The dam was built in 1922 and 1923 and the bridge atop was
completed July 16, 1923. A newer bridge was built just downriver in 2016 and barricades put across this bridge, relegating it to carrying pedestrian traffic only. The Willwood Diversion Dam was part of the Shoshone River irrigation project, constructed to irrigate surrounding farmland and pasture land. The dam raises the level of the river, allowing water to flow into the Willwood Canal system above the dam.
Willwood Diversion Dam is a concrete gravity structure with a height of 70 feet and a length of 476 feet. Willwood Canal, which begins at the diversion dam, is about 25 miles long and the diversion capacity is 320 cubic feet per second. Operated by the Willwood Diversion Dam, the canal is able to irrigate 11,400 acres.
Following is the story of the dam's construction.
Willwood Diversion Dam
Preliminary exploratory drilling on the Willwood Dam site, on the Shoshone River began using force account in 1921. Reclamation established a temporary camp in early July of 1922, and permanent structures began going up later that month.
Foundation excavation began August 7, and laborers commenced laying concrete on December 1. During the course of construction Reclamation twice changed to the dam design; the crest was raised five feet and engineers added an inspection gallery to check pressure on the foundation. The first concrete pour on the dam occurred January 1, 1923. Cold weather delayed work from January 28 to February 6 and from February 12 to 15, but few delays occurred due to breakdowns or lack of material. In spite of the fear that it could not be accomplished, construction of the dam's south half finished on March 8 before the advent of high water.
The concrete on the north side of the dam proved the easiest to place because of the short distance from the mixer to the site. This allowed the dam to be raised to elevation 4461, considered safe from high water, by March 28. Work on the apron began the next day and finished on April 5. Afterward construction continued on the dam finishing the concrete to the crest by May 23. The steel highway bridge built across the dam consisted of three 90 foot spans and was completed July 16.
Work on the tunnel for the eight foot diameter horseshoe diversion conduit began while excavation for the dam progressed. The conduit transferred water diverted by Willwood Dam to the Willwood Canal system. In 1923 the decision was made to increase the tunnel to nine feet.
The first 25 feet of the tunnel remained eight feet high. Ten feet of the tunnel provided the transition from eight to nine feet, and the remaining section continued at nine feet high. Tunnel excavation finished before the end of 1922. The concrete lining was in place by the end of 1923. Laborers began laying the concrete on the north half of the dam March 15...
Excavation of the Willwood Canal began August 28, 1922. The main canal and all lateral earthwork was completed in 1925. When finished the canal measured 24.8 miles in length.
From the U.S. Bureau of Reclamation, Page 12