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<br />OOlG46 <br /> <br />hydrologic boundaries, so that it is invalid to confine the analysis to <br /> <br />a hydrologic unit. This means that planning for a single reservoir can- <br /> <br />not meaningfully be conducted independently of others in the same social <br /> <br />"problem-shed." A systems approach recognizing social interdependence <br /> <br />among hydrologic units is required. This is particularly true because <br /> <br />so many substitution possibilities are available to recreational water <br /> <br />users. <br /> <br />Elements of the Problem <br /> <br />To be concrete, downstream non-consumptive water uses depend not <br /> <br />only upon streamflow levels below the reservoir under consideration, but <br /> <br />also upon many biological, geographical, and social variables. As an <br /> <br />illustration, consider three types of downstream non-consumptive water <br /> <br />uses which commonly occur on Colorado streams: trout fishing, kayaking, <br /> <br />and rafting. Each is adapted to different stream conditions, but none <br /> <br />is adapted to all conditions~ Each is affected by streamflow level, not <br /> <br />only in the sense of being possible only within a limited range of flow <br /> <br />levels, but also in the sense of yielding more or less user satisfaction <br /> <br />as a function of flow level. <br /> <br />A reservoir-induced change in streamflow in one stream may cause <br /> <br />two kinds of changes in the behavior of all three types of recreational <br /> <br />users. First, some substitution among streams will occur. Users will <br /> <br />shift among streams of different sizes, flow levels, and locations in <br /> <br />response to changes in flow level in one or more of those streams. <br /> <br />Second, some substitution among activities will occur. Users will shift <br /> <br />to or from nonwater-based recreational activities in response to flow <br /> <br />level changes whenever such changes make a favorite stream less or more <br /> <br />desirable for their preferred water-based recreational activity. In <br /> <br />2 <br />