Managed Flow Regimes and Resource Values: Traditional versus Alternative Strategies

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, I posely geared toward wet year water availability to address this issue, dry year requests will allow little or no out - of- stream use. • Administering out -of- stream requests can be complicated with a fixed -time request structure; out -of- stream users may have to adjust their use through the flow range. This same administrative complexity is present whenever natural flows fall below the level of existing water appropriations; junior water users have to adjust their use so that senior users receive their water. The difference here is that instream flow requests with multiple thresholds require more frequent adjustments than requests that use the same amount through an entire season. Summary Fixed -time requests are most appropriate when a small number of instream resources are protected, each with well- defined flow thresholds that are generally far less than nat- urally available flows. Fixed -time requests also make more sense in river systems where the water quantity is largely controlled by humans (systems with large dams, diver- sions, and storage capacity), and where there is less year -to -year hydrology variation. For example, weekend whitewater boating releas- es from a hydroelectric dam into a bypassed reach would be a good candidate for this kind of flow request. Similarly, fixed -time requests might work well when providing minimum base flows for a non - native, put- and -take trout fishery. Fixed -time requests are less appropriate on largely uncontrolled systems where there are multiple resource values, where a single threshold flow per opportunity may not ade- quately provide for recreation diversity, or when the timing of natural flows is highly variable. For example, the Dolores has sig- nificant storage capability behind McPhee Dam, but the large natural variation in flow timing and availability each year requires intensive management attention when plan- ning recreational boating releases (D. Mur- phy, BLM, personal communication). Although managers have identified specific thresholds that they want to provide, revisions are often required several times each spring as the availability of natural water quantities becomes more clear. IP E238 Trigger Requests A second type of request is based on trigger mechanisms, where requests begin or end at specific threshold flows. The request specifies an amount of water at a location but applies whenever the natural flow regime provides that flow, not just during a specified time peri- od. For example, with triggers at 2,500 and 1,300 cfs, a trigger request would ask for 1,300 cfs whenever available flows are between 1,300 cfs and 2,500 cfs. Once flows equal or exceed 2,500 cfs, that amount is requested. Figure 3 shows an example of an annual hydrograph (1995) and a trigger request based on thresholds at the optimum and low end of the range for each type of craft (see section on "Choosing Thresholds to Represent Recre- ational Opportunities" below). 4,000 3,000 2,000 1,000 1995 natural hydrograph water availability 0 16 N, 16 , " " 1 ^� 1 P -9 FIGURE 3. Trigger request and natural hydrograph for 1995 on the Dolores River. In this example, the trigger request pro- vides 26 days of optimal whitewater rafting and another 22 days of scenic rafting. Similar- ly, it provides 35 days of optimal whitewater canoeing and another 47 days of scenic canoeing. This is the identical number of days of white - water and scenic boating that a natural regime would have provided in 1995, but it still pro- vides water for out -of stream uses (the area between the natural flow hydrograph and the request hydrograph, as labeled). There is some loss of diversity with this request because flows are provided at only four distinct levels, but these still offer some variation and they arguably represent the major opportunities available under a natural flow regime. Advantages • Specific instream flow values are identified with specific threshold flow levels, which can be labeled and debated in a systematic fashion (see section on "Choosing Thresh- Rivers • Volume 7, Number 3 request 1111