River and Stream Aquatic Zones


Key Characteristics of Rivers and Streams

  • Stream or rivers form when water that does not find an underground aquifer drains off the land by either seeping through the soil or spilling over the surface into the river – or streambed.
  • Water in stream and river is in constant motion. The speed of the water depends on the slope of the land, the size and shape of the bed, the amount of water, and the amount of material carried by the water.
  • As river and stream water travels, it shapes its own channel by forcing itself into rock cracks and prying pieces loose, by rubbing sand, gravel, and rocks against the banks, and by dissolving rock substances such as limestone into the flowering water.
  • Streams and rivers have shaped the history by proving drinking water, food, irrigation, and transportation.

The Flowing of Rivers and Streams

river cross section

How fast is your water? The water in rivers and streams is in constant motion. It moves faster with a steep gradient, a narrow, curving or a high volume of water. Though the average speed of moving water is about 5-6 kilometers (3-4 miles) an hour (a fast walk for humans), the speed can range up to 31 km (19 miles) an hour during floods. Water moves fastest in the middle of the channel and around the outside of a curve, where it carves under the banks. The slower waters on the inside of a curve cause rocks and other carried materials to fall out.

Much of the land around us has been shaped by flowing water. Over time, rivers smooth out their beds. As rivers cut through softer rock faster than through harder rock, waterfalls and rapids are associated with young rivers. Over time canyons may be lifted up while the rivers continue to cut down the bedrock.

Streams and rivers often begin high in mountains, where their source (or headwaters) is melting snow. Their mouths form deltas and alluvial fans as they approach lakes, seas, or oceans. Near the head, the water flow is shallow and carries a lot of material. Later, as the stream or river drops its load, it will begin to meander (wander) (See picture above), cutting banks and making sand bars. If the river floods, it cuts across the meanders, creating oxbows and oxbow lakes.

Carrying by Rivers and Streams

river with stones

Water is a heavy carrier of materials, especially in the constantly moving waters of rivers and streams. Stationary or slow water drops its contents, but faster water gathers new materials and can carry them quite a distance.

Moving water erodes river and stream banks, by cutting away soil and rock. Fast-moving water can also pick up small clay particles that tend to stick to the bottom. Millions of tons of material wash down rivers and streams every year.

- Material can be carried on the bottom of the stream or riverbed. Bottom rocks of all sizes slowly tumble along in the currents. They carve through softer rock as the river flows, or form piles of rubble.

- Material can be carried in suspension. Silt and clay are carried along making the waters cloudy.

- Materials can be carried in solution. Materials dissolved, broken down into ions, and carried along with the water.


Deltas are fan-shaped deposits that form at the mouth of a river. As the moving water enters a still or slow-moving body (such as the ocean), it drops its load of particles and materials. As these fill up the river channel, the water moves around and forms new paths of flow. These secondary channels also slowly fill up with deposited materials, and the water fans out again, forming a delta. Deltas are continually changing their shape and depth as more materials are carried downstream and deposited at the river mouth.

Deltas vary. Great rivers with great deltas include the Tigris and Euphrates, the Nile, the Yangtse, the Ganges, the Mississippi and the Indus rivers. Silt from rivers continues to settle. The Columbia River in the northwestern United States, on the other hand, discharges into the ocean but does not form a delta because powerful waves and tides quickly distribute the sediment.

Environmental Challenges:

Toxins drain from land into local waters. Major contaminants include:
Hazardous waters
Gasoline and oil
Pesticides and herbicides

Dams and Distress

We straighten, channel and dam rivers in an attempt to control flooding and make water more accessible for drinking and irrigation. Unfortunately, this often creates new problems. The more we channel, the more floods will occur.

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