely to include a considerably modified Delta - perhaps including tens of thousands of acres of restored tidal wetlands and/or a peripheral canal - that is more supportive of environmental needs,” Hall said.

The most current ideas of what types of flow criteria are being discussed in BDCP are reflected in the table at

“This table reflects a sense of the range of flows that are thought to be needed in a modified Delta, with a peripheral canal and considerable tidal marsh habitat restoration," Hall said, "although it is important to note that no one in the process is completely comfortable with these criteria, and there is still a huge amount of uncertainty that will only be eliminated through long-term monitoring.”