Tuesday, June 5, 2007 - 8:00 AM
93

Allochthonous organic matter from tributary floods dominates organic matter inputs of the Colorado River in Grand Canyon

Emma J. Rosi-Marshall1, Theodore A. Kennedy2, Robert O. Hall Jr.3, Wyatt F. Cross3, and Barbara Ralston2. (1) Biology, Loyola University Chicago, Chicago, IL 60626, (2) Grand Canyon Monitoring and Research Center, USGS, 2255 N. Gemini Dr., Flagstaff, AZ 86001, (3) Department of Zoology and Physiology, University of Wyoming, Laramie, WY 82071

Prior to Glen Canyon Dam, the Colorado River in Grand Canyon was sediment laden and the organic matter budget was dominated by allochthonous carbon from upstream reaches.  Glen Canyon Dam reduced sediment transport by 93% and probably reduced organic matter inputs by a similar amount, thereby disconnecting the Grand Canyon reach from the upstream watershed.  To understand the sources and quantity of basal resources we constructed a budget of organic matter inputs (AFDM) for the 385-km reach of the Colorado River in Grand Canyon.  Annual allochthonous inputs from Lake Powell are 5x1010 gDOM and 5x109 gPOM.  The 25-km tailwater reach has high algal production (>500 g/m2/yr), but this drops dramatically downstream of the Paria River (0-50g/m2/yr), due to inputs of suspended sediment that reduce water clarity.  Annual allochthonous inputs from the riparian zone are 1.5x109 g, or 50g/m2 of river surface.  In contrast, particulate allochthonous inputs from a two-day Paria River flood contributed at least 3.3x1010 g, representing 900g/m2 of river surface, and annual inputs from all tributaries in the Grand Canyon reach likely exceed 5x1011 g, or 13,000g/m2.  Although tributary allochthonous inputs are substantial, it is unclear how much of this material is retained or contributes to secondary production.

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