More Valley and Stream Erosion

Earth Geology Erosion More Valley and Stream Erosion

More Valley and Stream Erosion

We tend to think of the valleys and canyons around us as mute and unchanging. And indeed, from the human time perspective very little change occurs. It seems as though they have always been the same. See Contemplating a Babbling Brook for more on this topic. But in the larger view, from the perspective of geological time, the valleys change, evolve and grow. The inexorable force of erosion gouges the valleys deeper and wider and constantly gnaws away at the headlands to extend their length. Sediment deposition clogs and changes the stream courses within the valleys where the stream flow slows and the gradient lessens.
Geologists classify valleys in two ways: first according to the overall pattern made by the stream valleys in a region, and second according to age based on the shape of the valley profile. The tributaries that feed the Bryant form in general a dendritic or treelike pattern, repeatedly branching upstream into smaller tributaries, much like the branches on a tree. However, in detail these apparently random branchings have a parallelism that reflects the joint and fault pattern of the rocks through which they flow. The structure of the bedrock exerts a subtle influence on the pattern of the watershed.
The stream valleys of the headwaters of Bryant and her tributaries are considered youthful. The headwater valleys have a relatively steep gradient and a cross-sectional profile that is "V" shaped with the stream occupying the bottom of the "V". FIGURE 1 Narrow "V" shaped stream valley on the flank of Iron Mtn. Gray area in the center is a small waterfall, white line is a dead tree trunk that is lying on and shows the slope angle of the valley wall.
In this region of youthful valleys, the predominant erosion is twofold. The streams erode down vertically, cutting their channels and the valleys deeper into the bedrock. The streams also simultaneously erode headward extending their length as they gnaw away into the surrounding hillsides and plateaus. This headward erosion occurs where the valley profiles are the steepest, and where the valleys are occupied only by intermittent wet-weather streams. This two part erosion proceeds at a pace determined by two factors: the amount of flow in the valleys, and the rate of weathering and breakdown of the underlying bedrock. The upper part of the Bryant has a stream gradient of around 125 ft/mi (see Topography). This steep gradient gives the storm runoff in these valleys considerable sediment carrying capacity. The more water there is and the faster it moves, the more sediment it can carry downstream. Farther downstream the stream gradient decreases significantly and the water can no longer carry the load. At this point, where the gravels and other sediments begin to drop out of the stream, a distinct change in valley character emerges. The stream and valley gain a level of maturity and the shape and character change. In a mature valley the stream profile in cross section is no longer "V" shaped, but flat-bottomed with the valley several times wider than the stream channel itself. The stream gradient is much less, on the order of 8 to 10 ft/mi. The stream may occupy any position in the valley but typically flows along one side of the relatively flat-bottomed valley. The flat bottom of the mature valley is called a floodplain because it is usually underwater during the largest floods and owes its existence to the deposition of sediments carried down from higher elevations during floods and deposited as gravel, sand and silt beds as the floods subside. These floodplain deposits record a complex history of erosion and sedimentation throughout the development of the watershed. These floodplain deposits are also prized as farmland because of the flatness and fertility of the sediment layers.

FIGURE 3 Lateral erosion along an intermittent stream channel has eroded into the edge of a field exposing a layer of dark soil on top of a buried gravel layer of old channel. Bank height is about five feet. Leaves in the foreground are from a small walnut tree that was undercut and toppled by the bank erosion.	FIGURE 4 Lateral erosion exposing dark silt floodplain deposits with layers of gravel. Bank height about four feet. Roots at upper right are of a small walnut tree being undercut by bank erosion.
FIGURE 5 Photo at the same location as Figure 1, looking upstream at a meander. The stream bank is being actively eroded along the outside of the curve to the right. Along the inside of the curve, to the left, is a coarse gravel deposit. In the center is a bed of fine gravel and sand. These gravel and sand bars, about three feet thick, were deposited by a single heavy storm during which the otherwise dry streambed ran bank full for several hours.
The major erosion in a mature valley is lateral as the stream shifts course back and forth across the valley and cuts into the valley walls to widen the valley itself. The shortest course for a stream is a straight line and water will follow that shortest course when possible. However, several factors work to create a meandering stream and deflect the flow away from a straight line. One factor is streambed clogging due to heavy sediment dumping during a flood. Sediment may pile up, causing the stream to seek a new channel across a different part of the floodplain. This usually results in minor shifts of streamflow, but can create significant changes.
Another possible factor is landslides or the collapse of an oversteep valley wall or bluff, which creates a dam that may divert the stream flow to the other side of a valley and result in a permanent meander. Another cause is the shifting of bedrock along faults that cross the valley. This sudden change can create sharp and unusual meanders in a stream channel. Abandoned channels are very common in floodplain development (Figure 6). FIGURE 6 Vertical and overhanging bluffs along an abandoned channel in Hunter Creek valley near Crystal Lake.
Once a stream channel begins to meander or curve there is a strong tendency to maintain and increase the curve. As a stream rounds a curve the greatest erosive force is located on the outside of the curve. The outside bank of the stream channel deflects the force of the running water along the curve and undergoes increased erosion. The inside of a curve is typically a zone of deposition as sediments are deposited from the slower current found there. In this way the stream channel tends to migrate laterally in the direction of the outside curve and the curve or meander grows. Occasionally erosion cuts through the neck of a sharp meander leaving an abandoned or cutoff meander valley (see The Meandering River). An interesting erosion feature observed along stream channels is the undercut bank. In this instance, the stream erodes beneath a resistant rock layer along the outside of a curve and creates a cave (Figures 7 and 8).

FIGURES 7 and 8 Erosion along a small stream has undercut a resistant sandstone layer and created a cave that is over 100 feet long and up to 20 feet deep with a ceiling height up to 6 feet. Located at the Ava Roadside Park on Highway 5 just south of the Bryant Bridge.

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