Grassy Lake Dam is a 118-foot-high zoned earthfill storage dam on Grassy Creek in Wyoming near the southern boundary of Yellowstone National Park. The reservoir has a total storage capacity of 15,500 acre-feet (active 15,200 acre-feet) which supplements the storage at Island Park. Storage at Grassy Lake Dam is augmented by a 0.7-mile canal from Cascade Creek which is fed from the Cascade Creek Diversion Dam, a rockfilled log crib weir that is 14 feet high.
Recent operation and maintenance modifications included the installation of a seepage blanket and drain at the toe of the dam in 1996 and 1997. In 2004-2006 the spillway was replaced due to advanced deterioration, additional toe drains and seepage blankets were also constructed.
The site is located on the Yellowstone Plateau. The geology is a complex sequence of volcanism and glaciation. The foundation and abutments are comprised of interlayered volcanic bedrock and glacial deposits, overlain by recent alluvium in the stream channel. Annual rainfall exceeds 48 inches in all but the driest years, and grouting, drains, and other measures were required during construction to control seeps and springs in the work area.
Prior to embankment placement, the foundation was stripped to bedrock or to dense, heavily over-consolidated silty glacial sediments over most of its area. Near the maximum section, where the embankment bears on bedrock, cut-off walls project upward into the fill 5 to 10 feet and downward 3 feet into rock, and the bedrock was grouted extensively. Elsewhere, a cut-off trench was excavated as deep as 25 feet into the silty glacial sediments.
At the dam site, the foundation was excavated to the Lewis Canyon Rhyolite in the central stream channel area (about 300 feet wide), except at the downstream toe where the bedrock dips sharply downstream and the dam is founded on alluvium consisting of sand, gravel, cobbles, and boulders with some stringers of fine, loose sand. The Lewis Canyon formation consists of multiple flows of rhyolite, glassy rhyolite, and obsidian, which are quite variable in nature.
The abutments of the dam are founded on dense, fine-grained lacustrine sediments, probably the result of glaciation and deposition on the irregular eroded surface of the Lewis Canyon Rhyolite. The sediments are composed predominantly of plastic silt and clay with small amounts of sand, having plasticity indices (PI) from 4 to 16. The "clay" fraction is primarily rock flour, rather than clay minerals. A gravelly glacial-alluvial deposit that was a source of troublesome groundwater seepage during construction is located within this formation on the right abutment between elevations 7172 and 7188. The remainder of the sediment is low in permeability, and acts as a confining layer for artesian pressure within the underlying rhyolite. The Lava Creek welded ash-flow tuff is present in the right abutment above approximate elevation 7200. It is hard and dense with irregular joints and strong flow banding. This rock unit was quarried upstream to provide rockfill and riprap for the dam. Later glacial deposits, colluvium, and slopewash are present on the slopes above and near the dam, but do not constitute foundation materials.