QGG J. HOOVER MACKIN AWARD: CHANNEL MORPHOLOGIC CHANGE ASSOCIATED WITH INVASIVE VEGETATION REMOVAL

Streams in the southwestern US (SW) show dramatic channel narrowing due to the spread of invasive vegetation such as tamarisk, Russian olive, and giant cane. Invasive vegetation is removed to benefit native vegetation, local wildlife, and restore channel morphology. Removal of woody plants promotes erosion by reducing root reinforcement of banks and hydraulic roughness, increasing flow velocity and shear stress. Where vegetation removal is followed by flooding, large increases in channel area and migration may result. River restoration removing invasive vegetation is now common practice, yet post-removal monitoring of channel change is lacking. We developed an inventory of channel morphologic response following invasive vegetation removal and link response to discharge magnitude and frequency. We hypothesize that removing invasive vegetation increases channel erosion such that flows above a threshold recurrence interval after removal increases channel cross-sectional area and migration. Also, removal method influences channel response such that whole-plant removal (WP) has a larger effect than cut-stump (CS) and other less invasive techniques. Data were compiled from the literature on SW rivers including vegetation type, method of vegetation removal, time since removal, number of high flows, and channel morphologic changes. Approximately 40 sites have been inventoried, and we used remote sensing and field measurements to analyze channel morphologic changes following removal at select sites. Along Chinle Creek in Canyon de Chelly National Monument (CACH), the WP and CS methods were used to control invasive vegetation in 2005-2006, and post-removal monitoring ended in 2008. Additional field data collected in 2019 indicate statistically significant increases in channel width in WP reaches compared to CS and control reaches. Remote sensing analyses of CACH show that WP reaches experienced greater variability in channel width and sinuosity compared to CS reaches, but preliminary analyses from other inventoried locations show minimal channel response to vegetation removal methods. Quantifying hydrologic thresholds that control post-removal channel response will help identify riparian areas sensitive to change, and results from this study will provide critical data on channel response.