No better symbol exists of how man has altered the river, in ways both bad and good, than Grand Coulee Dam. Built during the Depression, the dam's benefits were manifold. It put 7,000 people to work, created a reservoir for the biggest irrigation project the country had ever seen, provided flood control, and produced electricity that would power America's war effort. At the time, it was the largest concrete structure ever built. Today Grand Coulee generates more electricity than any other dam in the U. S., enough for a million households a year. . . . The dams have wreaked havoc on salmon, the creature that symbolizes the Northwest, whose epic migration from streams to ocean and back again to natal streams is one of the wonders of the natural world. Today perhaps 200,000 to 300,000 wild salmon remain on the Columbia, less than 3 percent. –Fen Montaigne, "A River Dammed," 2001 What is the main idea of this passage? The dam had little impact on the region. The dam helped the entire region grow. The dam had both positive and negative effects. The dam did not have any effect until many years later.