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Dam Construction It is important that the material in the dam be tied to the soil in the foundation. A cutoff or core trench should be cut lengthwise, along the dam's centerline. This trench should be deep enough that all soil, sand, gravel, and loose rock is removed down to either solid rock or clay. The trench should extend a minimum of 3 feet into impervious subsoil or be anchored into solid rock the length of the dam and into the valley walls at each end of the dam. It should have a minimum base width of 8 feet. This trench should be backfilled with clay, compacted in layers to the top of the dam, creating a clay wall within the dam. Failure to install a core trench and wall can result in seepage through, or even loss of, a dam. The dam should be constructed of imper- vious moist material that is compacted in continuous horizontal layers as it is installed. Dams pushed up with a bulldozer and not compacted have a greater chance of failing. All material should be compacted by either a sheepsfoot roller or an earthmover with rubber tires. The recommended dam top width is 12 feet for a dam less than 20 feet high. For taller dams, POND FEATURES POND FEATURES 14 • Nebraska Game and Parks Commission . . . . . . . . . . . . . . . . . . . . . . . . Trees/Rocks for Fish Habitat Trees/Rocks for Fish Habitat Island Island Staggered Pond Depths Staggered Pond Depths Rocked Mouth of Emergency Overflow Rocked Mouth of Emergency Overflow Watershed Watershed (Water Flow) Underwater Terrace or Hump for Silt Collection Underwater Terrace or Hump for Silt Collection Finger of Land Projecting Into Pond Finger of Land Projecting Into Pond (Crest of Dam) Trickle Tube Trickle Tube (Entrance to Water Outlet) (Side of Dam that Slopes Toward Pond) Watershed Watershed (Water Flow) Watershed Watershed (Water Flow)