| Summary: | The Little Colorado River is the largest tributary of the Colorado River in Grand Canyon and serves as spawning habitat for humpback chub (Gila cypha), an endangered species. The lower Little Colorado River, defined as the reach from Blue Springs to the Colorado River, is a dynamic stream, and its channel morphology and longitudinal profile are controlled by several factors, including debris-flow deposition, travertine deposition, and reworking by streamflow floods. I determined changes in the water-surface profile of the Little Colorado River by comparing data surveyed in 1926 by the U.S. Geological Survey and data extracted from 2002 digital-elevation models. Specific changes to the longitudinal profile can be attributed to travertine-dam formation, which appears to occur more quickly than previously assumed; debris-flow deposition; and boulder transport during occasional large mainstem floods. A debris-flow sediment-yield model was used to determine the worst case scenario of boulder inputs from tributaries blocking spawning runs. A one-dimensional steady state flow model shows the effects of changes in channel conveyance on flow velocities in the lower Little Colorado River. Velocity changes associated with debris-flow deposition could potentially affect the ability of humpback chub to move upstream and spawn in this river.
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