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

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Chapter 4 EXPLORING THE EFFECTS OF FLOW ON RESOURCE CONDITIONS Instream flow decisions are decisions affecting stream hydrology. However, a meaningful instream flow analysis also requires an understanding of the relationship between a river's flow regime and associated hydraulic, geomorphic, and riparian vegetation conditions. Often passed by because of their complexity, these relationships can be critical for exposing significant impacts to a river's biotic or recreational resources. Rivers are dynamic over both the short and long term; ignoring the latter can put valuable resources at risk. In order to understand and characterize the interplay between flow and various resource conditions, researchers need information about the following areas, each of which are discussed in subsequent sections of this chapter. ❑ Quantification of the river's hydrology (the amount and timing of flows). ❑ Quantification of the river's hydraulic geometry (how flows affect associated hydraulic variables such as depth, width, velocity, and wetted perimeter). ❑ Description of the river's landscape position and river type using a geomorphically -based river classification system. ❑ Integration of hydraulic geometry and related geomorphic processes or conditions, especially those affecting the responses of channel and depositional areas such as beaches, bars and floodplains to changes in hydrology. ❑ Assessment of how flow regimes influence the character and type of riparian vegetation. The reader should keep in mind that these are complex technical subjects which are only treated briefly here. For a more complete discussion, see standard hydrology texts such as Water in Environmental Planning (Dunne and Leopold, 1978), The Fluvial S sy tem (Schumm, 1977), or the U.S. Geological Survey guidelines on hydrology field techniques (see references at end of chapter). HYDROLOGY Hydrology refers to the amount of water in a river and the timing of flows (e.g., daily, monthly, or annually). Some of the most useful hydrologic descriptors for framing an instream flow quantification are mean monthly flows through they year and median daily flows for each month. Mean monthly flow indexes the amount of water available for instream allocation each month (Figure 8) and in the typical form shows how these are distributed throughout the year. Historical monthly maximums and minimums can also be useful for indexing the range of variability. In contrast, median daily flows provide more detail about changes through a given month. This situation is especially important in and and semi -arid stream systems, where infrequent high discharges can greatly skew 17 1600 1400 1200 1000 800 600 400 200 now J F M A M J J A S O N D Figure 8. Mean monthly flows provide an overview of the water likely to be in a river over the course of a year. (Data come from Birch Creek, Alaska).