Implications for the Removal of Invasive Species in Canyon de Chelly National Monument

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 evaluated. Root tensile strengths and distributions in streambanks were measured and used in combination in a root-reinforcement model to estimate the additional cohesion provided to layers of each streambank. Relations between tensile strength and diameter for Tamarisk and Russian-olive were similar to other woody, riparian species. However, where the roots of other riparian species have roots concentrated in the top 1m of the soil profile, roots of Tamarisk and Russian-olive extend throughout the bank profile (to 4.3 m). The additional cohesion provided by the roots in each 0.1m 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. That is, a cohesionless bank acquires the properties of a weakly cohesive bank due to the presence of roots, providing an average 2.8 kPa of cohesion. 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. Banks in the modeled sections are 4.3 and 3.9 m high, respectively and are dominated by sandy materials except near the toe where fine-grained deposits provide between 2.2 and 4.0 kPa of cohesion. 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. Results of iterative modeling runs show that the effects of root reinforcement provided by Tamarisk and Russian-olive have a significant impact on bank-stability and bank-failure frequency. Removal of riparian stands of Tamarisk and Russian-olive will lead to bank instabilities and accelerated widening in Canyon de Chelly National Monument.