Monday, July 14, 2008

Salmon's Wild Ride

What happens when a salmon swims into an irrigation diversion?

Nothing good. By the time the fish figures out it made a wrong turn in the stream, there’s not enough water to turn back, and it’s trapped—meaning certain death.

At least that’s how it used to be.

The contraption pictured above is installed on small streams to protect chinook salmon (and other fish) that swim down small stream diversions. Conservationists in the Salmon River Valleys recognize that small streams are vitally important to the remaining salmon and steelhead, but these streams are often overlooked as salmon habitat.

These little streams may not look like much at first glance, but salmon are not only a fish of large waters. After their long migration from the sea, it seems a shame for fish to be lost on the final part of their journey. Fish spawn in remarkably small streams connected to rivers like the Lemhi and Pahsimeroi:
These streams are also used by spawning steelhead, and contain populations of bull trout, cutthroats and resident rainbows—provided there is enough water. As fish migrate, they sometimes go into diversion channels, which, after all, look just like a regular stream channel:

A fish excluding device saves these fish. The water wheel concept is designed to divert the fish back to their stream, while ensuring adequate water still flows for irrigation.

Here’s how it works. Larger fish find an impasable metal barrier, so they turn around and swim back to the stream. But smaller fish can still pass—a fine mesh would accumulate so much debris in a day that the device would become a dam, negating its value as a diversion.

Smaller fish thus enter the pictured device and face a fine mesh barrier tube—rotated by the water wheel so that debris does not accumulate on the mesh.

Fish that make it to the mesh could be trapped, so a PVC pipe goes the opposite direction, returning some water to the stream. (Most of the water is run through the water wheel to the irrigation canal). The fish shoot down the tube and are deposited back in their natural habitat—subject to what must be a particularly wild ride, but far better off than the alternative.

The Nature Conservancy and partners consider it one of the highest conservation priorities to protect the salmon that are left. This includes a wide range of activities, including restoring habitat along spawning streams, ensuring adequate flows in rivers and devices like this one that protect both fish and agricultural practices.—Matt Miller
Photos by Jim Foster.

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