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Merritt Dam

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Merritt Dam and Reservoir and the Snake River drainage basin are located within the northern portion of the Sandhills Region of Nebraska.  The topography of this area is one of rough hills and hummocks, composed of fine, wind-blown sand, resting on low rolling plains.  Occasional valleys, which are generally broad and shallow, and areas of low relief create breaks in the dune topography.  In the lower reaches, the valleys often become narrow and deeply entrenched.  Merritt Dam and Reservoir are built on the Snake River where the valley narrows and becomes entrenched.  Maximum relief within the Snake River drainage basin is less than 200 feet.  The continental sediments representing the Tertiary system at the damsite are of the Pliocene Series (upper Tertiary).  The dam is built mostly within and on the Ogallala beds of Pliocene age.  The Ogallala beds lie unconformably on the Brule formation of the Oligocene series.  The Brule formation is a massive, light-colored siltstone.  It is considered impervious and the top of the Brule is regarded as the base of the possible seepage zones.  The Ogallala Group is represented by two formations, the Ash Hollow and the Valentine.  The Ash Hollow formation is exposed at the damsite and is composed of stratified sand, silt, volcanic ash, and clay, which occur alone and as mixtures.  The material is non- to well-cemented by lime.  The Valentine formation consists of compact, fine, greenish-brown sand and silts.  This formation was not encountered during construction as it occurs considerably below the stream bed. The sand dunes and limited amount of sediments in the small flood plain are of Recent or Pleistocene age.  Both the sand dunes and flood plain sediments are silty, fine sands.  The sand dunes are wind deposited and the flood plain sediments are reworked sand dunes.  The Snake River valley at the damsite is asymmetric, having the most recent stream location entrenched next to the left abutment.  The left abutment, with the exception of the lower portion, is dune sand and rises rapidly to elevation 3020.  It is in an over-steepened condition and slumping has occurred.  The lower portion of the left abutment is Ash Hollow formation.  The right abutment and valley floor and streambed are composed of the stratified silts, sands, volcanic ash and small amounts of clays of the Ash Hollow formation.  Several seeps and springs occurred slightly above and at the streambed both above and below the damsite before the dam was constructed.  The flow of the springs below the damsite is reported to have increased slightly with the filling of the reservoir.

Last updated: Apr 02, 2013