River responses to decadal-scale changes in discharge regime: the Gila River, SE Arizona

Rapid and large responses of semi-arid fluvial systems to alterations in discharge are well documented. Some of these changes have been sudden and have resulted in ‘metamorphosis’ of the channel. The Gila River in SE Arizona, USA, was identified as behaving in this way in the early twentieth century. Changes of the Gila River channel since 1920 have been investigated, and in particular, the last three relatively wet decades have been compared with the drier ones of the 1920s to 1950s. The research has shown that the morphological response to high flow events depends on sequences of events and critical combinations of conditions. Feedback effects of morphology can induce a ramped response and morphology can be further complicated by interactive effects with vegetation growth. Morphology may therefore not show a simple relationship to size of event or wetness of a period and therefore care should be taken when making palaeohydrological inferences from morphology.