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WWRC 90-15
Flushing Flows

Introduction and Background

The regulation of streamflow can both positively and negatively affect existing fishery habitats, fish populations, and channel characteristics. This occurs because, in most geographical areas, natural precipitation and run-off patterns have produced well-defined periods of low and high streamflow. The regulation of such flows can disrupt the biological communities which have adapted to the natural flow regime. In the western U.S., it was quickly recognized that uncontrolled development of water could result in the elimination of aquatic communities and dramatically alter channel morphology. This was alarming, since many of the systems in jeopardy harbored significant sport and commercial fishery resources, such as the salmon fisheries of the Pacific Northwest.

As a consequence, fisheries biologists and hydrologists have begun investigating the relationships between fishery habitat and streamflow with the ultimate goal of being able to prescribe flows necessary for the maintenance and/or enhancement of fish populations. To this end, a wide variety of methodologies for assessing the "instream flow" needs of aquatic life have been developed and used. Descriptions of many of these can be found in Stalnaker and Arnette, Wesche and Rechard, Estes, and EA Engineering, Science, and Technology, Inc. The net effect is that the regulation of water development projects is now often designed with consideration for existing fishery resources.

An important concept overlooked during many instream flow studies is the dynamic nature of the river system being regulated. Bovee noted that the instream flow investigator will ultimately be confronted with one of two problems related to channel dynamics: first, the determination of a flow regime that would prevent channel change and, second, the prediction of a new channel shape, should channel change be inevitable. From a biological perceptive, it is more desirable to accurately address the former than to risk the uncertain and possibly catastrophic consequences of the latter.

In regulated stream systems, an important option exists which can be used to maintain desired channel characteristics: the programmed release of a predetermined discharge for a given duration. Such releases, termed "channel maintenance" or, more commonly, "flushing flows" (for the effect of removing [flushing] fine sediments from gravels), can be applied to meet a variety of interrelated management goals. In this chapter the basis for and the theory behind such flows are discussed and methods used for making flushing flow prescriptions are presented. The last part of the chapter is devoted to providing guidance in developing reliable flushing flow recommendations.

In general, the primary uses of flushing flows include maintenance of channel geometry and the qualitative and quantitative maintenance or enhancement of fishery habitat. Indeed, 76% of respondents to a recent survey reported by Reiser et al. listed the removal of fine sediments from spawning gravels as a major purpose of flushing flow releases. In addition. 35% listed the removal of stored sediment from fish rearing habitats, while 37% cited channel maintenance as a major need. Given these results, it is not surprising that most of the flushing flow methods that have been developed (and discussed in this chapter) have focused on the maintenance of fish habitat.

While the need for flushing flow releases for channel maintenance has not received the attention that fish habitat maintenance has. the two are closely linked. Perhaps the primary difference between the two is more a matter of temporal and spatial variation than of physical process. The goal of a fish habitat maintenance flush is often a short-term improvement of habitat critical to a given life stage and/or species. Such a flow release can be either a routine annual event intended to coincide with the normal run-off period, or a one-time mitigative action to offset a perturbation resulting from an improper land-use activity (e.g., increased sediment load resulting from poor erosion control) or a natural catastrophic event such as a landslide or slump. In either case. the results of the flush can be measured immediately and evaluated with respect to the biological consequences of fine sediment deposition. While results of studies assessing the impact of sediment on aquatic biota are at times inconclusive and contradictory, in general they have demonstrated inverse relationships between the accumulation of fine sediment in fish spawning and rearing habitat vs. fish survival and abundance.

The concept of channel maintenance flows implies a longer time frame to determine success or failure, entails management of the entire active channel, and requires an understanding of the complex set of factors that influence channel morphology. For example, a fish habitat flushing flow may be termed a success if it reduces fine sediments in an important spawning area. However, a flow of the same magnitude and duration released in the same river may be inadequate for channel maintenance if flows are insufficient to flush fine sediments and plant materials from the edge of the stream. If such a situation persists over a period of years, riparian encroachment into the active channel might occur, resulting in a change in channel size and shape.

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