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Another option for ponds with a large watershed is a drop outlet structure which allows water to drain quickly. It allows water to trickle over the rim of an open, vertical pipe with the rim set at the desired water level. Water is then drained from the drop structure through a horizontal pipe through the dam. Ponds with these two types of outlet structures can be drained, either partially or completely, by either installing a gated valve or stop logs inside the drop outlet structure, or by installing a drain tube near the bottom of the dam. An accessible valve can then be installed on the drain tube, below the frost line in the earthen toe at the back of the dam. Water levels also can be manipulated by pumping or siphoning, but this can be time consuming and expensive. An optional watering line can be installed with the drain tube to provide water for livestock below the dam. Another type of outlet structure is the bottom withdrawal spillway, which discharges water from the bottom of the pond. This device is designed to carry much of the incoming muddy water through the pond, dam, and then downstream, resulting in a minimal rise in the pond water level. It also maintains good water quality by removing stagnant water, sediment and organic materials from the pond bottom. This will improve the productivity of the pond and extend the life of the pond by 50%. An optional livestock watering line can also be installed through the dam to facilitate livestock watering below the dam. Outlet structures, particularly drop structures, can prevent unwanted fish from entering the pond from downstream. Regardless of the type of outlet used, anti-seep collars should be installed around the pipe that passes through the dam to prevent the pipe from washing out. Trash guards or hooded inlets should also be installed on drain pipes, both for safety and to prevent them from becoming clogged with debris. Another necessary feature for water overflow, especially for ponds having outlet structures, is an emergency spillway. While the drainpipe carries water during normal runoff, an emergency spillway carries flood runoff away from the pond so the dam is not damaged or breached. The upper tube opening for outlet structures is generally set 12 inches below the earthen spillway . . . . . . . . . . . . . . . . . . . . second edition • Nebraska Pond Management • 21