Interactions among river flow, geomorphic processes, and riparian vegetation: Field studies and computational modeling of a sand-bed desert river

We are combining field studies and computational modeling to investigate interactions among flow, geomorphic processes, and riparian vegetation in an effort to improve both process understanding and the quantitative basis of ecosystem flow prescriptions. On the Bill Williams River, Arizona, we are examining the response of native and non-native riparian seedlings and channel morphology to prescribed dam-released floods to maintain a Populus-Salix riparian woodland system. Field studies documented seedling establishment patterns following a 2005 flood release and subsequent density reductions of the 2005 cohort following a 2006 flood release. Density reductions were greater among non-native Tamarix seedlings than native Salix seedlings in two study reaches, likely as a result of the substantially greater first-year height and diameter growth of Salix relative to Tamarix.  Mechanisms of seedling mortality differed as a function of reach-scale channel morphology and potentially basin-scale sediment supply. At the site closest to the dam, where dam-induced reductions in sediment supply may affect channel processes, the 2006 flood scoured seedlings and their substrates from mid-channel bars, whereas in a reach further downstream, Tamarix seedlings were buried as a result of aggradation. These results illustrate that in a dynamic, sand-bed river such as the Bill Williams, even relatively small floods generate sufficient hydraulic forces and geomorphic changes to substantially influence seedling establishment and mortality.  Computational modeling with the US Geological Survey’s Multidimensional Surface Water Modeling System was also used to assess spatially distributed velocity, shear stress, and sediment mobility associated with floods in the Bill Williams River and to quantify relationships between local-scale hydraulic forces and seedling response. Combining field studies with modeling of flood hydraulics and geomorphic processes provides a powerful means of quantifying the potential effects of high flows on riparian vegetation and therefore specific recommendations for prescribed flows to achieve restoration goals.