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~) -, 1';r Variability: Sta!lsjard deviation of date of peak to be 12 to 25 days from thb rneaji"ciaie:,of ~ay 31i, ~ .."". I..... en Purpose: Maintaining similar p~* timing.-'yill provide ascending and descending hydrograph limbs ~j~,~ similar~y, to the historical conditions that are suspected important for spawning of the endangered fishes, F. Category: Level: Purpose: G. Category: Control: Purpose: Target Base Flow (mean weekly nonspring runoff flow). 500 cfs from Farmington to Lake Powell, with 250 cfs minimum from Navajo Dam. Maintaining low, stable base flows enhances nursery habitat conditions. Flows between 500 and 1,000 cfs optimize backwater habitat. Selecting flows at the low end of the range increases the availability of water for development and spring releases, It also provides capacity for storm flows to increase flows and still maintain optimum backwater area. This level of flow balances provision of near-maximum low-velocity habitat and near- optimum flows in secondary channels, while allowing water availability to maintain the required frequency, magnitude, and duration of peak flows important for Colorado pikeminnow reproductive success, Flood Control Releases (incorporated in operating rule). Handle flood control releases as a spike (high magnitude, short duration) and release when flood control rules require, except that the release shall not occur earlier than September 1. If an earlier release is required, extend the duration of the peak of the release hydrograph, A ramp up and ramp down of 1,000 cfs per day should be used to a maximum release of 5,000 cfs. If the volume of water to release is less than that required to reach 5,000 cfs, adjust the magnitude of the peak accordingly, maintaining the ramp rates. Multiple releases may be made each year. These spike releases shall be used in place of adjustments to base flow. Historically, flood control releases were made by increasing fall and winter base flows, This elevates flows above the optimum range for nursery habitat. Periodic clean-water spike flows improve low-velocity habitat quality by flushing sediment and may suppress red shiner and fathead minnow abundance. Operating rules for Nav~o Dam were developed in cooperation with the Bureau of Reclamation to demonstrate how the dam may be operated to meet the flow recommendations. These suggested rules determine the timing and size of release flows to maximize the ability of the river to meet the SJRIP Biology Committee February 1999 8-8 Executive Summary Flow Report I I I I I I I I I I I I I I I I I I I