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NatureWater Lesson Plans
Problems & Solutions:
FISH PASSAGE

Problem

Dams are difficult or impossible for migrating fish to get by. Salmon and steelhead populations throughout the Pacific Northwest have suffered greatly because of large dams. Big dams now keep these fish from using thousands of miles of habitat that only decades ago were available to them. Smaller dams also create migration problems for the fish, although with well designed ladders and regulated flows, fish can swim by them.

The Red Bluff Diversion Dam on the Sacramento River is one place where fish passage has been especially troublesome. When the dam is in place, holding back water, young fry can get past it only by going under the gates. The water there is flowing very hard and very fast. The fish are helplessly swept under the dam, into the churning water below. Predators know the fish are vulnerable, so they gather below the dam, waiting for a meal. Before the little fry have a chance to get thier bearings, they are eaten. Many fish are lost this way.

When adult salmon and steelhead swim upstream to spawn, they must get by the dam again. Although there is a fish ladder on each side of the dam, the fish often don't find them. Fish tend to swim upstream in the main current, not along the stream bank, where the ladders are. The fish are on a time schedule - they must spawn before they wear out and before the female looses her eggs. If they are delayed too long at dams, they may die before spawning sucessfully.

If too much water is removed from the stream, or if there is a drought that dries up the stream, the fish have trouble migrating. Because water temperatures tends to rise as flows decrease, the fish often encounter stressful or even lethal temperatures.

Solutions

In the case of the Red Bluff Dam, an effective solution for most of the year has been found. The dam is raised from the water from mid-September until mid-May. This means that for nine months of the year, there is no longer a barrier at Red Bluff. However, fish that migrate up or downstream during the summer must still get by the dam. An additional temporary fish ladder has been used recently. It is in the middle of the dam, instead of on the side, so fish swimming upstream are more likely to find it.

Picture of Fish Ladder