River regulation is hypothesized to increase the dominance of non-native shrub species through changes in disturbance regimes.  We investigated the establishment patterns of Tamarix, a prolific invader of riparian areas, in the highly regulated system of the Colorado River through Grand Canyon National Park and considered how multi-scale hydrologic, geomorphic, and climatic processes have influenced Tamarix establishment in the past.  Dendroecological techniques were used to determine accurate ages of Tamarix individuals sampled from a 363-km stretch of riparian habitat, and geomorphic characteristics were documented at each site.  We created multiple logistic regression models to account for variation in Tamarix presence caused by hydrologic, geomorphic, and climatic variables. 

Results/Conclusions

The best-fit models contained the variables: flow regime type (e.g., pre-dam, fluctuating flows, low-fluctuating flows, post-dam floods), flow stage (positive effect of lower flood stages), peak annual flow (positive effect), and July temperature (negative effect).  More Tamarix establishment occurred in the post-dam flood flow regime than the pre-dam flow regime.  However, this could be an artifact of mortality caused by the 1983 high flows.  Post-dam floods in the spring of 1983 were associated with high rates of Tamarix establishment, while a period of constant establishment has occurred from 1987 until present.  Reduced Tamarix establishment is associated with years in which managed, test floods were conducted.  Contrary to previous studies, peak annual flow was not the most important factor for predicting Tamarix establishment.  Variables representing hydrograph recession and duration of inundation were more influential explanatory variables.  The 1983 flood was reminiscent of the pre-dam, unregulated hydrographs that were dominated by spring floods.  The high rates of Tamarix establishment associated with this semi-natural hydrograph calls into question the validity of using natural flows for river restoration.  It remains an open question whether flow prescriptions that mimic natural flow regimes are appropriate for managing riparian landscapes that are heavily invaded by non-native species.