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15 <br />1981). In these cases, the powerhouse is located below the dam, and <br />Y that reach of the river between the dam and powerhouse may be <br />subjected to very low flows depending upon the amount of water that is <br />diverted (L >> 0 and Q n A 9 << 1). Such a design can be incorporated <br />in projects that operate in both the store-and-release and <br />run-of-river modes. <br />In most cases, large-scale spatial changes (e.g., transbasin <br />diversions) and/or long-term temporal changes in stream discharge <br />patterns will not be associated with the development of small <br />hydropower resources, defined as those projects with a potential <br />generating capacity of <30 MW. Currently, this development is focused <br />on retrofitting existing dams for hydroelectric generation, and at <br />most of these sites, reservoir storage capacity is limited. <br />Consequently, modifications in the natural flow regime will likely <br />consist of short-term (e.g., AS/Qi < 24h; Eq. 1-1) rather than <br />- long-term temporal changes if the facility is operated in a peaking <br />mode. If, instead, it is operated in a run-of-river mode, no changes <br />in temporal flow patterns would be expected. Both peaking and <br />run-of-river facilities can, depending upon their design, result in <br />localized spatial changes in stream flow. <br />In summary, instream flow may be the most significant issue <br />associated with small hydropower development. Unlike the other issues <br />which are either very site-specific (e.g., fish passage, dredging) or <br />related only to projects operated in a particular mode (e.g., water <br />level fluctuations at peaking facilities), the instream flow issue can <br />affect both peaking and those run-of-river projects that utilize long <br />penstocks or canals. Moreover, maintenance of an instream flow regime <br />may reduce the potential energy that can be produced, because water <br />used to maintain instream flows is usually not available for power <br />production (U.S. Department of Energy 1981). In most regions of the <br />1 United States, hydroelectric generating systems are characterized by <br />the seasonal mismatch between energy demand and stream flow. For <br />example, in some basins like the Columbia River, which is fed by the