Providing Flows for Fish


Putah Creek below Putah Creek Diversion Dam, December, 2014, a drought year.  The stream flows are regulated in part to support a diverse native fish fauna, including Chinook salmon.

by Peter Moyle

A reality in California and the American West is that people are competing with fish for water. We humans are winning the competition.  However, because there are moral, aesthetic, and legal obligations to provide fish with water in streams, biologists like me often get asked the question “Just how much water do the fish need, anyway?” This, of course, is the wrong question because the best reply is  “all of it!” if you consider the stream flows under which each fish species evolved, that often varied from raging torrents to gentle summer trickles across a single year.   The question may then switch to, “well, what is the minimum flow we need to provide to keep the fish alive?”  This is also the wrong question because if you keep a stream fish assemblage on minimum flows for a long enough period, most native species will likely disappear. In their place will be trout raised in hatcheries and non-native species like fathead minnows and green sunfish; these fish will live in a highly degraded habitats, signified by dead riparian trees and stagnant pools. A more useful question is “what is the optimal flow regime that will allow a diverse native fish fauna and other biota to thrive, while providing water for use by people?”

Attempts to find an answer to the last question, usually by simple, cheap hydrologic methods, have dominated stream flow disputes over fish for decades.  While some regions now typically use holistic approaches, in the USA and few other countries there remains heavy reliance on mechanistic approaches such as the Instream Flow Incremental Methodology (IFIM) and its modeling companion PHABSIM.  Frankly, these methods don’t work very well, because they are based on some untenable assumptions and on simplified models that bear little resemblance to ecological reality. Without trying to explain them further, lets just say they are likely to work best in a ditch that contains only rainbow trout.  So what does work for most streams?  There are several good options, which are explained in a new book Environmental Flow Assessment : Methods and Applications by John Williams,  Peter Moyle, Angus Webb, and Mathias Kondolf (2019, Wiley Blackwell, 220 pages).  You will note that I am a co-author so you may want to read this summary with some skepticism.


It is not by accident that the two chapters of the book following the introductory chapters are primers on flow and life in rivers and streams.   These chapters make the point that flowing waters are beautifully complex both physically and biologically, so it is rather naïve to think that a simple hydrologic model will suffice to determine flows needs for desirable aquatic life  (or even just fish).

The next chapter (5) deals with tools for environmental flow assessment (EFA). It shows that we don’t have to be content with standard hydrologic models because there are so many options these days, of intellectual tools, models, concepts and approaches that be used for EFA. Examples of new approaches to EFA that the book describes include Bayesian networks and hierarchical Bayesian models, of growing interest in biological systems.  Of course, tools are only as useful as the users make them.  “Ultimately, successful EFA depends on clear and critical thinking; the human brain is the most important tool for environmental flow assessment (p. 51).”

Once the myriad of tools available for EFA are in hand, there are many options for methods to determine environmental flows. Chapter 6 classifies these options and provides a critique of how and when each one is useful; the methods range from very simple to very complex. Any of them can be misused, of course, but most methods, preferably holistic ones, can be adapted to local situations.  The latter is important because no two streams are exactly alike so when a study is proposed, diverse options should be considered.

This is also true for the use of models in flow assessment, which seem ubiquitous. They are often used as if they can, by themselves, provide definitive solutions to a flow problem.   Because of this, the book (Chapter 7) provides a lengthy discussion on modeling and model testing.  It‘s the kind of background we hope everyone involved in an EFA relies on from the beginning. But the most basic lesson here is that: “Models are best used in EFA to help people think, not to provide answers (p. 141).”

Nowhere in the EFA universe are models more important than when looking at the how rivers are regulated below dams. Indeed, most EFAs are done on regulated streams.  Historically, most EFAs were couched as water vs fish, with little attention paid to inevitable geomorphic simplification of fish habitat that dams cause. Dams reduce sediment inputs, stabilize stream channels, and eliminate most high flows events. These processes are important for allowing stream channels to support diverse riparian and aquatic habitats for fish and all other aquatic life (Chapter 8).  Maintaining a ‘living stream’ below a dam is hugely challenging, but possible if an adaptive attitude is maintained towards EFMs.

Much of the book is critical of common methods for EFA. However, Chapter 9 presents a highly workable approach for the typical case where limited data are available, so expert opinion becomes more important for developing a flow regime that favors desirable species, usually fish.  The trick is to use structured methods to turn expert opinions (including those from published papers and reports) into a conceptual model of the situation. The conceptual model can then be quantified as a Bayesian Network Model. This model can be continuously improved as more data becomes available.  Ideally these data would be collected from the stream being modeled, by monitoring the effects of an initial flow regime. This results in a ‘feed back loop’ of more data making the model more useful followed by further manipulation of the study stream.  Eventually, the greatly improved data set will allow a more powerful Bayesian hierarchical model to be used.

The final chapter is short but contains the statement  “We have also emphasized that EFA is human activity and so subject to human behavior.”   This idea should be kept mind even when evaluating abstract hydrologic models. The book ends with a long checklist of things to keep in mind when conducting an EFA.

Because the book is aimed a broader audience than the people who just conduct EFAs, we made an effort to keep the language as concise, jargon–free and as clear as possible, given our own deep interests in the issues. In fact, clear writing is key to good EFAs, especially plain-word summaries of technical reports.

It is hard to over-emphasize the need for improved and innovative assessments of environmental flows in California’s streams, because there are over 1400 large dams in the state. Most dams need periodic reassessment of their flows for aquatic life, especially fish.  Climate change, with longer droughts and bigger floods, will create more disputes over managing the limited water supply. Improved understanding of the EFA options available should make settling such disputes in an amicable fashion more likely.

Further reading

Williams,  J., P. Moyle, A. Webb, and M. Kondolf (2019), Environmental Flow Assessment : Methods and Applications (Wiley Blackwell, 220 pages).




About jaylund

Professor of Civil and Environmental Engineering Director, Center for Watershed Sciences University of California - Davis
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1 Response to Providing Flows for Fish

  1. Frances Griffin says:

    Not exactly a competition between people and fish. More like a competition between water -ntensive crops grown for export and the rest of us plus the environment. And if those “stagnant pools” are behind beaver dams, they are GOOD for salmon and for the rest of the fauna, plus restoring the water table to fight fire and slow down flooding.
    Good data here, but let’s not spread misconceptions.

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