Assessment of Coarse Sediment Mobility in the Black Canyon of the Gunnison River,Colorado |
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Authors: | Ian M Dubinski Ellen Wohl |
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Institution: | (1) Department of Geosciences, Colorado State University, Ft. Collins, CO 80523-1482, USA |
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Abstract: | The Gunnison River in the Black Canyon of the Gunnison National Park (BCNP) near Montrose, Colorado is a mixed gravel and
bedrock river with ephemeral side tributaries. Flow rates are controlled immediately upstream by a diversion tunnel and three
reservoirs. The management of the hydraulic control structures has decreased low-frequency, high-stage flows, which are the
dominant geomorphic force in bedrock channel systems. We developed a simple model to estimate the extent of sediment mobilization
at a given flow in the BCNP and to evaluate changes in the extent and frequency of sediment mobilization for flow regimes
before and after flow regulation in 1966. Our methodology provides a screening process for identifying and prioritizing areas
in terms of sediment mobility criteria when more precise systematic field data are unavailable. The model uses the ratio between
reach-averaged bed shear stress and critical shear stress to estimate when a particular grain size is mobilized for a given
reach. We used aerial photography from 1992, digital elevation models, and field surveys to identify individual reaches and
estimate reach-averaged hydraulic geometry. Pebble counts of talus and debris fan deposits were used to estimate regional
colluvial grain-size distributions. Our results show that the frequency of flows mobilizing river bank sediment along a majority
of the Gunnison River in the BCNP has significantly declined since 1966. The model results correspond well to those obtained
from more detailed, site-specific field studies carried out by other investigators. Decreases in the frequency of significant
sediment-mobilizing flows were more pronounced for regions within the BCNP where the channel gradient is lower. Implications
of these results for management include increased risk of encroachment of vegetation on the active channel and long-term channel
narrowing by colluvial deposits. It must be recognized that our methodology represents a screening of regional differences
in sediment mobility. More precise estimates of hydraulic and sediment parameters would likely be required for dictating quantitative
management objectives within the context of sediment mobility and sensitivity to changes in the flow regime. |
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Keywords: | Flow regulation Channel change Sediment mobility Colorado |
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