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. . communities may have changed considerably as a result of the volcanic activity and impoundment, with temporary loss of some species and formation of refugia in tributaries. Quaternary c7imate changes. Although Pleistocene glaciation occurred only on mountaintops on the Colorado Plateau, climatic changes associated with glacial advances significantly altered plant and animal communities (Wright and Frey 1965). During the height of the Wisconsin glacial interval, ca. 20,000 years ago camel, horse, mammoth, mastodon, Harrington's mountain goat, mountain sheep, elk, deer, and tapir undoubtedly occupied riparian habitats, and the giant Teratornis merriami sailed the skies. Groundwater isotope analyses from the central San Juan Basin suggest that late Wisconsin temperatures were 50 to 70 C cooler, with drier summers but wetter winters than at present (Phillips et al. 1986). Ice wedge features in montane prairie soil profiles suggest that permanent snowfields may have existed as low as 2,500 m elevation. Radiometric dating and identification of plant remains has been performed on Pleistocene and Holocene packrat middens and Shasta ground sloth dung from Rampart Cave in the lower Grand Canyon (Martin and Klein 1984), Harrington mountain goat remains from the Grand Canyon (Mead 1983) and southeastern Utah (Mead et al. 1987), and mammoth dung from Bechan Cave in Glen Canyon National Recreation Area (Agenbroad 1988). These studies demonstrated that during the full glacial interval, coniferous and other high-elevation plant species occurred more than 1,000 m downslope of their present elevations. Engelmann spruce, limber pine, and Douglas fir occupied the floor of White Canyon in Natural Bridges National Monument during the late Pleistocene, a canyon now dominated by riparian vegetation (Mead et al. 1987). Mammoth dung from Bechan Cave on Lake Powell revealed that 12,000 to 13,000 years ago a riparian gallery forest of water birch, elderberry, wolf-berry, and blue spruce, with Carex and grasses as understory, occupied the canyon floor, and bi~ sagebrush, oak, and dwarf juniper probably dominated the surroundlng terrain (Mead et al. 1986). Today, the canyon floor is occupied by blackbrush and rabbitbrush, with a water birch-elderberry-spruce community occurring 1,120 to 1,520 m higher in elevation. Pinyon pine and juniper occurred on the floor of the Grand Canyon 15,000 years ago in what is now Sonoran Desertscrub (Phillips 1984). Faunal communities during the full glacial also consisted of a mixture of species different than those existing today (Van Devender et al. 1977). At 1,000 m in the lower Grand Canyon, Sauroma7us obesus and Gopherus agassizi co-occurred with Sce7oporus undu7atus or occidenta7is, and Neotoma and Erethizon dorsatum coexisted with Marmota. These mixtures of taxa which now exist separately in either desert or montane habitats, suggest a . . . . . . . . 12 .