World Builders™                                                                               Session Two  -- Geology                       

The Life Cycle of a River

Rivers do important work by eroding the land and carrying nutrients to the oceans.

stream with rocks
    The place where a river begins is called its source.  Rivers begin high in the mountains, where melting snow gathers in small streams.  The water winds its ways through alpine meadows and begins its downward journey.

     A river's long course goes from high in the mountains down to sea level.  About 90% of the downward part is traveled  in the first 10% of its route.  The river begins by rushing downward, forming waterfalls and fast rapids when stones block the rush of water.  Later on, as more tributaries join the river, the wide expanse of water flows more smoothly and quietly.

This picture shows a young river flowing in its stony bed. You can see why people call this "white water". The water twists this way and that around the rocks.

Yellowstone FallsThe young river flows swiftly, with many rapids and waterfalls. It erodes its bed and cuts a narrow, V-shaped valley with steep sides. Notice the very steep sides of this valley and the powerful waterfall.

As the river flows, it is joined by other steams, its tributaries. The river has a greater flow of water now, and the river's bed is not so steep. The tributaries are also eroding the sides of the valley. The valleys become wider, and the walls of the valleys slope more gradually.

The valley floor becomes almost flat, and the water can spread out when the snows melt. In the mountains the streams form meadows where aspens and willows begin to grow. Further down the river's course the valley becomes a flood plain. When there is a lot of water catastrophic flooding can destroy human communities

river in meadow

 

 

This lovely mountain meadow shows a stream flowing in a very gently sloping bed.

meander in Grand CanyonIn the last part of a river's journey, it flows through a gently sloping flood plain. Because the plain is nearly level, the river can wander across it in a winding, snaking pattern. These loops and turns are called meanders. A meandering river flowing through a floodplain is called a mature river.

This picture shows the beginning of a meander pattern that this river formed when the green terraces in the picture were the floor of the floodplain. It has started cutting a V shaped valley again.

A mature river carries a heavy load of sediment. When the flow of water slows, the sediment settles to the bottom. Sand bars begin to appear.

The final stage in the life of a river is its joining with the sea. By now the river is traveling smoothly and carrying a lot of sand and clay. Merging with the sea slows the water flow even more, and the sediment drops out. This often leads to the formation of a delta, a broad, triangular river mouth with many sand bars and water channels. The delta becomes an estuary, an area with sheltered waters and a constant stream of nutrients from the land. The sand bars become islands with grass and trees that provide cover and nesting sites for birds.  Estuaries are important nurseries for the young of aquatic animals.  They are highly productive because they provide nutrients washed down from the land in an environment that is sheltered, and shallow enough to encourage the growth of water plants.


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Photographs from Corel CD-ROM s: for viewing only, not for downloading.   More Information.
Heading photograph from NASA, Earth from Space.

© 1996,1997, 1998, 1999, 2000, 2002, 2003.   Elizabeth Anne Viau. All rights reserved. This material may be used by individuals for instructional purposes but not sold. Please inform the author if you use it at eviau@earthlink.net .