The purpose of this earth cache is to show that the force of a river can change its direction.
A meander is a bend in a sinuous watercourse or river which forms when moving water in a stream erodes the outer banks and widens its valley, and the inner part of the river has less energy and deposits silt. A stream of any volume may assume a meandering course, alternately eroding sediments from the outside of a bend and depositing them on the inside.
The result is a snaking pattern as the stream meanders back and forth across its down-valley axis. When a meander gets cut off from the main stream, an oxbow lake forms. Over time meanders migrate downstream, sometimes in such a short time as to create civil engineering problems when they attempt to maintain roads and bridges.
A hypothetical stream bed following a tilted valley. The maximum gradient is along the down-valley axis represented by a hypothetical straight channel. Meanders develop, which lengthen the course of the stream, decreasing the gradient.
Meander formation is a result of natural processes. The waveform configuration of a stream is constantly changing with fluid flowing around a bend in a vortex. Once a channel begins to follow a sinusoidal path, the amplitude and concavity of the loops increase dramatically due to the effect of helical flow sweeping dense eroded material towards the inside of the bend, and leaving the outside of the bend unprotected and therefore vulnerable to accelerated erosion, forming a positive feedback loop.
The cross-current along the floor of the channel is part of the secondary flow and sweeps dense eroded material towards the inside of the bend. The cross-current then rises to the surface near the inside and flows towards the outside, forming the helical flow. The greater the curvature of the bend, and the faster the flow, the stronger is the cross-current and the sweeping.
Due to the conservation of angular momentum the speed on the inside of the bend is faster than on the outside.
Since the flow velocity is diminished, so is the centrifugal pressure. However, the pressure of the super-elevated column prevails, developing an unbalanced gradient that moves water back across the bottom from the outside to the inside. The flow is supplied by a counter-flow across the surface from the inside to the outside. This entire situation is very similar to the Tea leaf paradox. This secondary flow carries sediment from the outside of the bend to the inside making the river more meandering.
If you look to your right and across the river from the given coords, you will see what is known as a slip-off slope. The slip-off slope is the inside bank of a meander on a river where sedimentary material is deposited as a result of the slower flow rate. It is on the opposite side of the channel to a river-cut cliff or cut bank which is formed by the erosive forces of the stronger current on that side. The terms are used in connection with the formation of meanders on a river.
A typical example of a slip-off slope.
Deposition on the slip-off slope occurs due to the helical flow in meandering reaches that is constantly transferring sediment from the undercut slope on the outside of the meanders to the inside river banks.
(Adapted from http://en.wikipedia.org/wiki/Meander)
To claim this earth cache, please email the CO via the link above with the answers to the following questions.
1. At the given coordinates you can see the deposit left by the river Blackwater as the meander is created, known as a "slip-off". Can you describe the material that has been deposited?
2. Can you estimate the height of the bank on the side of the river where the deposit is and the side opposite the deposit? If there is a difference, why do you think this is?
3. Look at the speed of the river by the slip-off. Is the speed of the river slower or faster on that side of the river?