The fresh water bodies in which water moves continuously in one direction are called streams and rivers. The head water of a stream is cold and clear. It carries little sediment and a few mineral nutrients. The channel is narrow there. A fa -t current of water passes. over a rocky substrate. Numerous tributaries join to for a river farther downstream. Water is more turbid there. It carries more sediment and nutrients. The channel of the mouth of a river is wide. Its substrate is silty.
- Flow of water
Many factors influence the flow, the nutrient and oxygen content, and the turbidity of streams and rivers.
(a) Riffle: The shallow water move rapidly over a rough bottom. Surface riffles shows the turbulent flow of water. Deep water flows slowly over a smooth bottom.
(b) Pools: Pools of water are common Deep water moves rapidly over a flat bottom.
- Physical properties
Fallen leaves from dense, overhanging vegetation add large amounts of organicmatter. The erosion of rocks increases the concentration of inorganic nutrients in
the flowing water. The turbulent flow of any streams constantly oxygenates the water. But murky warm waters of large rivers contain little oxygen. The temperature of the water varies with altitude and latitude. The amount of water flowing in a stream or river varies seasonally. It increases with rainfall and snowmelt_
- Autotrophs in rivers and streams
The smatl organisms are washed away by the flow of water. Therefore, many fast flowing streams and rivers do not support stationary plankton communities
Photosynthesis takes place by attached algae and rooted plants. It supports the food chains. But dense vegetation on the banks of a narrow stream blocks the
sunlight. Therefore, organic material carried into the stream by runoff. It provides the most important input food for consumers. The high concentration of silt near the mouths of large rivers increases the turbidity. It can also block the passage of light. It makes the photosynthesis difficult.
- Animal population
There are dramatic variations in the physical environment of the streams and rivers. Therefore, the composition of animal communalities varies significantly
from the head waters of a stream to the mouth of the river. Some fishes like trout are present in cool temperatures, high oxygen, and clear water. Catfish and carp
are abundant in the warmer and murkier waters further downstream. Benthic (bottom-dwelling) communities also change. Many insect species are restricted to the relatively short stretches. Some large rivers are also inhabited by a variety of turtles, snakes. Crocodilians and porpoises are also present in the rivers of tropical regions.
- Evolutionary adaptations
The animals of stream and river show evolutionary adaptations. These adaptations enable them to resist flow of water. Small animals are typically
dorso-ventrally flattened. They can attach to rocks temporarily. Many insect species live on the underside of rocks or on their downstream side. They exploit a small habitat that is relatively free of turbulent flow. Other species are present in the quiet pools. The flow of water is slower in these pools than in riffles or runs.