Bumping Lake Dam is an earthfill structure on the Bumping River about 29 miles northwest of Naches. The dam, completed in 1910, is 60 feet high and contains 253,000 cubic yards of material. In 1973, the road crossing the spillway was replaced and a new concrete T-beam bridge was installed to replace a wood-truss bridge. Situated at the lower end of a natural lake, the dam formed a reservoir with an active capacity of 33,700 acre-feet constructed over a natural lake having unknown dead storage capacity.
Modifications to Bumping Lake Dam were undertaken in 1994-1997 under the Bureau of Reclamation’s Safety of Dams Program. These modifications included the installation of an interceptor drain at the downstream toe of the dam, construction of downstream and upstream stability berms, replacement of the concrete spillway, installation of a steel liner in the outlet tunnel, and replacement of the outlet channel lining. The outlet gates, gate house, and gate operators were replaced in the early 1990s.
The stratigraphy at Bumping Lake consists of Evans Creek Glacial drift (Qde), overlain by the Bumping Lake lahar (Qm) and alluvium (Qal). The alluvium present at the site is composed of reworked drift and lahar materials and is contained to the lowest portion of the valley. The shells of the dam are founded on the alluvium, between approximately stations 13+00 and 15+00. Tumac Mountain is located approximately 11 miles south of the dam. Recent tephra studies near the Tumac Plateau indicate the most recent major eruption occurred between 20,000 to 30,000 years ago. Observations during the excavation of the test pits indicate the materials overlying the glacial drift were volcaniclastic in nature. The material also exhibits an interbedded ash layer, indicating multiple emplacements of the Bumping Lake lahar. The lahar materials mantle the glacial drift to a depth of up to 13 feet near the center of the valley floor and decrease to a thickness of approximately 4 feet at the ends of the dam. The lahar underlies the shells of the embankment, and appears to be continuous for the entire length of the structure, except in the lowest portion of the valley where the shells are founded on alluvium. The cutoff trench appears to have fully penetrated the lahar and was terminated in glacial drift. The thickest portion of the lahar occurs near the outlet works, believed to be the location of the natural channel at the outlet of the ancestral Bumping Lake. The lahar mainly consists of sand with lesser amounts of silt, gravel and basalt cobbles. The lahar contains a large amount of organic debris composed mostly of charred wood. The lahar is classified as silty sand with gravel and cobbles. The uppermost portion of the unit is composed mostly of silty sand to silty sand with gravel. The interbedded volcanic ash, approximately 5 to 10 mm thick, occurred at depths ranging from .4 feet in the upstream test pits to 1.4 feet near the downstream toe of the dam. Just below the ash, the coarseness of the lahar material increases gradually from silty sand with gravel to silty sand with gravel and cobbles. The Evans Creek drift is composed of gravel sand and silt with cobbles and boulders. The gradation of the drift deposit varies greatly, but ranges in classification from poorly graded gravel to poorly graded sand with varying amounts of silt.