Three dams help form Carter Lake Reservoir. The Reservoir is one of two final storage locations for water diverted from Colorado's West Slope via the Colorado-Big Thompson Project. Water exits the reservoir through Dam #1 and the St. Vrain Supply Canal serving communities ranging from Longmont to Boulder and Fort Morgan, Colorado before water is returned to the South Platte River system.
Dam No. 1, the largest, occupies a gap across the Dakota sandstone hogback, and ifs founded on Morrison shale and limestone and the Sundance sandstone. The abutments are Dakota sandstone and shale, and all the strata have about 15 degree dip downstream.
The geology of Dam No. 2 is somewhat more complex. Due to sharp echelon folding, the hogback structure ends abruptly against a southward plunging anticline. The result is that the foundation strata dip to the southwest into the right abutment, with dips varying from 5 degrees on the right abutment to 70 degrees on the left abutment. The dam rests on upturned edges of the Morrison shale and limestone, Sundance sandstone and the Lykins formation.
Dam No. 3 rests entirely on Lyons sandstone.