Red Willow Dam, located on Red Willow Creek about 10 miles northwest of McCook, is an earthfill embankment with a structural height of 126 feet that forms a reservoir of 86,630 acre-feet. An ungated concrete spillway is located in the right abutment. An outlet works through the base of the dam provides for river and irrigation releases for downstream diversions. The reservoir behind this dam is Hugh Butler Lake.
Public Information Portal Red Willow Dam/Hugh Butler Lake, Neb.
Lying beneath the dam and abutments is the Ash Hollow member of the Ogallala Formation (To), a Mio-Pliocene nonmarine deposit of variably cemented sand, silt, clay, and gravel. The Ogallala is a major aquifer in the area.
Overlying the Ogallala at the abutments are discontinuous beds of Pleistocene Crete Sand (Qcf), which fill shallow paleochannels on top of the Ogallala Formation. (Though detected only during investigations of the left abutment, it is assumed that the Crete Sand is intermittently present at both abutments.) Pleistocene Loveland and Peorian loess (Ql), consisting of silt and fine sand, rest on top of Crete Formation and form the upper part of the abutments.
The foundation of the dam consists of approximately 40 feet of alluvium (Qu and Qal) deposited in the old Red Willow Creek channel on top of the Ogallala Formation. Beneath the Ogallala Formation, at a depth of approximately 120 to 130 feet, is Pierre Shale. A considerable portion of the alluvium consists of saturated, well-sorted sand which may be potentially liquefiable. The upper alluvial zone (Qu) is described as alluvial silt and has much lower horizontal and vertical permeability than the alluvial silty sand (Qal) zone that lies below it.
The spillway and outlet works, located in the right abutment of the dam, are both founded on the Ogallala Formation. Excavation for the outlet works was primarily within the Ogallala Formation, whereas excavation for the spillway was primarily through loessial materials.
There are no known faults in formations exposed in the area, nor are there any landslides of significant size known to exist in the area around the reservoir.