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13 How Low Can You Go? The Needs and Considerations for Outlets Photo 1: Releasing 300 ft3/sec through a Low Level Outlet at Dam in Utah Introduction Whether your dam is a modern wonder of the world or a low-head earthen dam tucked away in a rural community, your dam impacts the environment and populations on both sides of the structure. One of the more routine operations of managing a dam is reservoir drawdown. This article will discuss the reasons for drawing down a reservoir, the methods available for reservoir drawdown, the potential impacts, and other things to consider when drawing down a reservoir. Dam ownership, regulation, and operation vary from the federal government, state government, local municipalities, utility providers, and in some cases private individual or group owners. Because no two dams are the same and their ownership, operations, obligations, and impacts are very specific to their individual circumstances, this article will discuss these topics in generalities. Reasons for Drawdown Reservoir drawdown simply means the release of water stored behind a dam such that the reservoir water level decreases. The reasons for drawing down a reservoir can vary widely based on the purpose of the dam. Dams are built for a variety of purposes: to develop irrigation supply for agriculture, municipal water supply for populated communities, power generation through use of hydroelectric turbines; to store and attenuate water during times of high precipitation or snow melt runoff, to develop/restore various types of ecosystems, or combinations of all of these services. Dams also provide recreational opportunities as well as attractive residential property around their reservoirs, although these are generally ancillary benefits and not primary reasons for constructing a dam. The purposes, schedules, rates, and magnitudes of reservoir drawdowns are generally described in a dam’s Operation and Maintenance Manual (O&M) and/or Emergency Action Plan (EAP). Drawdowns can occur for recreation, environmental considerations, water supply agreements, or dam safety concerns. Drawdowns mandated by Dam Safety regulatory agencies can be driven by poor operating conditions or damage of the dam, stability concerns, design issues, maintenance, or repairs. Reasons for reservoir drawdowns are discussed further below. Supply – The stored water behind dams may be released downstream to supply irrigation canals, pump stations, and/or water treatment plants located downstream. Water releases are also sometimes used to maintain specific waterway stages so they may be serviceable to recreational and commercial boating. In the case of routine systematic drawdowns, release schedules are often based on water distribution agreements negotiated between dam owners and water use stakeholders. Seasonal Operation – Reservoir operation is often cyclical based on seasonal weather patterns. Dam operators often drawdown their reservoirs to provide storage space prior to seasons of high precipitation and/or snow melt runoff. The high inflows then refill the reservoir. This is typically followed by a season of low precipitation, which usually coincides with the above-described demands for supplementing downstream water supply and the reservoir is gradually drawn down. And the cycle repeats. Flooding – Normal operating levels for reservoirs are established to provide additional freeboard and storage for infrequent but significant precipitation events. During these events, the increased inflow can be stored, reducing the peak flood discharge downstream. After the flood, the reservoir is lowered at a controlled rate to the normal operating level.