Big rivers monitoring within Dinosaur National Monument: A summary of monitoring results to detect change in channel condition

Scott, Michael; Perkins, Dustin; Merritt, David; Scott, Julian; Cooper, David

The Northern Colorado Plateau Network (NCPN) big rivers monitoring program is designed to detect long-term, directional change in riverine and riparian ecosystems resulting from human activities and/or broad-scale and persistent climate shifts. Early detection of such changes can help land and water managers to better anticipate, manage, and perhaps mitigate the effects of future changes in water supplies. In this report, we use relevant new research, along with hydrologic and vegetation analyses and geospatial mapping, to refine the NCPN big rivers monitoring program and detect channel narrowing or widening along the Green and Yampa rivers in Dinosaur National Monument. Channel narrowing is a widely documented response of rivers to natural or human-caused changes in the flow regime. Using flow data from the seven largest rivers in the Upper Colorado River Basin, we show how dam construction and flow management have reduced flow variability on all of these large rivers except the Yampa and White rivers, making these rivers relatively unique natural resources on the Colorado Plateau. Based on previous geomorphic research, we stratify our monitoring sites by channel type. Monitoring at sentinel sites involves integrated measurements of total and species-specific plant cover; quantitative assessments of erosion and deposition based on digital elevation models; mapping and textural analysis of distinct alluvial deposits (facies); and repeat photography. Using a large network of vegetation quadrats, we identify a concise list of plant species that serve as indicators of ecological conditions across the range of fluvial geomorphic conditions in the monument. Finally, we present a conceptual model showing how largely unvegetated active-channel features along a relatively unregulated river like the Yampa are converted to vegetated floodplain during channel narrowing. This model is the basis of our monitoring design, which involves intensive monitoring at sentinel sites and rapid monitoring assessments, or rapidos, across a broad spatial range of geomorphic features with known sensitivities to narrowing. Results from two sentinel sites over the first few years of monitoring demonstrated measurable, site-specific responses to annual variation in streamflow. Erosion and deposition occurred across the two sentinel sites we examined, with large net erosion occurring on the unregulated Yampa River between 2011 and 2014, compared with small net deposition on the partially-regulated Green River over the same period. Vegetation showed similar patterns of cover variation across active floodplain surfaces at both sentinel sites. In contrast, there was site-specific cover variation on active-channel surfaces between sites. Additional years of measurement will be needed to establish cause-and-effect relationships at sentinel sites. Rapid-assessment techniques detected clear differences between geomorphic features on the Green and Yampa rivers. However, refinements need to be made in order to detect year-to-year differences at the same sites. The establishment of a baseline of measurements over a number of years will provide the basis against which to evaluate any future trends in channel form.

Big rivers monitoring within Dinosaur National Monument: A summary of monitoring results to detect change in channel condition

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