Instream Flows for Recreation: A Handbook on Concepts and Research Methods

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A FINAL COMMENT ON EXPLORING FLOWS AND RESOURCE CONDITIONS Preceding sections of this chapter have briefly explored how different flow regimes can affect resource conditions. As the discussion should have made clear, many of these effects are longer term and may be difficult to understand and document. Faced with such complexity, it is all too easy to ignore or pass lightly over the subject. In the classic case, researchers may list the important functions of certain flows (usually floods), but then note that no one has been able to quantify these flow needs on the river in question. In the absence of this information, they then go on to recommend a bankfull flow (the one or two year flood) for a period of a few days to a couple of weeks every year or two to play it safe. There is nothing inherently wrong with this approach, which has some validity for almost any river. Nearly all natural rivers have evolved with periodic flooding and only the most naive believe that floods are always a destructive natural force. Like many western forests that depend upon natural wildfire for their rejuvenation, many rivers (or sections of river) require floods to sustain their natural features. The issue, then, is less about whether some kind of riparian or channel maintenance flow is needed, but how large those flows should be, when they should occur, and what will happen if they are not provided. The stock recommendation of a weeklong bankfull flow every year may make a good starting point for this discussion, but in many situations a closer look is warranted. For many rivers, a "bankfull" flow is insufficient to engage most fluvial adjustment processes. For other rivers, especially those immediately downstream from dams (and therefore "sediment- starved "), prolonged periods of high flows may further diminish sediment 29 dependent resources. A major decision facing instream flow researchers is deciding when the services of hydrologists, geomorphologists, or riparian specialists are necessary to explore these sorts of issues. The brief presentations in this chapter do not provide readers with the all the tools to conduct in -depth studies on the indirect effects of flow. They do, however, provide some guidance on when those studies should be conducted and what they need to explore. For example, it should be clear that it is more important to examine these issues on heavily regulated streams (those with larger dams, diversions, or withdrawals) than those which experience more natural variation in flows (streams with run-of- the -river hydropower or smaller diversions). Similarly, rivers with important alluvial features such as beaches, bars, gravel riffles, and sloughs are at greater risk of channel changes as a result of modified flow regimes than rivers or river segments that are have more bedrock channel features; studies are thus more important on the former. Finally, rivers that feature riparian vegetation dependent upon the river (e.g., and western streams with their cottonwood groves) deserve a closer look than rivers where the riparian vegetation is virtually indistinguishable from the upland vegetation. The heart of the matter is an explicit determination of whether various channel or riparian features play an important role in providing high quality recreation on the river, and whether a change in flows seems likely to result in significant changes to those features. If the answer is obviously "no," studies need only state this argument and move on to other issues. If the answer is "maybe" or "yes," further investigation by qualified scientists is warranted.