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Perenni a 1 streams are preferred for hydro sites because equi pment can operate much or all of the year, thus maximizing the return on the invest- ment. SSH is more dependent on natural flow than are larger projects, which can compensate for seasonal fluctuations in flow by using large impoundments. Storage of water at an SSH project is typically minimal, unless the storage is for a use other than hydropower, in which case, the water is typically unavailable for SSH. The value of a site for a SSH plant also is related to the season in which the maximum discharge occurs. For example, if the plant is to be tied into a regional electricity grid and the electric utility needs most of its output in the summer to meet air conditioning needs, a hydro plant that has its maximum output in the summer will be more useful than one that generates most of its power in the winter. Figure 6 shows the general seasons of maximum and minimum flows in the conterminous United States. The broad regional patterns shown are complicated by local factors, particularly the presence of withdrawals for uses such as irrigation or power plant cooling. The seasonality of flow is determined mainly by climate, but can be affected by storage within the system, either in the ground water, in snow, or in upstream reservoi rs. The uptake of water by vegetation and irrigation return flows can be important factors in terms of the seasonality of flows. Headwater streams, which are appealing for SSH projects because they are re 1 at i ve ly easy to dam, are especi ally sub- ject to seasonal flow fluctuations. Lowland streams, which receive contri- butions from many streams, usually have more predictable and constant flows. However, because of their wide valleys, lowland streams are more expensive to dam and, if dammed, they create a large, shallow impoundment without much head. A significant consideration for the maintenance of SSH projects (and, therefore, site suitability) is the formation of ice, which clogs screens, damages equipment, and is often associated with floods when it thaws. 3.1.2 Types of Small-Scale Hydro Projects and Their Environmental Impacts Small-scale hydroelectric installations fall into three broad categor- ies, based on the work required to bring them on-line: restoration; retro- fit; and new. Restoration (or rehabilitation) entails bringing a currently unused hydroelectric plant back into service. This process may involve relicensing, rebuilding a breached dam, dredging a reservoir, or digging canals, much as would be required for a new plant; on the other hand, only mi nor repa i rs to the ex is t i ng plant may be needed. The second proj ect type, the retrofit, involves placing hydroelectric generating equipment in an existing dam or other water project structure in order to take advantage of fall i ng water energy that wou ld otherwi se be unused for energy produc- tion. Fairly limited construction work typically is needed, but temporary cutoffs and diversions of flow are usually required to complete a retrofit. The last type of project, the new project, requires construction of a new dam or diversion structure, a powerhouse, and all necessary auxiliary facilities. Construction for new projects is often extensive, and the e~sent i a 1 character of the ri ver at that site is permanent ly altered as a 22