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14 Sediment Flushing – A side effect of a dam located along a stream or river (as opposed to an off-channel impoundment in which water is piped to the reservoir) is the accumulation of sediment that would otherwise travel downstream. In addition, the presence of a dam will also significantly lower the natural flow rate and energy through a waterway, allowing sediment to settle to the lake bottom and accumulate adjacent to the dam, rather than stay suspended and be transported downstream. In some cases, dams have scheduled releases designed to flush sediment trapped behind the dam and/or to flush sediment built up in the downstream channel. However, the presence of the sediment can also result in undesirable or unintended release of sediment during drawdowns initiated for other reasons; these can impact the downstream waterway, if not properly controlled. Managing unintended sediment release may include using higher level intakes, controlling the discharge rate, and limiting the drawdown level. Environmental/Biological – Because dams significantly alter the natural flow and water quality conditions of a river/stream, it is often required as part of a dam owner’s operation to release water for downstream environmental considerations. These drawdowns are usually governed by regulatory agreements. The releases are designed to benefit the downstream ecology and habitat by maintaining temperature and flow conditions beneficial to the aquatic species, supporting wildlife spawning, eradicating invasive species, and building beaches. Inspections, Repairs, & Modifications –To inspect, repair, or construct modifications to a dam, it may be necessary to lower the reservoir significantly to provide safe access for these activities. If the outlet system is the item in need of repair, a temporary means of drawdown may be required, such as a siphon (see Methods of Drawdown, below). Damage/Distress – A dam or its appurtenant structures can be damaged due to natural events such as an earthquake or extreme flood, a human-induced event such as vandalism or sabotage, or long-term wear and erosion. This damage may be severe enough that the dam is at risk of failure. Under these cases it is critical to be able to lower the reservoir as fast as safely possible to reduce the loading on the dam and risk of failure. Methods for Drawdown All dams are, or should be, equipped with outlet structures or systems for releasing water. Dams can be outfitted with different combinations of discharge structures with varying degrees of redundancy. For the purposes of this article, overflow spillways will not be discussed, as they are designed to pass flood flows in an uncontrolled manner (i.e., without human or mechanical operation) rather than to lower reservoirs by significant levels in a controlled sequence of operation. This article focuses on the need and use of mechanical outlet works systems designed to discharge water for the reasons described above. Often these outlet works consist of, from upstream to downstream, an intake structure, control valve/gate system, conveyance conduit, and discharge structure/basin. Different agencies/regulators, both federal and state have guidelines for minimum drawdown capacities of outlet works systems. These regulations often differ between jurisdictions and between hazard classifications. For example, for high hazard dams, the Colorado State Engineer’s Office requires that the outlet works system be capable of releasing the top five feet of reservoir capacity within five days. Photo 2. Outlet Commissioning Test. Photo from the Dam Safety Branch of the Colorado State Engineer’s Office. It is not uncommon to have multiple sets of outlet works systems, where one is set up for routine discharges and another designed to significantly lower