Alterations to river flow can impact natural and cultural resources within river valleys and riparian zones. The USGS Grand Canyon Monitoring and Research Center and the River Ecosystems Science Branch of the Southwest Biological Science Center are actively working with the National Park Service, Bureau of Reclamation, and local Native American Tribes to monitor these impacts on two iconic dryland river systems, the Colorado River and Rio Grande. Within Grand Canyon National Park, Arizona, regulation of the Colorado River by the Glen Canyon Dam since 1963 has changed the river’s flow regime, reduced sediment inputs, and altered plant communities. One of the effects of the long-term sediment reduction and vegetation change has been a decline in the condition of archaeological sites within the river corridor. Researchers are working to better understand the connection between river sediment and adjacent landscape change at those sites. Using geomorphic change detection between high-resolution, ground-based light detection and ranging (lidar) surveys, they assess how dam operations and experimental vegetation removal affect physical changes in archaeological site condition through reworking of river sediment by wind. Regular monitoring of sites is showing that dam operations, including annual controlled floods that rebuild sandbars, can improve river corridor habitat and help maintain a protective cover of windblown river sand over sensitive archaeological resources.
Another long-term effect of the upstream Glen Canyon Dam has been increased density and cover of native and non-native riparian plants along the Colorado River in the Grand Canyon. Researchers are providing new details on spatiotemporal patterns of these changes by combining 20-centimeter resolution multispectral imagery and 1-meter resolution digital topography to develop a vegetative classification for 26 common species within the riparian zone throughout the entire national park. Change detection of imagery classifications between 2002 and 2013 shows how vegetation encroachment has led to large changes in plant community composition in the riparian zone of the river.
The Rio Grande in Big Bend National Park, Texas, has similarly faced changes in riparian vegetation communities and sediment resources following decades of upstream flow regulation, including an increase in invasive giant cane (Arundo donax) populations. The image classification methods are being used with 20-centimeter resolution multispectral imagery and airborne lidar collected in 2012 to investigate the extent of the giant cane invasion and effectiveness of management treatments to remove cane over the past decade. These classifications are providing the foundation for evaluating both past and future vegetation changes by incorporating compatible data with the National Agriculture Imagery Program for change detection analyses to track the effects of management.
Detailed vegetation classification derived from high-resolution aerial imagery and topography data acquired in the Colorado River in Grand Canyon National Park. Photographs by Laura Durning representing seven of the 26 species included in the analysis of vegetation within the historical Colorado River corridor (river kilometers from Lees Ferry, 77.6).