Streams and Rivers (part 1)

 Streams and Rivers

Standing on a bridge in Albuquerque, New Mexico, you look through your binoculars at the waters of the Rio Grande - the "Big River." The name fits this broad, deep stretch of water. But 700 kilometers upstream, the Rio Grande looks very different. The river begins as trickles of melting snow high in the San Juan Mountains in Colorado. As more water joins the river, it carves deep, narrow canyons out of the rock.

By the time it reaches Albuquerque, the river has grown wider. It continues into Texas, winding back and forth across the dusty desert valley. In places, the river is so shallow that it may even dry up during the summer. When the Rio Grande finally empties its water into the Gulf of Mexico, it is sluggish and heavy with mud.

Spanish explorers once gave different names to three parts of the Rio Grande. They thought they had seen three different rivers! In this section, you will discover how rivers change, and how they change the land around them.

How do Rivers begin?

Have you ever helped out at a car wash for your school or youth group? Think about what happened to the water that sloshed onto the pavement. First, the water ran in little trickles, which then joined together into a larger stream. The water followed the slope of the pavement down to the street or into a storm drain. A river begins in much the same way - trickles of water run over the ground and join together in larger streams.

When rain falls, some of the water evaporates immediately. Some soak into the soil. The remaining water that flows over the ground surface is called runoff. Runoff also comes from melting ice and snow, like the runoff that forms the beginnings of the Rio Grange.

Factors that affect runoff

What determines whether water soaks into the ground or flows over it as runoff? One factor is the nature of the ground surface. Water soaks into some types of ground covering more easily than others. How much water soaks in depends on the amount of space between the particles that make up the ground cover. For example, there is more space between the particles of soil than between the particles of soil and between the particles of pavement. As a result, water soaks into the soil more easily than into pavement. Since plant roots also absorb water, ground that is covered with grass or trees absorbs water more easily than bare soil.

The rate of rainfall is a second factor that affects the amount of runoff. During a heavy downpour, so much rain falls in a short time that it can't all soak into the ground. Instead, some become runoff.

A third factor is whether the land is flat or hilly. The force of gravity pulls water downhill, just as it pulls you downhill on a sled or skateboard. Water flows faster down a steep slope than over flat ground. Because the water is moving so quickly, it runs off instead of soaking in. As runoff flows along a trench or channel, it forms a stream. This is the beginning of the process that forms a river.