Upward of 2.5 million wild salmon once populated the rivers and streams of the Snake River basin in Idaho. Today, all of Idaho's salmon species are listed as threatened or endangered under the Endangered Species Act. There are many factors that affect salmon throughout their life cycles, but the main reason for their sustained decline is dams.
Since completion of the four lower Snake River dams in particular the populations of Idaho salmon have crashed.
How dams kill salmon Dams and their reservoirs pose the most daunting problem for juvenile salmon, called smolts, during their migration to the ocean:
Smolts rely on river currents to flush them downstream during spring runoff. Dams still rivers into reservoirs. Without a guiding current, smolts have difficulty finding their way downstream.
Reservoirs warm water temperatures, which stresses these cold-water fish.
Stressed and confused smolts are easy prey for birds and predatory fish that thrive in reservoir environments.
Some smolts are caught in dams' deadly turbines.
Dams and reservoirs cause the smolts' journeys to take much longer, disrupting their biological transformation from fresh water to salt water. Historically, the migration would take about a week on swollen spring rivers. Now it can take a month or more for a baby salmon to reach the ocean.
Adult salmon migrating upstream must climb stair-step ladders to get over eight dams plugging the Columbia and Snake rivers. Scientists estimate that between 15 percent and 30 percent of returning salmon die during the passage through eight fish ladders and reservoirs.
Dams and reservoirs are the single biggest killer of Idaho's salmon.
Four dams too many Smolts are able to make it past a few dams and reservoirs on their ocean-bound journey, but the cumulative effect of passing eight dams and going through eight reservoirs is too stressful. Fisheries biologists say that removing just the four lower Snake River dams will give wild salmon the fighting chance they need to recover.
For example, in the late 1950s before construction of the lower Snake River dams, Idaho's salmon had to navigate only three dams on the Columbia River. Salmon populations were relatively healthy then, even if they had to deal with a few dams.
But between the early 1960s and mid 1970s, one more dam was added to the Columbia and four to the lower Snake River that made the journey more than twice as treacherous. After the last dam on the lower Snake was completed in 1975, Idaho's salmon populations began a journey toward extinction that continues today.
For years, a majority of fisheries biologists have said the surest and probably only way to restore Idaho's wild salmon is to remove the four lower Snake River dams. Learn more about dam removal.