Tectonics is an important determinant of river type (Figure 1.2). It generates the relief that drives the erosional machine; it generates earthquakes, which provide vast amounts of sediment to a river; and it causes avulsion and gradient changes. All of the above influence river type and behavior. Significant effects of tectonics are visible in the Amazon basin and on the Hungarian Plain (Schumm et al., 2000), and as an upstream control its downstream impacts can be significant. The effects of local, active tectonics, which impact limited reaches of a river, are discussed in Chapter 11.

High relief mountains, that are tectonically active and that were or are glaciated produce large quantities of coarse sediment, which in turn, produce wide, high width–depth ratio, braided bedload rivers. In contrast, low relief, stable mountains, especially in humid regions, produce low width–depth ratio, sinuous or straight channels.

As is generally the case, the effect of relief and slope on sediment yields and hence upon the type of river found downstream is modified by other variables, such as lithology and climate (Figure 1.2). Ahnert (1970) collected data for 20 European and US river basins with a range of relief from 89 m to 2869 m. He concluded that there is a direct correlation between relief and rate of erosion and sediment yield. In contrast, for small drainage basins in western, semiarid US the relation is exponential (Figure 5.1).