Figure 1
Braided channels are associated with higher stream gradients, coarser material (more bedload transport), and larger width-to-depth ratios (Figure 1) than meandering streams.
Figure 1
The width-to-depth ratio and the percent of fine material
in a river are approximately inversely proportional (Figure 2). The
classic
explanation for this relationship is an efficiency argument: it is more
efficient for rivers which carry most of their material near the bed as
bedload to have most of their turbulent mixing and shear stress
concentrated
near the bed. A river with a large width-to-depth ratio accomplishes
that.
Figure 2
Figure 2b shows a braided stream in Alaska.
Figure 2b
Grain size variations in vertical well logs can be used
to reconstruct the history of channel migration and avulsion in a fluvial
depositional environment (Figure 3). Active channel sequences are
distinguished
from abandoned channel sequences in part based on larger average grain
sizes resulting from the larger competence of active channels. Active
channel
sequences also typically exhibit fining-upwards sequences as a result of
decreasing channel gradient as the active channel fills.
Figure 3
Based on historical and geologic data, changes in channel habit over time have been documented. Figure 5 illustrates the evolution of the South Platte River resulting from a gradual decrease in discharge from 1850-1950 due to irrigation and flood control projects upstream. In 1850 the channel was a wide, stable channel with subaqueous bars (A). Initially the flow depth decreased rapidly without a decrease in channel width (B). The channel banks responded more slowly but eventually the width decreased to form a braided stream (C) and finally the meandering stream of today (D).
Figure 5
The Mississippi River has dramatically changed in the last 18kyr due to the huge increase in sediment produced upstream by continental glaciation and due to rising sea level. The Mississippi's base level was 100m lower at 15ka than today. This episode of rapid sea level fall incised a large trench in the present-day Gulf of Mexico. As the sea level rose and the glaciers retreated, the Mississippi River changed from a straight channel with a slope about 10 times larger than that of today to a braided stream that avulsed frequently. The lower Mississppi Valley was filled with coarse sediment characteristic of braided streams. As the sea level rose the gradient decreased and the deposits became finer. Eventually a relatively stable meandering channel was formed. Meandering channels are very unstable within their floodplains but the formation of oxbow lakes once the meanders become large enough to cut themselves off limits the wandering of a meandering stream.
Figure 6
.
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