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.", '. < ;: Ecolo~ , Rioarian EcoSYStem The ecology of the Colorado River corridor was transformed by closure of Glen Canyon Darn. Carothers et al. (1979) discussed zonation of Grand Canyon's riparian vegetation. These observations were further clarified by Turner and Karpiscak's (1980) rematching of historic photographs throughout the Colorado River riparian corridor. Johnson (1991) summarized trends in post-darn vegetation change in this system, and Stevens and Ayers (L, E. Stevens and T. 1. Ayers, 1993. The impacts of Glen Canyon Dam on riparian vegetation and soil stability in the Colorado River corridor, Grand Canyon, Arizona, unpubl. report to Grand Canyon National Park) reported on the results of test flows under the GeES Phase II program. Recruitment in the old-high-water-zone (OHWZ) community has declined due to the decreased frequency of large magnitude floods. This zone, the only perennial riparian vegetation community of the unregulated river, was watered by sporadic flood-flows; the assemblage was adapted to drought and rare, high floods. Lower riparian zone vegetation is now dense and is composed of different species than the upper riparian OHWZ community. Saltcedar and sandbar willow have colonized many bars down to a stage associated with a discharge of 300 m3/s. Marsh assemblages, formerly rare to absent in this system, have colonized patches of silt- and clay-rich alluvial sediments near the elevation of mean baseflow in backwater return-current channels. Vegetation encroachment, daily stage fluctuations related to hydroelectric peak power production, and in-filling by slope processes tend to obscure sand bar and backwater topography typical of the unregulated river. Under reduced levels offlow fluctuation and ramping rates of the current Interim Flows, maCfophytic vegetation has rapidly colonized newly stabilized, low-elevation deposits of the Colorado River corridor (L. E. Stevens and T. J. Ayers, 1993, The impacts.ofGlen Canyon Dam on riparian vegetation and soil stability in the Colorado River corridor, Grand Canyon, Arizona, unpubl. report to Grand Canyon National Park). At nearly every site examined, Stevens and Ayers reported that the "new dry" zone at water stages between discharges of 566 and 890 m3/s sustained a surprising amount of primary colonization by emergent herb, macrophytic, and clonal perennial vegetation. However, these researchers noticed die-back of sandbar willow rarnets attributable to desiccation of higher surfaces inundated by stages associated with discharges of about 1100 mJ/s at two study sites. Premature chlorosis of this marsh-indicator species was noted at one other site. High discharges of the Little Colorado River in winter 1993 scoured large areas of low-lying vegetation and aggraded backwater channels. .- .' :..:; r. ;.-; .' ~ ~ '~: " :' \. ~-:. '. ?" :",.~ , "-" -,{,o Aauatic Ecosvstem The change in characteristics of the Colorado River below Glen Canyon Dam has fundamentally altered theaquatic food base. The overriding factors influencing these alterations are dam operations which create clear, cold water, and tutbidity from tnbutaries which result in a gradual decrease in benthic standing biomass below the confluence of the Paria River. The filamentous green alga. Cladophora g/omerata, dominates the benthic community between the dam and Paria River because of the clarity of the water and substrate availability, while Osci//atoria spp., crustose bluegreen algae (cyanobacteria) dominates downstream of the Little Colorado River. Standing biomass and abundance of other benthic organisms is closely ;~! ~ ~ 1 -'"i ~~ ,. 9