In Chapter 5, the effect of tectonics on sediment yields and river systems was considered. Here, the focus is on local effects of deformation of the valley floor (Figure 1.2). This can include not only active tectonics, but subsidence owing to groundwater and petroleum removal, and local uplift as a result of valley incision (Bell, 1999). These examples can be considered as pseudotectonics, but they can have the same effects as active deformation.

Active tectonics can take several forms. Deformation can be along faults or pairs of faults (horst and graben), which should have the same effect on rivers as a monocline, dome, or basin. In addition, an entire valley may be tilted upstream, downstream, or laterally. The possibilities are great, but, in reality, the primary effect of tectonics will be a local steepening or reduction of gradient or lateral (cross-valley) tilting. In order to adjust to these changes, the river must either degrade, aggrade, or change sinuosity. Small streams in the Mississippi River valley show clearly the impact of active tectonics on their morphology (Boyd and Schumm, 1995; Spitz and Schumm, 1997), and indeed, even large rivers, such as the Mississippi, Indus, and the Nile, change patterns in response to active uplift and faulting (Schumm and Galay, 1994; Schumm and Winkley, 1994). In addition to these primary influences, there will be secondary effects, as rivers respond to changed gradient (aggradation or degradation), and there will be tertiary effects, as decreased or increased sediment loads influence reaches downstream of the deformed reach and as aggradation or degradation in the deformed reach progress upstream.