How Do Rivers Change the Land?

Running water is a major factor in changing the surface of Earth.

Earth’s surface has been constantly changing. Some changes occur within minutes while other changes may take millions of years. One of the most important causes of change on Earth is running water.

Rivers begin high in the mountains or hills as small tributaries, or “feeder streams.” Tributaries are fed by runoff--water that runs off Earth’s solid surface. The areas from which the water is drained are called watersheds. Tributaries keep merging until they form larger streams or rivers.

The force of gravity keeps water flowing downhill. Where slopes are steep, the water in streams and rivers flows very fast. Rivers flowing along gentle slopes move more slowly. The moving water carries sediment (pieces of material) with it.

The amount of sediment a stream or river can carry depends on how much water there is and how fast it is flowing. Larger, faster-flowing streams can carry much larger loads of sediment.

• The force of running water with its load of sediment can erode the stream bed.

• When the stream or river slows down, sediments are dropped or deposited on the river floor.

• Where river valleys are nearly flat and sediments are thick, rivers form bends. These bends or S-shapes are called meanders.

• The outside of a meander will lose sediment because the water moves more swiftly. The opposite is true of the inside of a meander. This is the area where sediment is deposited.

As the landscape continues to flatten, flood plains may form along the banks of a river. Some of the world’s most important agricultural areas are found in flood plains. A river might overflow its banks following a heavy rain. As the water covers the surrounding land, it releases sediments. Over time, these sediments build up, creating fertile farmlands.

All rivers eventually end as they empty into a lake or ocean. The place where a river empties into the ocean is called the mouth of the river. The water slows down so much at its mouth, that it unloads most of its sediment there.

The sediment forms a fan-shaped deposit called a delta. Deltas are very important agricultural areas. That is because they are a source of fertile soil.

Delta

Delta

A delta is a low triangular area of alluvial deposits where a river divides before entering a larger body of water.

Tributary

• Not all rivers lead to the ocean. Some rivers join other rivers. A tributary is a river or stream that does not reach the ocean. Instead a tributary will join another river or stream. The land where rivers meet can change very quickly. But even the smallest stream affects the land around it.

Water Gaps, Canyons, and Valleys

• River valleys form from small channels that are deepened and widened by erosion. Small gullies become deeper and wider as their walls are eroded and the sediments are carried away.

• Deep V-shaped valleys are often called canyons.

• A U-shaped valley is the shape left after a valley has been over-deepened by a glacier. The original V-shaped valley, which would have been made by a river, is widened and deepened after the ice has eroded the sides and bottom of the valley.

Grand Canyon

• The Grand Canyon formed over millions of years.

Water Gap

• A water gap is an opening or notch which flowing water has carved through a mountain range. Water gaps often offer a practical route for roads and railroads to cross a mountain ridge.

The End!