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Earth Geology Topography The Meandering River

The Meandering River

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FIGURE 1: Topographic map of the Horseshoe Bend Area, Ozark County. Click to see more details.

The Bryant and its major tributaries are very sinuous streams. The stream valleys and the stream channels within the valleys do a considerable amount of wandering or meandering back and forth. These "meanders" (Figure 1) are an erosional feature that is typical of a mature stream. The meanders of the Bryant are entrenched, that is they form deep meandering valleys that are two to three hundred feet deep. 

The stream valleys are typically asymmetric. The outside of the meander is normally a very steep slope or a bluff whereas the inside of the curve is a much more gradual slope. The reason for this asymmetry is simple physics. Along the outside of the curve the river is forced to turn and the force of the moving water is concentrated along this bank and erodes it away. The inside of the curve is a larger region of little erosional force and is the site of deposition. Gravel bars typically are found along the inside curve of a meander. The river channel will move laterally over time in the direction of the outside curve of a meander. This causes the meanders to grow more pronounced with time.  
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FIGURE 2: Portion of a topographic map of Pine Hill and the junction of the Bryant and North Fork. Click to see more details.

A very interesting feature is found at the junction of Bryant and North Fork. Pine Hill (Figure 2) is an elongated triangular hill just to the north of the junction. Around the north side of Pine Hill is a semicircular valley that is actually a dry meander. At one time the North Fork actually went around Pine Hill in a very tight meander. The junction between Bryant and North Fork at that time was probably in the vicinity of Parker Cemetery. Lateral erosion along the outside curves at the neck of the meander finally cut through the narrow ridge between creating a "cutoff meander" that now loops around Pine Hill. This cutoff probably occurred several thousand years ago.

A fault has been mapped by the U.S. Geologic Survey which goes through the long dimension of Pine Hill (Fig. 3). This fault may be the reason that this long narrow meander developed here (see below). 
geologic fault line
FIGURE 3: Portion of geologic map of the junction of the Bryant and North Fork.

Although most meanders observed in the Bryant and its tributaries probably developed by a normal erosional process, there are a few whose features are different enough to require a different explanation.  

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FIGURE 4: The extreme southeast part of the Mansfield 7.5 minute quadrangle. Two very sharp meanders are visible, one in the Bryant and the other in Dry Creek. Click to see more details.
Two unusual very sharp meanders occur in the Bryant Valley in northern Douglas County. One is where U highway crosses Bryant ("A" in Fig. 4) and the second is about one mile south at the junction of Bryant and its tributary, Dry Creek. In each of these places a very sharp, finger-like, east-west, ridge about a half mile long projects into the Bryant valley creating a very narrow sharp meander. A similar sharp narrow ridge projects into the Dry Creek valley just a mile upstream from its junction with Bryant ("B" in Fig. 4). 

Several strands of the Mansfield Fault Zone have been mapped in this area and apparently cross the Bryant and Dry Creek courses. It appears that movement on these faults may have caused deflections of the streams or the fault zones have created resistant zones that caused the deflection of the streams and resulted in the unusual meanders. The geologic map of the area (Figure 5) shows a number of faults in the local area.