A dam is a large barrier built across a river or stream to hold back water. Dams create reservoirs (artificial lakes) that store water for drinking, irrigation, and generating electricity. The Hoover Dam, built during the Great Depression, holds back the Colorado River and provides water and power to millions of people across the American Southwest.

Dams work by blocking the natural flow of water, forcing it to build up behind the barrier. Engineers design dams to withstand enormous pressure: a large reservoir might hold billions of gallons of water pushing constantly against the structure. Some dams release water through turbines, using the force of falling water to spin generators and create hydroelectric power. Other dams have gates that operators can open to control flooding or release water downstream when needed.

Building a dam changes the landscape dramatically. The reservoir behind it floods valleys and sometimes even towns, while the river below the dam flows differently than before.

As a verb, to dam means to block or hold back water.

Beavers are nature's dam builders: they construct dams from branches and mud to create ponds where they build their lodges. A beaver dam might be just a few feet tall, but it transforms a trickling stream into a calm pond, showing how even a small barrier can reshape flowing water.