PARENT SESSION
Oral Session #85: Ecosystem Restoration in the Grand Canyon.
Presiding: S. Gloss and B. Gold
Thursday, August 8. 1:00 PM to 3:45 PM. Mohave Meeting Room, TCC.
Grand Canyon debris flows: implications for aquatic habitat structure and availability below Glen Canyon Dam?
MELIS, THEODORE*,1, WEBB, ROBERT2, GRIFFITHS, PETER2, 1 U.S. Geological Survey, Flagstaff, AZ2 U.S. Geological Survey, Tucson, AZ
ABSTRACT- Debris flows occur in 736 tributaries along 444 km of the Colorado River, and in at least 74 tributaries along 22 km of the lower Little Colorado River (LCR) in Grand Canyon. From a restoration and management perspective, the lower LCR is a critical reach because it contains abundant spawning habitat for endangered humpback chub (Gila cypha). Glen Canyon Dam operations have dramatically reduced main-channel temperature, as well as annual peak flows that caused backwatering in the LCR up to several km above the confluence. Unique spawning conditions of the lower LCR result from a combination of perennial discharge from springs, precipitation of travertine, and locally derived gravel winnowed from debris flow deposits. Debris flows along the wider channel of the Colorado River maintain boulder-rich debris fans and cobble bars that comprise main-channel habitat. Numerous main-channel debris fans located up and downstream of the LCR result in low-velocity eddies that are used as habitat by juvenile and adult chub. As main-channel debris flows continue to occur under regulated flows from Glen Canyon Dam, fans increasingly aggrade and impinge on the river, enhancing and expanding eddies. As a result, adult and juvenile habitats in the vicinity of the LCR confluence (located 126 km below the dam), may increase in both quality and abundance. However, similar-magnitude debris flows in the narrower LCR could restrict access of mainstem adults to critical spawning habitat when such events alter the longitudinal profile of that river. Statistical model results for debris-flow probability in a similar number of main-channel and tributary drainage areas are noteworthy. Our model predicts that 49% of tributaries within the lower LCR have a greater than 75% probability of producing at least one debris flow during the next century. Model results for an equal number of Colorado River tributaries supporting eddies with high chub fidelity indicate that 55% have a greater than 75% probability for debris flow in the next century (74 basins from 118 to 156 km below the dam). Is it reasonable to assume that limited flow strategies designed to achieve river restoration in Grand Canyon can mitigate potentially catastrophic recruitment failures in humpback chub caused by naturally occurring debris flows in the lower Little Colorado River?
KEY WORDS: Debris Flows, Glen Canyon Dam, Little Colorado River, Gila cypha