Rivers are more than just water rolling along between two earthen banks. They are worlds of their own, with very special attributes, properties, and associated features. Anyone who lives near a major river system knows that they often seem to be very complex, and sometimes very unpredictable. Any change or disruption in any part of the river system affects the entire river.
There are three main parts to every river system. The collecting system consists of the branches or tributaries in the headwater region, upstream. These tributaries, other rivers and streams, collect the water that runs off of fields and areas at higher elevations, and carries debris, sediment and water toward the main river. Further downstream the number of tributaries decrease, and the length of the tributaries increase. Also, the steepness, or the gradient, decreases further downstream, and streams become deeper and wider. Rainfall, or melting snow, upstream in the system, will determine the depth of the main tributary or river and determine the potential for flooding.
The transporting system is the main trunk stream, or river, that carries all the water and sediment from the tributaries to the ocean. It is here that erosion and the depositing of sediment occur.
The dispersing system consists of the distributaries at the mouth of the river. At this delta, sediment and water enter the ocean, a lake, or a basin. Everything from weathered rock to fine silt, chemical runoff, and other debris are deposited here. When an abundance of nutrients are included in the dispersal, problems can arise in the ecosystem at the delta. The Mississippi River delta often becomes a “dead zone” when nitrogen and phosphorous are washed downstream from all of the waterways in states further north, producing an abnormal algae bloom. This algae depletes the oxygen level in the area and alters the food chain. The degree of this condition is determined by the weather and flooding.
Along the river and tributaries there are also floodplains. These flat areas border the stream and are normally underwater during a major flood. As the water recedes after the flood, the stream leaves a deposit of sediment called alluvium in the floodplain. When sediment is deposited in the river in other areas, natural levees may be formed as well as sand bars, called point bars, or back swamps where water is retained.
Rivers are complex structures that seem to take on a life of their own, and every part of the river system is vital to its function.