Assessment and Characterization of Ephemeral Stream Channel Stability and Mechanisms Driving Erosion In The Grand Valley, Colorado, 2018-21

The purpose of this study is to provide information regarding the stability of ephemeral streams on the north side of the Grand Valley, Colorado. The ungagged ephemeral streams in this semiarid region are of particular interest because (1) the underlying bedrock geology is a sedimentary rock deposit that has been identified as a major contributor of salinity to the Colorado River and (2) despite infrequent flows of short duration, monsoon derived floods in ephemeral streams can carry substantial amounts of sediment downstream. These points of interest are important because salinity in the Colorado River has increased and because salt loading in these environments is closely associated with sediment loading. In an effort to minimize sediment contributions to the Colorado River from public lands administered by the Bureau of Land Management (BLM), a comprehensive sediment control approach is used to reduce sediment transport through cost-effective land management techniques and practices.

In 2018, the U.S. Geological Survey (USGS), in cooperation with the BLM, began an assessment of ephemeral streams to characterize stream channel stability and identify mechanisms driving erosion. In doing so, the USGS developed a method for automatically extracting channel geometries from existing remotely sensed terrain models and based on estimated flood stage stream discharges, hydraulic characteristics were calculated. Furthermore, utilizing a statistical model, the channel geometries and hydraulic characteristics were used to estimate channel stability for the ephemeral streams.

Based on a stream channel stability assessment, cross-section stabilities were known for a subset of 1,406 visited locations but desired for 13,415 cross sections which were delineated from remotely sensed terrain models. The application of Manning’s resistance equation in combination with multiple Logistic Regression models demonstrated that channel stability can be estimated with an 0.85 goodness of fit for a validation dataset when using a combination of drainage area, width to depth ratio, sinuosity, and shear stress as the explanatory variables. Using the multiple Logistic Regression model and defined explanatory variables, stream channel stability was extrapolated for the remaining unvisited cross sections. The mapping and categorizing of ephemeral streams in this arid rangeland can be used by BLM to evaluate and prioritize areas to target for remediation or changes in management strategies to reduce sediment loading to the Colorado River.

The study found channel stability within the ephemeral streams to be spatially variable, longitudinally discontinuous, and dictated by changes in channel bed slope. The stable ephemeral streams were relatively wide, shallow, and often had smaller drainage areas with less potential for producing shear stresses suitable to overcome channel adhesion. A change in channel bed slope provided the means necessary to generate shear stresses appropriate to initiate erosion and a subsequent stability transition to incising channels. Channel widening arises when either bank heights of incising channels reach a critical height for mass failure, or when channel curvatures cause higher sidewall stresses. Regardless, widening channels promote sinuosities and subsequently bring about shallower channel bed slopes.