|Athro, Limited Gubik Field Trip Cretaceous Sediments Channel Deposits|
|Channel and Overbank|
Meandering streams cut a channel into a flood plain and deposit several characteristic sediments. Most notable of these are 1) channel fill sediments left by point bars that are deposited in the slowly moving water on the inside of bends and 2) the overbank deposits. Overbank deposits are primarily soils developed in fine grained sediments draped on the floodplain during floods. Examples of point bars at several scales are shown below.
A point bar in a small creek in Alabama. When this stream floods, the fast moving water in the
thalweg erodes into the bank on the outside of the bend, while slower moving water on the inside of the bend looses some of its sediment load, building up the point bar. The point bar is thus built of cross bedded sediments, and laterally accretes into as the meander bend migrates. As time goes on, flood after flood eats into the bank. As a result of this erosion, the meander bend migrates and the point bar follows it.
A point bar in a meandering stream in Yellowstone. As the stream migrates across the flood plain, the point bar leaves a deposit of laterally sands in the former path of the channel. When the stream floods, it carries large quantities of sediment in suspension. Sands or gravels may fall out of suspension on the point bar and accumulate there. As a flood ebbs, water velocity on the flood plain becomes quite slow, and very fine grain sediments fall out of suspension there. During the rest of the year, soil horizons form in these fine grained sediments on the flood plain. This process not only maintains a rich soil on the flood plain, it also produces a characteristic fluvial signature - the point bar and overbank deposits lithify to become packages of cross bedded sands within fine grained sediments containing aerosols
The basic fluvial geomorphology of meander bends can be seen at many scales. Here is an aerial view of a point bar produced by late Pleistocene floods in the Columbia River in Washington. From the ground, this large scale fluvial geomorphological feature appears to be a gently rolling river bank. From the air, huge ripple marks become visible.