Rivers and Surface Water

Rule one of survival: find water. You can last weeks without food, but only about three days without water. That's why nearly every town in history was built next to a river. Idaho Falls exists because of the Snake River. It's literally in the name.

Earth has had basically the same water for billions of years. It just keeps moving around in the water cycle:

1. Evaporation: the Sun heats water, turning it into vapor that rises into the air.
2. Transpiration: plants "sweat" water vapor out through their leaves.
3. Condensation: vapor cools and turns into tiny droplets, forming clouds.
4. Precipitation: water falls as rain, snow, sleet, or hail.
5. Runoff and infiltration: water flows over the ground into streams, or soaks into the soil.

Here's a reality check: about 97% of Earth's water is salty ocean water. About 2% is frozen in ice. That leaves less than 1% as fresh liquid water we can actually use. Don't waste it.

A river starts small. Rain and snowmelt trickle into tiny streams, which join into bigger ones. Smaller streams that feed a bigger river are called tributaries.

All the land that drains into one river is its watershed (or drainage basin). Imagine pouring water on a giant tilted table. Everywhere the water ends up in the same spot is one watershed. The high ground that separates watersheds is a divide. The Continental Divide runs along the Idaho-Montana border. Rain on the west side eventually heads to the Pacific; rain on the east side heads to the Atlantic.

The Snake River's watershed covers most of southern Idaho, and the Snake eventually flows into the Columbia River and out to the Pacific.

• Young rivers (near the source): fast, steep, and cut down hard. They carve deep V-shaped valleys, rapids, and waterfalls.
• Mature rivers: slower, wider, and start to curve.
• Old rivers: slow and lazy. They wander in big loops called meanders. Sometimes a meander gets cut off and becomes a curved pond called an oxbow lake.

Where a river finally slows down and drops its sediment at a lake or ocean, it builds a delta, a fan-shaped pile of mud and sand.

The faster water moves, the more sediment it can carry. A slow river drops mud. A fast, flooding river can roll boulders the size of cars.

When more water enters a river than its channel can hold, it spills over onto the flat land beside it, called the floodplain. Floods drop rich soil, which is why floodplains make great farmland. They're also dangerous places to build houses.

In 1976, the Teton Dam in eastern Idaho failed while filling for the first time. About 80 billion gallons of water poured out, flooding Rexburg, Sugar City, and other towns. Eleven people died and thousands of homes were damaged. Ask your grandparents. Some of them remember.

People manage floods with dams, levees (raised walls along rivers), and by keeping wetlands, which soak up water like sponges.

• Lakes form wherever water fills a low spot: glacier-carved basins, volcanic craters, or behind dams. Bear Lake, on the Idaho-Utah border, is famous for its turquoise color, which comes from tiny bits of limestone floating in the water.
• Wetlands (marshes, swamps, bogs) are soggy land that filters pollution, prevents floods, and provides homes for tons of wildlife.