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

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the distribution of daily flows in any given month, thus providing less useful mean monthly measures. Both mean monthly and median daily flows can be readily determined from stream gage data (typically compiled by USGS). When stream gage records are unavailable for the river in question, analytical methods can often be employed to construct the needed flow statistics using gage data from similar streams in the region. On highly regulated river systems such as the Colorado or Columbia, hydrologic models have been developed to determine long -term records under various dam operating rules. Several other hydrologic variables are also useful for describing a river's annual flow regime. A flood frequency analysis describes flood size in relation to its probable frequency of occurrence (Figure 9). This information is often of particular importance in evaluating now 25,000 20,000 15,000 10,000 5,000 10 100 Recurrence Interval in Years Figure 9. A peak flow analysis (also known as a flood frequency analysis) shows how often floods of a certain size are likely to occur. Data come from the Dolores River prior to construction of McPhee Dam. 18 fluvial and riparian processes which generally depend on high flow events. In similar fashion, low -flow frequency analyses quantify the lowest flows likely to occur over periods of days or months (Figure 10), helping identify periods of critical water shortage. While analysis of annual hydrologic regime is necessary for an instream flow assessment, in many cases it is not sufficient. It is often important to know stream discharge or flow at a specific site on the river during field work. If a river is gaged, the flow records will be available for a particular gaging station. If such a station is located up- or down -river from the reach of interest, however, extrapolation of flows may be required. In the case of an ungaged river, field measurements of stream discharge using standard techniques may also be necessary (see the sidebar on page 19). now 40 30 20 10 0' 1 10 100 Recurrence Interval in Years Figure 10. A low flow analysis shows how often low flows are likely to occur. Data come from the Dolores River prior to the construction of McPhee Dam.