Environmental Flow Assessment

Methods and Applications

John G. Williams, Peter B. Moyle, J. Angus Webb, G. Mathias Kondolf

• Editorial: Wiley-Blackwell
• Año: 2019
• ISBN:
  9781119217381
• Edición: 1
• Categoría: Physical Sciences
• Categoría: Hydrology

Provides critiques of current practices for environmental flow assessment and shows how they can be improved, using case studies.

In Environmental Flow Assessment: Methods and Applications, four leading experts critique methods used to manage flows in regulated streams and rivers to balance environmental (instream) and out-of-stream uses of water. Intended for managers as well as practitioners, the book dissects the shortcomings of commonly used approaches, and offers practical advice for selecting and implementing better ones.

The authors argue that methods for environmental flow assessment (EFA) can be defensible as well as practicable only if they squarely address uncertainty, and provide guidance for doing so. Introductory chapters describe the scientific and social reasons that EFA is hard, and provide a brief history. Because management of regulated streams starts with understanding freshwater ecosystems, Environmental Flow Assessment: Methods and Application s includes chapters on flow and organisms in streams. The following chapters assess standard and emerging methods, how they should be tested, and how they should (or should not) be applied. The book concludes with practical recommendations for implementing environmental flow assessment.

• Describes historical and recent trends in environmental flow assessment
• Directly addresses practical difficulties with applying a scientifically informed approach in contentious circumstances
• Serves as an effective introduction to the relevant literature, with many references to articles in related scientific fields
• Pays close attention to statistical issues such as sampling, estimation of statistical uncertainty, and model selection
• Includes recommendations for methods and approaches
• Examines how methods have been tested in the past and shows how they should be tested today and in the future

Environmental Flow Assessment: Methods and Applications is an excellent book for biologists and specialists in allied fields such as engineering, ecology, fluvial geomorphology, environmental planning, landscape architecture, along with river managers and decision makers.

CHAPTER 1
An introduction to environmental flows

Summary

Environmental flows are flows in a river required to sustain aquatic ecosystems and other beneficial uses of free‐flowing rivers. Environmental flow assessment is a general term for studies that can inform management of flows. Such assessments are surprisingly difficult to do right, constrained by the natural variability of the environment through which rivers flow and the diverse needs of organisms that live there. They are also made difficult by social constraints that pit human demands for water against those of the environment, and by aspects of human behavior.

1.1 What are environmental flows?

The 2007 Brisbane Declaration of the 10th International River Symposium and Environmental Flows Conference states that: “Environmental flows describes the quantity, timing, and quality of water flows required to sustain freshwater and estuarine ecosystems and the human livelihoods and well‐being that depend upon those ecosystems.” We will use this definition, taking “freshwater ecosystems” to include riparian areas. “Instream flows” is an older term that means much the same thing, but we prefer “environmental flows” because it implies a broader view of what should be assessed; instream flow assessments historically have been concerned mainly with the physical environment of only a few species, especially salmonids. We take environmental flow assessment (EFA) to be the process of trying to translate the Brisbane definition into usefully precise estimates of environmental water needs and the effects of modified flows on ecosystems and human well‐being, to inform decisions such as:

• Whether to reserve some portion of the flow in a stream for environmental uses, and if so, how much, and on what kind of schedule;
• How effects of an existing project on streams or estuaries can be mitigated (or not) by releases of environmental flows or restrictions on water withdrawals;
• Whether and how to modify existing water projects to improve environmental conditions;
• Whether and how to build a new water project.

Environmental flow assessment is hard to do well. This book is about the scientific and social difficulties with EFA and how to address them as best one can. In this chapter, we first explain why EFA is so difficult, and address problems with the EFA literature.

1.2 Why EFA is so hard; scientific issues

1.2.1 Stream ecosystems are dynamic and open

Twenty‐some years ago, three of the authors of this book participated in a small workshop on environmental flow assessment at the University of California at Davis, which concluded that “…currently no scientifically defensible method exists for defining the instream flows needed to protect particular species of fish or aquatic ecosystems” (Castleberry et al. 1996). Despite major progress with analytical and statistical methods over the last 20 years, especially those described in Chapter 9, we still believe that at best an EFA should be regarded as a first cut, to be implemented within the context of adaptive management. Why is this problem so hard? Scientists have a truly wonderful understanding of the nature of energy and matter, the evolution of the universe, the atomic structure and properties of molecules, the structure and activities of cells, the origin of species and the evolutionary relationships among organisms, and much more. Why, then, is it so hard to assess the consequences of taking some of the water out of a stream, or changing the timing or temperature with which water flows down the stream?

