FIGURE 5.14 Effect of Monroe Uplift on Big Colewa Creek. A, Change of channel depth. Three reaches show different degrees of response to uplift. B, Change of channel gradient and slope of valley floor. C, Change of sinuosity. From Burnett and Schumm (1983).

FIGURE 5.15 Vertical benchmark movement along a National Geodetic Survey route between Jackson, Mississippi, and New Orleans, Louisiana. From Brown and Oliver (1976) with permission of the American Geophysical Union.

degradation has yet to affect the profile of Big Colewa Creek.

The Big Colewa Creek channel can be used to summarize the effect of uplift on an alluvial channel. It can be divided into three zones of activity along its valley (Figure 5.14A). The lower zone, from the mouth to valley km 32, has a high average bank height of about 6 m. The middle zone, from valley km 32 to 65, shows a clear upstream decrease in the average bank height from 6 to 2 m. The upper zone, above valley km 65, has a constant low average bank height of 2 m. Degradation may have occurred in the lower zone, whereas entrenchment is still in progress in the middle zone, and in the upper zone entrenchment has not yet occurred.

Figure 5.14B shows changes in valley slope and channel thalweg slope with valley distance. The valley slope remains high from the mouth to valley km 40, and then it suddenly decreases. The break in slope, at valley km 40, defines the apparent location of the uplift axis. The thalweg slope is high from the mouth to valley km 55, and then it also suddenly decreases. The fact that the two curves do not coincide suggests that the channel has incised through the axis of uplift.

Sinuosity is approximately 1.2 in the upstream stretch of Big Colewa Creek (Figure 5.14C). Downstream between valley km 50 and 55, sinuosity increases to about 1.7, and then it gradually decreases to 1.5 at the mouth.

In summary, numerous relations between the morphology of the streams and terraces and the underlying Cretaceous and Tertiary structures of the Monroe Uplift indicate that the area is still active tectonically. The patterns of changes and the present stream morphology provide information on the response of streams to active uplift within their valleys.

Wiggins Uplift In contrast to the Monroe Uplift, active displacement of the Wiggins Uplift (Figure 5.9) is clearly displayed by geodetic surveys (Figure 5.15). Bogue Homo Creek is analogous to Big Colewa Creek in this area, and it displays similar morphologic differences. Above the axis of uplift the channel is anastomosing, and the main channel is relatively straight. Immediately below the axis the channel has incised below the former floodplain to form a low terrace, and sinuosity is higher. Numerous cutoffs have occurred, and locally braided reaches have developed as a result of increased sediment loads resulting from incision.

A gaging station on Tallahala Creek provides evidence of channel incision at an average rate of 12 mm/yr since 1940 (Figure 5.16). This is three times the rate of measured uplift; however, a channel probably adjusts episodically to continuous uplift (Ouchi, 1983).

A factor that makes comparison of channel behavior



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