Figure 5.1. Changing slope at fan-head leading to fan-head trenching (Schumm 1977).
Figure 5.2. Headcuts and nickpoints.
The result of headcut or nickpoint formation and migration is, of course, lowering of the stream
bed. Erosion of the bed material will be dramatic as a headcut or nickpoint migrates under a
bridge. During a major flood in Tujunga Wash, California, erosion above the headwall of a
gravel pit led to the failure of three highway bridges. In cohesive material or erodible rock,
headcut migration normally lowers the channel abruptly to its new position. In alluvial material,
however a nickpoint produces degradation that persists for some distance (Figure 5.2b). In
both cases, scour continues until the gradient has been reduced and bank erosion has
widened the channel to the point that deposition can begin. As the nickpoint or headcut
migrates farther upstream, the quantity of sediment delivered to the reach at which a stream
crossing is located increases greatly due to the erosion of the bed upstream and subsequent
erosion of the banks of the stream. Therefore, a period of degradation may be followed at a
site by a period of aggradation.
5.4
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