Saltation bounces particles of sand up the shallow upstream side and over the peak of the ripple, eroding the upstream side and depositing sand on the downstream side. These ripples have a characteristic profile with a shallow slope on the upstream side and a steeper slope on the downstream side. At low velocities, the creep of sand along the bed will, if anything, tend to smooth out the bed.Īt higher velocities, sand ripples begin to form: small ridges of sand with the ridge at right-angles to the current. Geologists can discover the relationship between these factors both by observing actual rivers, and by laboratory experiments using flumes, allowing them to control variables such as the velocity of flow and the size of the clasts.įor example, consider the effect that a river's velocity has on a bed consisting of average-sized sand grains. The sedimentary structures formed by a river at a particular point will depend on its velocity, its depth, and the sediment type.

Note also the oxbow lake: its peculiar orange color is probably caused by salt-loving bacteria in the water, and should not be taken as being typical of oxbow lakes. The photograph to the right shows the Rio Negro as seen from space: as you can see, the landscape is scarred with former meanders where the river used to flow before it shifted its bed. The shifting of meandering and braided rivers across the landscape as a result of their own deposition of sediment produces a flattened, sediment-rich landscape known as a flood plain. The floodplain of the Rio Negro, as seen from space. Eventually the meander may touch itself and the river will suddenly find itself with a new, straighter, shorter path, leaving the meander isolated as an oxbow lake. If this tendency goes far enough, the meander approaches a loop doubling back on itself. The result of this is that a meandering river will become more meandering over time. As a consequence of the hydrodynamics of this situation, the current is faster and causes more erosion on the outside of the loop, while sediment will tend to be deposited on the inside of the loop, forming point bars. Meandering rivers wind from side to side in large loops ( meanders). the ridges of sediment breaking the surface of the water) are not permanent features, but shift around over time. They are formed when a river with a lot of sediment repeatedly deposits the sediment and erodes it so the braids and bars (i.e. At low gradients, two characteristic forms they can take are braided and meandering rivers.īraided rivers, as the name suggests, consist of a number of channels which separate and rejoin around bars of sediment. Rivers, unless artificially banked, rarely flow in completely straight lines. 4 Note on superposed and antecedent riversīraided and meandering rivers.

Meander river