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Irregularities in the bed of a stream or river can cause the speed of water flow to be different at the two banks of the river. Where stream velocity is fastest, the water erodes the bank. Where velocity is low, sediment is deposited because the water there has less kinetic energy and is, therefore, unable to carry the sediment. Continued erosion and deposition moves the stream channel down the stream bed, and aerial photos record the progression.
Secondary FlowsHigher velocity portions of a stream tend to be driven to the outside of a meander (1). On the outside of the meander, the surface of the water has a tendency to be slightly higher, or super-elevated, because it has gained momentum and acceleration. Here, the flow is forced down the outer bank, which results in a steeper velocity gradient and greater bed shear stresses. The flow returns to the surface toward the inside of the meander where it is less turbulent (2). When stream flow reaches the outer bank of a meander, the increased acceleration erodes it away, causing toe scour and undercut banks (4). Sediment is deposited at the slower moving inside bend (5). Helical flow maintains the channel profile as the river erodes its way across the valley floor (6). For example, the larger the cross-sectional area of a river, the slower helical flow will be absorbed by friction. The meandering of a river or stream, which maintains a steady channel gradient and matches the waterway's flow characteristics, is a result of helical flow.
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The material lining the banks is not uniform the entire length of a river system. If river water runs into resistant sediments, the movement of the meander can slow downstream. As other meanders continue to migrate through softer sediments upstream, they eventually intersect the slower-moving meander and cut off the channel between the two, forming an independent loop that will become an oxbow lake.
Excerpted from: Meanders, Aquatic Hydraulics: Helical Flows