Some Notes on Arrowrock Dam


Arrowrock Dam is one of three major dams along the course of the Boise River. It’s the oldest of the three, completed in 1915.

Arrowrock Dam, Boise River, Idaho

Arrowrock Dam, Boise River, Idaho

At 348 feet tall, Arrowrock was the highest dam in the world at the time. It’s primary purpose is irrigation; it holds the winter’s rain and snowmelt so that water is available all summer for farming in the lower valley. Its secondary purposes are flood control and hydroelectricity.

This spring, it’s the flood control is on Boisean’s minds. This past winter snow and rain in the Boise River drainage amounted to about 200% of average precipitation. Water levels in the Boise River, usually 1,500-1,700 cubic feet per second (cfs) are currently about 9,500 cfs, causing minor flooding and a fair amount of anxiety.

Arrowrock Reservoir, along with its sister reservoirs, Lucky Peak and Anderson Ranch, are pretty much full. The Bureau of Reclamation, the federal agency that operates the dams, has no choice but to send water down the river.

WC thought it would be interesting to see what Arrowrock looks like when it is completely full. And it’s impressive. Three of the six spillway gates are open, and torrents of water are pouring over.

Arrowrock Spillway, looking south, Boise River

Arrowrock Spillway, looking south, Boise River

From the gates the water pours down a manmade cascade to the level of the tail race, a very impressive sight.

Arrowrock Spillway, Boise River

Arrowrock Spillway, Boise River

From here, the water runs down to Lucky Peak Dam and Reservoir. It’s at 93% of capacity. And there’s a lot of snow left in the mountains, and thunderstorms moving through as WC writes this. It’s a good news/bad news scenario: the good news is there is ample water for irrigation and even water to partially recharge the aquifers in the Snake River Plain. The bad news is that most of the remaining snowmelt and any rain are going to have to go down the Boise River to the Snake River, which means more and continued flooding in Boise and towns further downstream.

The Boise area is a flood plain, and indeed flooded regularly before Arrowrock was built. There have been very high water events since, most recently in 1984, but none as severe as the floods of the pre-dam era.

All dams involve environmental tradeoffs. Arrowrock, or perhaps the much smaller Boise River Diversion Dam before it, ended salmon runs up the river. But the dam and its sister dams made large scale farming in a desert possible, and greatly reduced flooding downstream. Reasonably clean electricity was made possible. There are no perfect solutions, as much as WC likes salmon.

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