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January 24, 2014

How Do Rivers Form?

From the Mississippi to the Colorado to the Potomac, natural waterways are vital to America’s environment, economy, and recreation. But few people know or wonder how these rivers formed their courses.

Utah: Canyonlands by Marivel87 (deviantART).

The birth of a river is dependent on its water-source, which might be snowmelt, precipitation, or groundwater from an underground reservoir. Whatever the origin, a river’s subsequent journey comes down to geography and physics. Water will always flow downhill from a source, because of good ol’ gravity. As it moves towards a larger body of water, the river channel usually widens, joined by other streams and rivers, gaining a lot of momentum.

The curvature of a river results from the process of erosion altering the water’s momentum. The process of riverine erosion involves wearing away rocks and soil along the watercourse, as well as breaking down particles and sediment on the banks. Slower-moving water spends energy carrying rocks and sediment downstream, whereas faster currents speed by on the outside bend. The slower current is therefore moving on the inner bend, where deposition (the depositing of rocks and sediment) is usually found. This difference between fast and slow currents creates the curvy courses that we see when we look at rivers on a map.

“The lake is being fed by the pristine glacial Godley River, seen in the image proper, as a network of braided twists and knots. The water in the lake has flowed 30 kilometres south from the Mount Cook region in the Southern Alps.” — Humayun Qureshi

Scientists are also finding that soil creep and rates of incision affect how and why a river’s course will take on certain characteristics. Soil creep (rock and soil moving downward) and incision (a river cutting through bedrock) play a part in the formation of river branches. When incision overrides soil creep, a river overcomes a certain “tipping point” and becomes more likely to branch off.

Though the course of a river is determined by its geological environment and energy, riparian health is an entirely different matter. Each of us can see our community’s rivers choked up by trash and careless dumping. Creeklife aims to connect those who are concerned with the health of local rivers to real-life actions that help out the ecosystem.

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Posted by Mark Contorno.

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