Oxbow lakes on New Zealand's Taieri River have been converted into water meadows. This flow of the boundary layer is significantly different from the speed and direction of the primary flow of the river, and is part of the river's secondary flow. As it flows along the floor of the river, it sweeps loose material toward the convex bank. It responds to this pressure gradient, and its velocity is partly downstream and partly across the river toward the convex bank. The boundary layer that flows along the river floor does not move fast enough to balance the pressure gradient laterally across the river. A pressure gradient toward the convex bank provides the centripetal force necessary for each parcel of water to follow its curved path. As a result, at any elevation within the river, water pressure is slightly greater near the outside of the bend than on the inside. The curved path of a river around a bend makes the water's surface slightly higher on the outside of the bend than on the inside. The primary flow might be expected to fling the dense particles to the perimeter of the bowl, but instead the secondary flow sweeps the particles toward the center. However, the secondary flow of the boundary layer across the floor of the bowl is inward toward the center. The primary flow of water in the bowl is circular and the streamlines are concentric with the side of the bowl. This is the mechanism that leads to the formation of point bars and contributes to the formation of oxbow lakes. The dense particles quickly sweep into a neat pile in the center of the bowl. Set the water into circular motion with one hand or a spoon. Partly fill the bowl with water and sprinkle dense particles such as sand or rice into the bowl. The effect of the secondary flow can be demonstrated using a circular bowl. The process of deposition of silt, sand and gravel on the convex bank is clearly illustrated in point bars. Gathering of erosion products near the concave bank and transporting them to the convex bank is the work of the secondary flow across the floor of the river in the vicinity of a river bend. This process can occur over a time from a few years to several decades, and may sometimes become essentially static. When deposition finally seals off the cutoff from the river channel, an oxbow lake forms. When this happens a new, straighter river channel develops-and an abandoned meander loop, called a cutoff, forms. The narrow neck of land between the two neighboring concave banks is finally cut through, either by lateral erosion of the two concave banks or by the strong currents of a flood. Continuous deposition on the convex bank and erosion of the concave bank of a meandering river cause the formation of a very pronounced meander with two concave banks getting closer. In contrast, both lateral erosion and undercutting occur on the cut bank or concave bank (the bank with the greater radius). In the vicinity of a river bend, deposition occurs on the convex bank (the bank with the smaller radius). When a river reaches a low-lying plain, often in its final course to the sea or a lake, it meanders widely. Because oxbow lakes are stillwater lakes, with no current flowing through them, the entire lake gradually silts up, becoming a bog or swamp and then evaporating completely. The entrances to the abandoned meander eventually silt up, forming an oxbow lake. The river then follows a shorter course that bypasses the meander. This takes place because meanders tend to grow and become more curved over time. An oxbow lake forms when a meandering river erodes through the neck of one of its meanders.
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