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Floodplain Plant Community and Stream Channel Response More than Ten Years Following Tamarisk and Russian Olive Removal in Canyon De Chelly National Monument, Arizona; Lindsay Reynolds

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Floodplain Plant Community and Stream Channel Response More than Ten Years Following Tamarisk and Russian Olive Removal in Canyon De Chelly National Monument, Arizona
Lindsay Reynolds1*, Kristin Jaeger2, Keith Lyons3, Celeste Weiting7, Julianne Scamardo8, David Cooper4, Ellen Wohl5, and Sara Rathburn6
1Colorado State University, Fort Collins, CO, USA;;
2United States Geological Survey, Tacoma, WA, USA;
3National Park Service, Canyon de Chelly National Monument, Chinle, Arizona, USA;
4Colorado State University, Fort Collins, CO, USA;
5Colorado State University, Fort Collins, CO, USA;
6Colorado State University, Fort Collins, CO, USA;
7Colorado State University, Fort Collins, CO, USA;
8Colorado State University, Fort Collins, CO, USA;
Riparian areas throughout Canyon de Chelly National Monument (CACH) in northeastern Arizona were invaded by the exotic tree species tamarisk (Tamarix ramosissima, T. chinensis, and hybrids) and Russian olive (Elaeagnus angustifolia) starting in the early 20th century. By 2005 tamarisk and Russian olive dominated the floodplains. Prior to exotic tree invasion, streams in Canyon de Chelly were wide, shallow and braided, with native cottonwood and willows along the margins. Some reaches in the lower, mainstem Chinle Wash, still remain wide and shallow, however, the two primary tributary canyon streams have narrowed and incised 1-5 m over the last 50 years. Both the invasion of exotic trees and the incision of stream channels throughout CACH have created challenges for land managers and Navajo residents. Lower water tables and the lack of regular overbank flooding associated with stream down-cutting have dramatically altered the landscape, hampered farming and traditional grazing, and threatened road-stream crossings throughout the canyon. In an effort to restore native vegetation and an active floodplain in the canyons, the National Park Service, in collaboration with the Navajo Nation, began clearing tamarisk and Russian olive throughout the canyon system in 2005. Between 2005 and 2008, we monitored four intensive study sites in CACH for stream channel characteristics and floodplain vegetation prior to and following exotic plant removal. We compared cut-stump (above-ground stem removal only) and whole-plant (above-ground stem and below-ground root) removal of tamarisk and Russian olive trees to control sites that were not treated. We found that channel change, largely through channel widening, was greatest in the whole-plant removal treatments, but that the cut-stump treatments were more beneficial for promoting native plant communities. However, both the floodplain vegetation and channel morphologic changes were limited by the existing degree of channel incision, the bed and bank material, and subsequent entrenchment. Since 2008, most large floods have been insufficient to inundate historic floodplain surfaces in the intensive study sites, potentially limiting morphologic change through bank erosion, channel widening and reestablishment of native riparian plants. In the summer of 2019, we focused new sampling efforts on quantifying floodplain vegetation composition and channel entrenchment in exotic removal sites. We resampled four, 1 km reaches of floodplain plant communities and associated stream channel in Canyon de Chelly after more than 10 years since exotic tree removal. Preliminary results show that floodplain and terrace vegetation continues to be dominated by exotic and weedy species, but native grasses and herbs persist in large numbers and are doing especially well inside grazing exclosures and native seeding areas. Channel widening is occurring in plant removal sites that have coarse bank materials and less initial incision, leading to the formation of new inset point bars and small inset floodplains. However, channel incision has continued in sites with deeper initial entrenchment and clay layers armoring the banks. Initial findings show that bank material, specifically the presence of clay, may exert a local control on on-going channel incision and widening processes.

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