![]() ![]() Therefore, in addition to growing laterally, the bends also gradually migrate down the valley. Due to the slope of the channel, erosion is more effective on the downstream side of a meander. Thus by eroding its outer bank and depositing material along its inner bank, a stream moves sideways without changing its channel size. ![]() At the same time the reduced current at the inside of the meander results in the deposition of coarse sediment, especially sand. The sideways movement occurs because the maximum velocity of the stream shifts toward the outside of the bend, causing erosion of the outer bank. Meanders change position by eroding sideways and slightly downstream. Where the bends of two meanders meet, they bypass the curve of river, creating an oxbow lake which may then be infilled with overwash sediment. ![]() Photo courtesy of the National Park Service. A meandering river in the Gates of the Arctic National Park and Preserve in Alaska. This ongoing process of deposition and erosion spreads the meanders further left and right of the valley center. A meander is more likely to occur in a slow moving, shallow river or stream on a floodplain. Where Can a Meander Be Found Almost all rivers have sections that include meanders. A large river can create a large meander that may be several miles long. This creates an erosional surface on the outer edge (a cut bank) and a depostional surface on the inner edge (a point bar). Meanwhile, the faster flow against the outer (concave) bank causes erosion. The size of a meander depends on the size of the river or stream. Phil Reiker, NPS Geologic Resources DivisionĪ meandering stream has a single channel that winds snakelike through its valley, so that the distance 'as the stream flows' is greater than 'as the crow flies.' As water flows around these curves, the outer edge of water is moving faster than the inner. 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.Oxbow lake formation in a meandering stream This clip shows perfectly the processes and features involved in the formation of a meander. For example, the larger the cross-sectional area of a river, the slower helical flow will be absorbed by friction. Helical flow maintains the channel profile as the river erodes its way across the valley floor (6). Sediment is deposited at the slower moving inside bend (5). When stream flow reaches the outer bank of a meander, the increased acceleration erodes it away, causing toe scour and undercut banks (4). The flow returns to the surface toward the inside of the meander where it is less turbulent (2). Here, the flow is forced down the outer bank, which results in a steeper velocity gradient and greater bed shear stresses. How is a meander formed Water flows slowly over shallow areas (riffles) in the riverbed and faster through pools, deeper sections of the river. 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. Higher velocity portions of a stream tend to be driven to the outside of a meander (1). They are typically found in the middle and lower course of a river. Continued erosion and deposition moves the stream channel down the stream bed, and aerial photos record the progression. Meanders and Oxbow Lakes A meander is a winding curve or bend in a river. ![]() Where velocity is low, sediment is deposited because the water there has less kinetic energy and is, therefore, unable to carry the sediment. Where stream velocity is fastest, the water erodes the bank. Once the velocity is increased and the load has become fine the river has a. 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. Meanders form at the middle coarse of the river and are sinuous bend in a river. ![]()
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