Channel dynamics in Canyon de Chelly National Monument, Arizona, with emphasis on the effect of invasive plants

This research evaluated relative regional and local-scale influences on historic and contemporary channel change in Canyon de Chelly National Monument, Arizona, U.S., with emphasis on the invasive, exotic plant species, tamarisk (Tamarix spp.) and Russian olive (Elaeagnus angustifolia ). In the last 70 years, the channel has experienced narrowing and incision that is apparently concurrent with widespread exotic plant establishment. To place this research within a broader context of regional trends in channel change, average alluvial erosion rates were statistically compared between the southwestern and southeastern U.S. Causes of historic and recent channel change were evaluated using a field-surveyed channel longitudinal profile, an existing aerial photograph analysis, and historical climate and land use information. Channel response to exotic plant removal by two methods was quantified to determine the more effective method for promoting bank widening. One method involved cutting the above-ground portion of the plant applying an herbicide (cut-stump). The second method involved removing the entire plant (whole-plant). Channel adjustment was quantified over a 3-year field period and a 6-year simulation period using the hydraulic model, CONCEPTS.Systematic differences in erosion rates do not exist between the two regions, suggesting that inferences drawn from channel dynamics in southwestern U.S. may be applicable in other regions despite differences in hydroclimatology, geology, and land use. Channel morphology in Canyon de Chelly reflects ongoing complex response to the combined effect of catchment and local-scale controls. Catchment-scale factors include water and sediment yield fluctuations from climatic and grazing conditions beginning in the 1800s. Local-scale controls include channel bed and bank properties and the presence of in-channel structures. Short-term field results indicate significant increases in channel widening in the whole-plant reaches relative to the control reaches. Minor increases in channel width occurred in the cut-stump removal reaches. The whole-plant method provides the highest potential for channel widening, however, substantial geomorphic channel change will only occur with either the combined effect of severely weakened banks and repeated large flows of longer duration than the typical monsoon (< 24 hours), or a sediment regime shift that increases sediment delivery to the channel.