Spatial variation in channel morphology and sediment dynamics: Gila River, Safford Valley, Arizona

This paper aims to identify and explain the extent and pattern of spatial variability in channel and floodplain morphology and sediment dynamics of a semiarid river which is known to be highly dynamic. The study reach is a 100 km length of the Gila River in Safford Valley, southeast Arizona, and was mapped from large-scale aerial photographs of one date. Three major zones were identified in the valley floor: channel, active and phreatophyte zones. The width of the zones was measured at cross sections 1 km apart down the valley. The active zone is an important indicator of the extent of fluvial activity and is found initially to increase in width downstream from the canyon exit at the upstream end of the valley, then to vary in dimensions, but with a significant increase in width in the downstream quarter of the valley. Although the whole length of channel has been affected by the same major hydrological events, morphologically distinct reaches are identifiable. Various factors are hypothesized and examined as having a possible influence on this morphological variability through effects on sediment dynamics, including gradient and sediment supply; aspects of land and channel management are also assessed. The increase in active zone width at the lower end may reflect earlier clearances of phreatophytes, though it could also be due to more recent interaction of vegetation growth with groundwater and moisture levels.