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\J[)2J95 13 BIOLOGICAL RESOURCES The Glen Canyon Dam Environmental Impact Statement (GCD-EIS) recommended that high flows be utilized as a means of restoring the Colorado River ecosystem and its associate resources. This recommendation was tested during the 45,000 cfs 1996 Test Flow. This report documents the state of the aquatic food base, fisheries, and wetland and riparian habitats and assemblages in Glen and Grand canyons. The information presented here is derived from the research and monitoring activities conducted in relation to the BHBF, and presented at the April 1997 Flood Symposium held in Flagstaff, Arizona. At this time, limited monitoring data are available from the river corridor for the first part of 1997. < Aquatic Food Base Flow rate, turbidiry, substratum distribution. solar insolation, and water temperature are the primary factors influencing the development of the aquatic food base in the Colorado River ecosystem. The structure of the aquatic food base in the Colorado River ecosystem was altered by the closure of Glen Canyon Dam in 1963, and the corresponding changes in river discharge (i.e., virtual elimination of extreme seasonal fluctuations), organic budget (i.e., greatly diminished allochthonous inputs), suspended sediments (i.e., virtual elimination from the,G1en Canyon reach), water temperature (i.e., cold and constant vs. warm and fluctuating) and, reduced nutrient concentrations. In response to these changes, an aquatic foodbase developed in the tailwaters of Glen Canyon Dam that is based on the filamentous green alga, Cladophora glomerata and associated epiphytic diatoms, as well as chironomid midges and the freshwater amphipod, Gammarus lacustris. These biotic communities, while present, are greatly modified and reduced in the variably turbid and turbid waters below the confluences of the Paria and Little Colorado Rivers. The 1996 Test Flow strongly scoured the benthic foodbase in the mainstream; however, recovery was extremely rapid (one month at some sites; Shannon et al. unpublished). Cladophora glomerata, other aquatic macrophytes (e.g., Chara and PotamOl!ton) have become established in areas of fine sediment deposition since 1995, possibly as a result oflow nutrient concentrations from Lake Powell. The 1997 discharge regime of high steady flows has resulted in colonization of the varial zone and increased benthic standing biomass throughout the river corridor (Figure 4). In addition, the typical pattern of decreasing benthic standing biomass with distance from Glen Canyon Dam (normally resulting from increasing turbidity over distance downstream) has been replaced by more variability within sites across seasons, and among sites. High steady flows in 1996-1997 probably enhanced benthic growth by reducing turbidity and hydraulic variability, and benthic biomass reached its highest level since monitoring began in 1990 (Shannon et al. 1997, unpublished). Higher. steady mainstream flows may dilute the tributary-derived suspended sediment load, and may provide a more consistent habitat and light regime, enhancing benthic productivity. Whatever the mechanism. Final Draft - 12/12/97 - For AMWG Review