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1 1 1 t ri 1 1 applicability to the current subject. Nor will the focus be on mean annual or more frequent events as these represent flows lower than the desired protection level. Flood hydrology, or the occurrence of larger (in volume and rate of flow) flows more infrequently than occurs during an average year, up to and including the 100-year flood, is the hydrologic characteristic of most interest. These more frequent yet reasonably possible events have been recently and comprehensively studied (Ayres 1996a and b). Hydrologic computations were completed for this site utilizing both streamflow statistical information and rainfall-runoff modeling. The 205 square mile drainage basin was divided into five sub-basins representing consistent physical/meteorologic/biologic characteristics and studied in detail. The result is that snowmelt dominated streamflow characterizes the hydrology up to approximately the 100-year event as illustrated by the following table of peak flow rates. Table 2-2. Rainfall and Snowmelt Peak Rates for Elkhead Reservoir Rainfall and Snowmelt Peak Runoff Rates Elkhead Reservoir Inflow Frequency Rainfall Peak Runoff Rate (cfs) Snowmelt Peak Runoff Rate (cfs) 2-year 21 1,300 10-year 425 1,800 100-year 2,850 2,500 In fact, there are no obvious rainfall only or rainfall dominated annual peak flows in the relatively short period of record, yet rare, significant rainfall/runoff events remain as a theoretical possibility. 2.1.3 Current Reservoir Operation and Fishery Management Elkhead Reservoir does not have a defined operating plan. As previously described, controlled releases are made from the reservoir for water supply purposes via the primary outlet structure to Elkhead Creek for augmentation or diversion further downstream and from the "Starr Ditch Intake" directly to the Starr Ditch. Both outlet structures are gated and positioned on the dam. All other flow passes over the service spillway at the darn. Records of historical reservoir operations are not available and the reservoir has actually not been regularly operated for water supply purposes since its construction. Reservoir drawdowns have occurred periodically for maintenance, but otherwise the reservoir level stays very stable within one foot of the service spillway crest during an average year. As a result, the historical outflow hydrograph is very similar to the inflow hydrograph, less evaporation, with minor attenuation of peak flows during rainfall events and during spring months when some reservoir filling occurs. It is important to note that several studies have proposed significant physical changes to the reservoir and dam which could result in corresponding operational changes. Since those changes are not immediate, the current operational characteristics will be the focus of this study. The combination of the hydrology and the dam/reservoir operations results in water regulation for typical year consisting of snowmelt runoff beginning in mid-April with water going over the service spillway and continuing until about mid-July; the remainder of the year the water level remains at the service spillway crest. The primary outlet can pass up to its capacity of 180 cfs during the spring Control Structure Feasibility Evaluation 2-5 Miller Ecological Consultants, Inc., February 18, 1997