Examples of valley floor variability (Figure 1.2) and its effects on channels have been given under the discussion of history, tributaries, and tectonics. Convexities and concavities of the valley floor impact a channel, as it incises or deposits, to form a linear longitudinal profile. Many examples show the effect on channel morphology with pattern being the most obvious. The Mississippi, Nile, and Indus rivers provide good examples of this (Figures 5.8, 11.7, 11.12) as discussed in earlier chapters.

However, a different type of channel control exists in semiarid and arid regions. Studies in Wyoming and New Mexico (Schumm and Hadley, 1957) have demonstrated that discontinuous gullies develop in small drainage basins on convexities of the longitudinal profile (Figures 12.1, 12.2). The convexities develop at tributary junctions where the influx of sediment cannot totally be removed and at locations in these dryland valleys where the loss of flow by infiltration causes deposition in long reaches between tributaries (Figure 12.3).

Discontinuous gullies form on the steeper downstream part of the convexity. Depending on sediment delivery to the site, the gullies may be buried or they can extend upstream linking other discontinuous gullies to form a major arroyo (Figure 12.2). Love (1983) shows how different the valleys were in the nineteenth century and at present (Figure 12.4).