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. . . . . . . . . . . . . . . . . . . . second edition • Nebraska Pond Management • 15 the width should be increased an extra 2 feet for each additional 5 feet of height. The actual dam height depends on pond size, along with the size, slope and usage of the watershed. All dams should have at least 3 feet of extra height, or freeboard, to prevent flood waters and waves from overtopping them. The dam should be constructed with slopes that will not slump or slide. The steepness of the slope on the pond side of the dam should not exceed 3:1 and the steepness of the slope on the downstream side should not exceed 2.5:1. Muskrats and beavers sometimes burrow into a dam. These holes may eventually cause the dam to fail due to erosion. An adequate freeboard and width is needed to eliminate leakage or dam failure. Since beavers and especially muskrats prefer steeper slopes for burrowing, another option is to construct steeper banks on the nearby pond banks than those found along the dam. Lining the dam face with rock 2 feet above and 3 feet below the water surface will also deter burrowers. Pond Bottom Design The least desirable design for a bass-bluegill- catfish pond is one with a bowl-shaped bottom, with no irregular features. If core samples indicate soil is suitable (will not leak), bottom features can be made that will benefit fish production as well as angling opportunities. Trenches and drop-offs can be built to diversify basin fish habitat. Deeper water near shore will benefit fish and shoreline anglers, but should be avoided in wading and swimming areas. Underwater terraces and humps can also be incorporated, particularly in the upper reaches of the pond. They will provide additional structure and slow incoming water, allowing sediment to settle out in the upper reaches of the pond. Those underwater structures should be considered when it isn't feasible to install sediment/nutrient entrapment structures above the pond. Excess fill soil can be used to create additional shoreline by building fingers of land that extend into the pond (either below or above water surface), or small islands. This will increase shoreline access and produce additional fish habitat. Make sure adequate depth is present adjacent to these structures to prevent excessive growth of aquatic vegetation. Water Control Structures Ideally, the pond owner should have complete control of the water entering and leaving the pond. Inlet and outlet structures are two of the most important structural features of a pond. When incorporated into new pond designs, they will help prevent and control many common problems. An outlet structure enables the owner to drain the pond to make repairs, manage fish populations, control nuisance aquatic plants, and encourage desirable aquatic plants. Most waterfowl management efforts require some water level manipulation and a flooded food source, so a water control structure needs to be included in the pond design. Installing only an earthen overflow spillway not only prevents water control, it can also result in erosion and dam failure if it is not properly designed and maintained. An inflow or inlet control structure may be necessary to prevent waters containing pollutants or undesirable fish from entering the pond. When streams are used as a water supply, the stream should be diverted around the pond and an inlet pipe, which can be screened or closed as needed, should be installed. One type of outlet control structure is a trickle tube. The upper opening is installed at the planned water level. The tube is then sloped downward with the lower opening at or near ground level at the back of the dam. The tube should be large enough to carry most of the runoff, or at least large enough to draw the water level down in a short period of time once storm flows subside. Additional information on beavers and muskrats is provided on page 78.