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 Tamarix 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 Tamarix and Russian-olive were similar to other woody, riparian species. However, where other species have roots extending down about 1m, roots of Tamarix 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 Tamarix 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 Tamarix and Russian-olive, respectively. 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. The implications of vegetation removal on bank stability and failure frequency were evaluated in two incised reaches, Upper White House (UWH) and Sliding Rock (SLR) by modeling bank-toe erosion and bank stability with and without vegetation. Banks in the modeled sections are 4.3 and 3.9m 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. Stability analyses including the effects of vegetation, however, are simulated with an additional 1.9 to 3.6 kPa of cohesion in each layer. The effects of a series of 1.0 and 1.5-m flows on bank-toe erosion, pore-pressure distributions and bank stability were evaluated. Flows of this magnitude can be common. For example, three flows of at least 1.0m occurred during July and August 2006. Stability analyses were conducted for the most critical failure conditions at peak flow (equal flow and groundwater elevations) and for the drawdown condition following recession of stage where the heightened groundwater levels were maintained. Results of iterative modeling runs show that the effects of root reinforcement provided by Tamarix and Russian- olive have a significant impact on bank-stability and bank-failure frequency. With vegetation, instability in the UWH and SLR reaches do not occur until recession of the 7th and 5th 1.0m flows, respectively. This is supported by dendro-chronologic evidence of widespread failures during the wet, winter and spring of 2005. In comparison, simulations for the two reaches assuming vegetation removal showed instability following recession of the 4th and 1st 1.0m flows, respectively. A similar pattern emerged for simulations of 1.5m flows where model runs with vegetation showed instability upon recession of the 2nd event for both reaches while instability for the non-vegetated cases occurred simply from elevated groundwater levels and the consequent loss of matric suction during the 1st flow event. Removal of riparian stands of Tamarix and Russian-olive will lead to bank instabilities and accelerated widening in Canyon de Chelly National Monument. The rate that this occurs will be a function of the frequency of moderate flows that have the ability to cause losses of matric suction from lateral infiltration. For this reason, longer-duration flows such as snowmelt hydrographs should be more critical than monsoon-type convective events.