Earth Science: Rivers – High and Low Courses
Nearly a third of the approximately 24,000 cubic miles (100,000 km3) of yearly precipitation flows into the oceans via creeks and rivers. During this process, rivers shape the environment through erosion and sedimentation.
The highest flow rate and strongest force of erosion occurs in the early or youthful rivers and depends on the water level, the width of the river, and the structure of the stream bed. Due to vertical erosion, a notch-shaped river valley or vale can be formed.
A valley that is carved deeply into the surrounding area like a gorge but with steep slopes is called a canyon. The Grand Canyon in Colorado is the largest canyon worldwide where sudden drops in the terrain of the river bed create narrow fast- flowing streams and roaring waterfalls.
Meanders and the mouth of a river
As soon as the terrain flattens, the flow rate and force of erosion decreases significantly and a river is able to bypass larger obstacles. This is called a middle-aged river. As the current is stronger toward the outside of a bend, a middle-aged river begins to undercut the stream bank and widen its bed, creating an almost vertical bank. Due to the low flow rate on the inside of a bend, gravel and sand is continually deposited here.
This creates a point bar with a shallow slope. Over time, meanders are formed. Named after a river with many bends in Turkey, meanders move downstream in great curves. When there is a large flow of water, a river may occasionally break through the neck of a loop, leaving behind an oxbow lake. Where slow-flowing or older rivers drain into the ocean, large sediment plumes will often push into the ocean and form a delta.
Large deltas such as the Nile Delta or the Mississippi Delta can only be generated near calm and shallow seas. Rivers draining into the sea at a coast with a large tidal range form estuaries, allowing tidal waves to reach far into the interior of the land.
High water
When heavy rain falls or rapidly melting snow surpass the capacity of a river, the river overflows and lower parts of the river valley get flooded.
Human intervention has increased the risk of flooding through the destruction of vegetation, clear-cutting of forests, and the over-regulation and straightening of river courses.
RIVERS MADE BY HUMANS
One of the most impressive inland waterways is the Saint Lawrence Seaway, completed during the 1950s in North America. Several locks allow for continuous shipping along a system 2,500 miles (4,023 km) long, from the Great Lakes in the interior all the way to the Atlantic Ocean.
The Kiel Canal (in Germany Nord-Ostsee-Kanal) was completed in 1895 and is one of the world’s most frequented artificial waterways.