If you took out a dam in Montana the following chain of dam failures would cut the United States in half all the way to the Gulf of Mexico and destroy US agricultural output. The US produces 40-50% of the world Soybean and Corn supply. Long term you're probably talking billions of deaths due to food shortages.
Dams are dangerous beasts. The KMT breached some dikes in the 1930s for the purpose of environmental warfare, and in the process killed half a million peasants and displaced millions more.