The Little Dam That Wouldn’t

When a dam is built, the purpose is to create a reservoir, thereby allowing for the accumulated water to be used to create irrigation for local farmers. In theory, that was the plan for the dam in Hot Springs County, Wyoming that would become known as Anchor Dam, and the resulting “reservoir” to be nicknamed “the reservoir that wouldn’t hold water” and occasionally, “boondoggle.” The dam is located 35 miles west of Thermopolis, Wyoming.

The US Bureau of Reclamation built the dam in the 1950s. It was intended to provide a reliable supply of irrigation water, but after three years and $3.4 million spent on construction, it became clear that the geology of the area would not allow the reservoir to fill. After more than a decade of continuing attempts at remediation, the final cost of the dam topped $7 million, and its reservoir still holds very little water. The concrete thin-arch dam was completed in 1960 by the United States Bureau of Reclamation as a water storage project. The 208-foot-high dam structure impounds the water of the South Fork of Owl Creek, with the spillway designed as an unnecessary central overflow. Owl Creek rises in the Absaroka Mountains of northwestern Wyoming, and it was the site of early agriculture in the area. As homesteaders settled the region in the 1890s, it would become Hot Springs County. No area can support agriculture or livestock without water, so they knew something would have to be done. Ranchers used irrigation ditches to bring water from Owl Creek to pastures or to help grow livestock feed crops like alfalfa.

As construction began, the crews discovered “solution cavities” in the bedrock forced the re-positioning and re-configuration of the dam, causing delays and added expense. Solution caves or cavities are formed by the dissolution of rock along and adjacent to joints, fractures, and faults. These cavities are created by water passing through soluble rocks, leading to underground cavities and cave systems. The same karst solution cavities prevented Anchor Reservoir from filling its design capacity of 17,400 acre-feet. The reservoir has never been full. Since the dam was constructed, more than 50 sinkholes had been identified in the underlying Chugwater Formation geology of the reservoir basin. Some of the sinkholes were 30 feet in diameter and 35 feet deep. The site’s lack of “hydraulic integrity” was well known to Bureau scientists before and during construction, which begs the question…why was this project ever approved?

Bureau of Reclamation policies mandated that the Owl Creek farmers must organize an irrigation district, which would oversee distributing water to members and collecting payment. This was not well received, and a number of farmers in the area did not want to join, for two reasons. First, not everyone thought they needed the extra water, and they thought that their business would suffer if had recurring repayment costs for the project. Under Wyoming’s “first-in-time-first-in-rights” system of water law, farmers and ranchers with water rights filed earlier in time are allowed to take all the water they are entitled to before their neighbors with later rights get any. On Owl Creek, the farmers and ranchers who joined later and therefore had lower priority water rights tended to favor the proposed reservoir.

The Bureau of Reclamation rule that farmers getting project water could not own more than 160 acres, or 320 if married, was a source of contention, however. This rule was designed with the small family farm on fertile soil in mind. In the high altitude, short season, arid Owl Creek Mountains, 320 acres was nowhere near enough for cattle ranching, meaning that the rancher would not contribute. Efforts by local dam proponents to force reluctant ranchers to join the irrigation district prompted a series of lawsuits that held up progress on Anchor Dam until 1954. In 1954, Wyoming’s US senator, Frank Barrett, succeeded in having Anchor Dam exempted from the acreage limit. Immediately, a group of ranchers changed their minds and joined the irrigation district. With that, the Bureau of Reclamation began seriously planning for construction. Nevertheless, it was all for not, because in the end, the reservoir only filled enough to provide some irrigation benefit through July and August of each season, as it still is today. The Anchor dam is still operated by the local Owl Creek Irrigation District.