Drylands are regions encompassing hyper-arid, arid, semi-arid or sub-humid climatic conditions (see also, Chapter 1). They include cold and warm subtropical deserts, savannas, and the Mediterranean environments. Our focus is here on warm drylands, which are generally characterized by the existence of a well-defined dry season dominated by subtropical high pressure (Malanson, 1993), and a rainy season with average precipitation of less than 700 mm/year. Such regions cover approximately 50% of the continents, with about 20% of the world's population living in these areas (Le Houerou, 1982; Nanson et al., 2002). This explains the growing scientific interest in the study of drylands. Here we focus on the interactions between fluvial geomorphology and riparian vegetation. These interactions act at different spatial and temporal scales, suggesting the existence of an intrinsic and remarkable sensitivity of riparian ecosystems to hydrological and geomorphological modifications. Riparian ecosystems have often been affected by heavy anthropogenic disturbances, with great reductions in spatial extent (up to 80%, as in certain U.S.A. sites) with respect to presettlement times (Smith et al., 1991; Tooth, 2000a,b; Salinas et al., 2000; O'Connor, 2001; Pettit et al., 2001).