Abstract

Experiments in a 60-ft-long tilting, recirculating flume were conducted to study river incision in simulated bedrock, which was a mixture of sand and kaolinite. Slope, sediment feed, and water discharge were controlled during the development of four channels. After an increase of slope at constant discharge, the following sequence of erosion occurred: (1) development of longitudinal lineations, ripples, and potholes; (2) enlargement of the lineations into prominent grooves; (3) coalescence of the grooves into a single, narrow, and deep inner channel.

The inner channel was incised below base level and a sequence of bedrock lows and highs formed. Bedrock scour lows had a weakly regular spacing during incision and a randomly clustered spacing following aggradation.

Incision around stabilized alternate bars in a sinuous sand-bed channel resulted in destruction of the bars and maximum scour where the flow was locally constricted. In an initially sinuous bedrock channel, scour depth was greater at bends than at crossings. Provided all of the available sediment load was entrained, the bed was eroded more at convex banks of bends than at concave banks. However, after deposition occurred, the maximum erosion shifted to the concave bank. These results indicate that lateral or vertical incision at bends of incised meandering streams is controlled by the amount of available sediment load entrained by channel-forming discharges. The results also suggest that incised meanders superposed from an earlier pattern on a peneplain should rarely occur in nature, if epeirogenic tilting caused the incision.

Representing the locus of deepest scour by a bedrock stream, inner channels may be the locations of heavy mineral concentrations as well as gravel deposits. The experimental results help to explain inner channels discovered at damsites, provide an explanation for some paleochannels in California and South Africa, and suggest that, like the Dalles type of river channel, bedrock floors of valleys will be uneven in both transverse and longitudinal sections.

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