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15 the reservoir. The latter is generally termed a low-level outlet. Low-level outlets are operated when the reservoir needs to be drawn down below the invert of other discharge structures. With a reservoir at normal pool, a low-level outlet system is typically under a much larger magnitude of static and dynamic hydraulic pressures than the other outlet structures on the dam. Ideally, a reservoir would be drawn down using other available outlet structures until the reservoir has reached the lowest achievable elevation before operating the low-level outlet. This will help reduce stress on the system. A low-level outlet system is typically only used to dewater a reservoir for inspection or construction activities or in the case of emergencies. When designing a low-level outlet, predicted sediment accumulation should be considered. If the low-level outlet is buried beneath enough sediment that it cannot release water effectively, dredging will be required to maintain serviceability. A low-level outlet should be equipped with an adjustable control valve or gate, such that water release can be throttled. Since most reservoirs are bowl- shaped, the rate of drawdown will increase at lower levels if the discharge rate remains the same. Having adjustable valves or gates will allow the dam operator to control the reservoir drawdown at constant rates by slowing the release flow as the pool draws down. For cases where there is no low-level outlet system, or the outlet system is unsafe for use, a siphon can be installed, providing the ability to lower the reservoir. A siphon is generally installed over the dam or spillway, providing a safe and easily constructible, but usually temporary, outlet option for dam owners. A detailed discussion about siphons was presented in Volume 1: Issue 1 of Western Dam Engineering: Technical Notes. Impacts of Drawdown Dams and the reservoirs they impound have significant impacts on the communities and environments around them. The drawdown of a dam’s reservoir is one of the more impactful aspects of dam operation. Reservoir drawdown can have far reaching effects on the environment downstream, upstream, and even the dam itself. The magnitude of these impacts or whether they exist is dependent on the size of the dam, volume of the reservoir, discharge capacity of the dam, and sensitivity and capacity of the downstream channels. Below is an overview discussion of the different impacts that may occur when drawing down a reservoir. Downstream – The most obvious impacts of drawing down the reservoir are the impacts it has downstream. The downstream channel will experience higher flows and will introduce more energy to the system. This also impacts any basins that the immediate downstream system ties to. Significant reservoir drawdown will impact the performance of downstream drainage and irrigation systems, introducing a much larger volume of water that may exceed their capacity, overwhelm those systems, and result in unintended flooding. Environmental – Downstream flood plain habitats and species can be significantly impacted by large reservoir discharges. Raising the level of the downstream channel can have negative impacts upon vegetation, significantly increasing the flow and velocity within the channel can wash away certain species of wildlife and disrupt migration patterns. In some cases, large reservoir drawdowns can alter the chemical composition of the aquatic environment by changing the PH levels, salinity, or temperatures, rendering the environment unlivable for certain species. Sedimentation – Releasing water from the reservoir will often release sediment accumulated behind the dam. Sediment may be deposited in the downstream channel, reducing the channel capacity and exacerbating downstream flooding. The release of trapped sediment can also significantly increase turbidity in downstream habitats, and can often have detrimental effects on aquatic wildlife. The US Army Corps of Engineers regulates releases from reservoir through their Regulatory Guidance letter 05-04 (USACE, 2005). Upstream/Reservoir Rim – Reducing the reservoir level significantly can also change the upstream habitat, affecting bird nesting and accessibility of water for wildlife. Some reservoirs used for recreational activity such as boating may become unnavigable, inaccessible, or unsafe. If there are structures such as residential properties, immediately surrounding the reservoir, a