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APPENDIX A Hydrology 101 / AOP Procedures A-I General Background In the West, reservoirs serve as a moderating control over river flows by storing spring snowmelt runoff and releasing the water during the remainder of the year, The spring peaks can be quite large and releases are usually substantially smaller. The size of this difference directly affects the change in reservoir storage throughout the year. If this difference is large, the reservoir gains storage quickly in the spring and gradually loses storage during the remainder of the year. Water year 1995 is a good example of this phenomenon. The monthly pattern of releases also affects reservoir storage. If the releases resemble a "natural' or unregulated hydrograph with high releases in the spring, reservoir storage would not change but the releases would vary widely, Conversely, if releases are held constant throughout the year, it is the reservoir levels that would fluctuate greatly. Typical dam operation consists of balancing these reservoir storage and release objectives. Often, storage targets dictate releases. Releases are adjusted in response to inflow to meet these target storage levels. In the case of Glen Canyon Dam, statutes and compacts govern the nature of annual releases as well as storage levels. A minimum objective release of 8.23 MAF was set by the 1970 Operating Criteria in an effort to meet "Law of the River" commitments to the Lower Basin States and Mexico. During periods of low inflow, this minimum objective release has the effect of drawing down Lake Powell and stabilizing Lake Mead storage. The drawdown of 1988 - 1994 was dir-ectly a result. Storage equalization provisions in the 1968 Colorado River Basin Project Act also serve to keep the two large reservoirs relatively equal in content At present this is a one-way equalization, occurring only if Lake Powell contents are greater tban Lake Mead contents. The 1968 Act also contains an Upper Basin "storage insurance" clause, eliminating equa1ization if Lake Powell content drops below a level determined to protect Upper Basin consumptive uses, and a spill avoidance clause designed to avoiding wasting project water. With these controls in place, determining monthly releases is sometimes a difficult matter. It begins by estimating future inflow, then adjusting future releases until storage objectives and release commitments are met. Five factors are key to these decisions: delivery requirements, reservoir inflow, storage, forecasts and potential forecast errors, and the downstream environment of the Grand Canyon. To facilitate these determinations required by the various Acts and Compacts, the Colorado River Management Work Group was established by Reclamation in 1986, This group was originally comprised of the Basin States, Reclamation and Western, but has