Destabilization of streambanks by removal of invasive species in Canyon de Chelly National Monument, Arizona

As part of a study to investigate the causes of channel narrowing and incision in Canyon de Chelly National Monument, the effects of Tamarisk and Russian-olive on streambank stability were investigated. In this study, root tensile strengths and distributions in streambanks were measured and used in combination with a root-reinforcement model, RipRoot, to estimate the additional cohesion provided to layers of each streambank. The additional cohesion provided by the roots in each 0.1-m layer ranged from 0 to 6.9 kPa for Tamarisk and from 0 to 14.2 kPa for Russian-olive. Average root-reinforcement values over the entire bank profile were 2.5 and 3.2 kPa for Tamarisk and Russian-olive, respectively.The implications of vegetation removal on bank stability and failure frequency were evaluated in two incised reaches by modeling bank-toe erosion and bank stability with and without vegetation. The effects of a series of 1.0- and 1.5-m-deep flows on bank-toe erosion, pore-pressure distributions, and bank stability were evaluated first. In addition, bank stability model runs were conducted using iterative modeling of toe erosion and bank stability using a discretized flow and groundwater record for one year. Results showed that the effects of root-reinforcement provided by Tamarisk and Russian-olive have a significant impact on bank-stability and bank-failure frequency. Because the bank materials are dominated by sands, cohesion provided by roots is significant to bank stability, providing an average 2.8 kPa of cohesion to otherwise cohesionless bank materials. Bank retreat rates at one site following vegetation removal have approximately doubled when compared to the control reach (from an approximate rate of 0.7–0.8 m/y between 2003 and 2006 to 1.85 m/y during the year modeled). Vegetation removal along the entire riparian corridor in Canyon de Chelly may lead to the introduction of significantly more sediment to the system through bank widening processes, although it is not known whether this change alone would be sufficient to cause a shift in channel morphology to the wide-braided channels that were once characteristic of this canyon.