About That Neglected Infrastructure: Salmon Falls Creek Dam

WC has been off birding again. This time the excuse was two good friends from St. Louis, Missouri. Birding took us to Salmon Falls Creek, in far southern Idaho, just a few miles from the Nevada State line, in the Jarbridge Mountains. The trip included driving across Salmon Falls Creek Dam. And if you wanted a poster child for aging, neglected infrastructure, this dam would be a candidate.

Salmon Falls Creek Dam is located near the Idaho-Nevada border, south of Twin Falls, Idaho. It drains the Jarbridge Mountains and a chunk of the Snake River Plain.

Salmon Falls Creek Drainage Map

Salmon Falls Creek Drainage Map

About 224 feet tall, with a width of 450 feet, the dam creates a 15-mile long reservoir, holding at peak about 230,650 acre feet (0.28450 km3) of water in a long,very narrow gorge. The dam was built in 1910, to provide irrigation water in the powder-dry Snake River Plain south of Twin Falls, Idaho. The dam is privately owned by the Salmon Dam Canal Company.

Salmon Falls Creek Dam, vie from western side, March 25, 2017

Salmon Falls Creek Dam, view from western side, March 25, 2017

WC isn’t an engineer and doesn’t pretend to be, but it can’t be good when water is streaming out of the rocks along the edge of the dam. The top of the dam serves as a single lane road for Three Creeks Road. The railings along the top of the dam are badly eroded. There are fissures and cracks in the downstream face of the dam. One of the cracks is at least six inches wide.

Salmon Falls Creek Dam, center top of dam face

Salmon Falls Creek Dam, center top of dam face

This is a gravity arch dam. WC’s limited reading on the subject suggests that water streaming out of the sidewall of the dam, especially where the arch meets the wall, is a serious problem. The dam’s integrity depends, in part, on the arch pressing against the canyon sides when under the pressure of water. If the connection between the dam edge and the canyon wall is weakened – lubricated – by water, the dam is at risk of failure.

Any dam built on volcanic rocks can be problematic. The lithified ash layers are highly permeable; the basalt is often highly fractured. The evidence is in the water flowing out of the easterly canyon wall, immediately below the dam. Reportedly, the larger flows further down have been running for 30 years or longer; the smaller flow at the dam appears to be newer.

Salmon Falls Creek Reservoir

Salmon Falls Creek Reservoir, about 200 feet deep

This is a “high hazard” dam. That’s a technical term, not describing the condition of the dam but the effects of a dam failure. But for WC’s money, this is a highly hazardous “high hazard” dam and WC won’t be buying any property in the downstream canyon, or along the Snake River immediately downstream of the confluence.

Detail of road curb at top of Salmon Falls Creek Dam

Detail of road curb at top of Salmon Falls Creek Dam

Note the cascade of water in the background, flowing out of the canyon wall where it meets the concrete arch dam. Not good.

You can argue about whether the dam should exist at all. It provides irrigation water to about 25,000 acres, but at the cost of pretty severe pollution downstream, where most of the flow is undiluted agricultural run-off. The 187 shareholders presumably benefit from the dam, at a modest cost per acre of irrigated land. But that’s not the point of this post. If the dam is to exist, it needs to be safe. And this dam looks unsafe. In Idaho, you only have to say “Teton Dam” to remind folks that dams fail, that engineering can be wrong, and that lives are at stake.