Wednesday, December 02, 2009

Meandering along a river

We all know that rivers run from higher elevaions to lower elevations with the assistance of gravity.
Most natural rivers have twists and turns in them, very long straight sections are quite rare.
As the seasons change, the flow of water varies and fluctuates along the various flood plains.

The surprising fact about a meandering rivers' creation is that the geometric shape is similiar
around the world despite the different geophysical conditions. Geomorphology gives a
clear, detailed explanation of the causes and the development processes of meanders.

A river continually picks up and drops solid particles of rock and soil from its river bed.
In areas where the river flow is fast, more particles are picked up than dropped.
Conversely, in areas where the river flow is slow, more particles are dropped than picked up.
Areas where more particles are dropped are called alluvial or flood plains, and the
dropped particles are called alluvium. Even small streams make alluvial deposits,

The speed of the current close to the bank is usually slower than the one in mid-stream,
because of the friction with the river bank. When a disturbance to the straight water flow occurs,
as a result of an obstacle or a change in soil conditions in different parts, the water detours
the obstacle and an arc is formed in the river bed. The water flow is accelerated
and as a result the alluvium process intensifies in the external side of the arc.

Deposition of sediment occurs on the inner edge as the river sweeps and rolls
sand, rocks and other submerged objects across the bed of the river towards
the inside radius of the river bend, creating a slip-off slope called a point bar.

Erosion is greater on the outside of the bend where the soil is not protected by deposits.
The current on the outside bend is more effective in eroding the unprotected soil.
The inside bend receives steadily increasing deposits of sand and rocks,
and the meander tends to grow forming a small cliff called a cut bank.

The meanders extend the watercourse of the river; creating a reduction of the overall
flowing speed in this part of the river. As a result, there is a gradual tapering off the
centrifugal force until it diminishes altogether. The curvature radius eventually stops growing.
When the curvature radius reaches its maximum size, the meander is called a mature meander.

The more one descends down stream, the intensity of the stream grows,
and therefore the meanders reach maturity when the curvature radius is larger.
The process of meander creation is one which creates a balance in the stream's speed.
Rivers tend to balance the current speed along the river bed.

Over time,  meanders migrate downstream, sometimes creating problems for
local municipalities attempting to maintain stable roads and bridges.

Rivers deserve respect and people need to understand their power. Please wear your vests !  
Paddling a swollen river from a recent rainstorm or large snowmelt can cause loss of life.
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