Overview

Willow Creek Dam is an earthfill structure on Willow Creek about 15 miles southeast of Gibson Dam. In addition to storing water from Willow Creek, the reservoir is fed from the Sun River through the Willow Creek Feeder Canal. The structure is 93 feet high, has a crest length of 650 feet, and contain 275,000 cubic yards of material. An open spillway channel 700 feet wide at the ground surface has a capacity of 10,000 cubic feet per second. The outlet works tunnel run through the right abutment. The reservoir has a capacity of 32,400 acre-feet of water.

Geology at the dam site consists of Cretaceous age bedrock of the Two Medicine Formation overlain by Pleistocene glacial moraine deposits and recent alluvium and colluvium along the channel of Willow Creek. Bedrock consists of nearly horizontal bedded, decomposed to soft shales and siltstones, and fine-grained sandstones. The shales and siltstones are usually poor laminated and tend to air slake rapidly. The sandstones are moderately hard to hard, and high angle jointing is prominent and leads top formation of blocky, tabular fragments. Glacial moraine deposits in the area are quite coarse and generally consist of sand and gravel with minor amounts of cobbles, boulders and silt and clay fines. Alluvium along the creek channel is a highly variable mixture of material ranging in size from clay fines to boulders. A thin layer of material at the base of the dam, identified as alluvium, is composed of mostly cobbles and gravel with almost no fines. Glacial deposits form the foundation of the dikes and spillway.