Feedbacks between Riparian Vegetation and Morphodynamics in Sand-Bed Rivers: Insights from Field Studies in the Western United States and Models

Feedbacks among river flow, geomorphic processes, and pioneer riparian vegetation are fundamental to the evolution of sand-bed rivers, and understanding of such feedbacks is essential to management and restoration. This talk will report on investigations of ecogeomorphic feedbacks that combine field studies, hydraulic modeling, and laboratory simulations. Field studies have examined the response of woody riparian seedlings and channel morphology to prescribed dam-released floods that have been designed in part to maintain a native riparian woodland system on the Bill Williams River, Arizona, USA. Through monitoring of floods over a 7-year period, we have observed temporal and spatial variations in channel response. Floods have produced geomorphic and vegetation responses that varied with distance downstream of a dam, with scour and associated seedling mortality closer to the dam and aggradation and burial-induced mortality in a downstream reach with greater sediment supply. We also have observed that as vegetation grows beyond the seedling stage, its stabilizing effect on bars and its drag effect on flow progressively increases, such that floods of similar sizes but at different times may produce markedly different downstream responses as a function of vegetation characteristics. We also observed greater mortality among nonnative Tamarix spp. (tamarisk) seedlings than among native Salix gooddingii (Goodding’s willow) seedlings. Combining field observations with laboratory and computational modeling is being used to draw linkages between hydraulics, channel change, and plant response at the patch and bar scale. These studies are also being used to inform management of flow releases from dams, non-native species, and riparian ecosystem restoration.