The reasons have been known for some time: ecosystems are open, dynamic systems that are “…in a constant state of flux, usually without long‐term stability, and affected by a series of human and other, often stochastic, factors, many originating outside of the ecosystem itself” (Mangel et al. 1996, p. 356). For such reasons, Healey (1998) argues that questions such as “How much can a river's hydrology be altered without endangering its ecological integrity?” are trans‐scientific, sensu Weinberg (1972); trans‐scientific questions: “… can be stated in the language of science but not answered by the traditional means of science.” These ideas have been restated recently by Harris and Heathwaite (2012) and by Boyd (2012, p. 307): “Predicting the dynamics of real ecosystems – or even of components of these ecosystems – will remain beyond the reach of even the best ecosystem models for the foreseeable future.”

A long‐term study on the South Fork Eel River in Northern California (Box 1.1) illustrates these points. Although the highly predictable seasonality of flow is a major factor structuring the food web in that river, year‐to‐year variation in the timing and magnitude of high‐flow events results in substantial variation in the structure of the food web and its response to mobilization of the bed by high flows; for practical purposes, predictions of the response can only be probabilistic, not deterministic.

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Box 1.1 Variable Effects of High Flows on a River Ecosystem

Eighteen years of field observations and five summer field experiments in a coastal California river suggest that hydrologic regimes influence algal blooms and the impacts of fish on algae, cyanobacteria, invertebrates, and small vertebrates. In this Mediterranean climate, rainy winters precede the biologically active summer low‐flow season. Cladophora glomerata, the filamentous green alga that dominates primary producer biomass during summer, reaches peak biomass during late spring or early summer. Cladophora blooms are larger if floods during the preceding winter attained or exceeded “bankfull discharge” (sufficient to mobilize much of the river bed, estimated at 120 m³ s⁻¹). In 9 out of 12 summers preceded by large bed‐scouring floods, the average peak height of attached Cladophora turfs equaled or exceeded 50 cm. In five out of six years when flows remained below bankfull, Cladophora biomass peaked at lower levels. Flood effects on algae were partially mediated through impacts on consumers in food webs. In three experiments [with caged fish] that followed scouring winter floods, juvenile steelhead (Oncorhynchus mykiss) and …[coastal roach, Hesperoleucus venustus] suppressed certain insects and fish fry, affecting persistence or accrual of algae depending on the predator‐specific vulnerabilities of primary consumers [that were] capable of suppressing algae during a given year. During two post‐flood years, these grazers were more vulnerable to small predators (odonates and fish fry, which… [steelhead stocked in the cages always suppressed] …[As a result, the abundant grazers] had adverse effects on algae in those years. During one post‐flood year, all enclosed grazers capable of suppressing algae were consumed by steelhead, which therefore had positive effects on algae. During drought years, when no bed‐scouring winter flows occurred, large armored caddisflies (Dicosmoecus gilvipes) were more abundant during the subsequent summer. In drought‐year experiments, stocked fish had little or no influence on algal standing crops, which increased only when Dicosmoecus were removed from enclosures. Flood scour, by suppressing invulnerable grazers, set the stage for fish‐mediated effects on algae in this river food web. Whether these effects were positive or negative depended on the predator‐specific vulnerabilities of primary consumers that dominated during a given summer. (Power et al. 2008, p. 263 edited for clarity)

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As another example, consider the valuable and well‐managed sockeye salmon fishery in Bristol Bay, Alaska, for which long‐term catch records are available for three major fishing districts, corresponding to areas of spawning and rearing habitat. The catch is a good proxy for the number of spawning fish, known since about 1950 (Hilborn et al. 2003). Although there has been little human disturbance in the spawning and rearing areas except for climate change, the relative contributions to the catch from the different districts has varied widely over time, as described by Hilborn et al. (2003, p. 6567):

The stability and sustainability of Bristol Bay sockeye salmon have been greatly influenced by different populations performing well at different times during the last century. Indeed, no one associated with the fishery in the 1950s and 1960s could have imagined that Egegik would produce over 20 million fish in 1 year, nor could they imagine that the Nushagak would produce more than the Kvichak, as it has in the last 4 years. It appears that the resilience of Bristol Bay sockeye is due in large part to the maintenance of all of the diverse life history strategies and geographic locations that comprise the stock. At different times, different geographic regions and different life history strategies have been the major producers. If managers in earlier times had decided to focus management on the most productive runs at the time and had neglected the less productive runs, the biocomplexity that later proved important could have been lost.

Hilborn et al. (2003) were thinking of fisheries management, but the same point would apply to managing the freshwater habitat in these regions; there have been major geographical shifts in productivity in this undisturbed habitat, and no one knows why.

1.2.2 Fish evolve

We are used to thinking of evolution as a slow process, but this is not always the case. Stearns and Hendry (2004) wrote that: “A major shift in evolutionary biology in the last quarter century is due to the insight that evolution can be very rapid when populations containing ample genetic variation encounter strong selection (citations omitted).” It is now clear that significant evolution can occur within the time spans commonly considered in EFA, and fish populations may respond to changes in the environment in unexpected ways. For example, in several California rivers, rele...