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. . failure event approximately 4,200 years ago and left lacustrine sediments and the remains of eight mollusk species (Hamilton 1979). Modern canyon lakes are surrounded by riparian Salix, Popu)us (cottonwood), Fraxinus, Betula, Celtis, Juglans, and Acer. Although Holocene lakes may have also been surrounded by riparian tree species, riparian perennial plant pollen were not be well preserved in the sediments. The reliability of pollen as an indication of predominant vegetation was also studied in surface soils from Chuska Mountain forests, the arid steppe of the Chinle Valley, and Chinle Wash alluvium (Fall 1987). She concluded that alluvium provided a poor representation of dominant vegetation because slackwater settling rates differ between taxa, favoring Pinus pollen, while other species' pollen (e.g., Populus) decomposed quickly. Summary Throughout an enormous span of geologic time the sedimentary rocks of the Colorado Plateau provide incontrovertible evidence of significant changes in regional environments. The geologic history of the Colorado Plateau included extensive pre-Cambrian through Mesozoic marine, coastal and deltaic habitats, to present- day, moderate to high elevation arid lands environments. As such, the Colorado Plateau has provided an evolutionary theater for the development of riparian flora and fauna, with speciation and adaptation of life history traits in spatially and temporally diverse ecological settings. Present-day native species appear extremely well-adapted to the environments they inhabit in the upper Colorado Basin, but anthropogenic changes in discharge regimes, water quality, as well as the introduction of exotic species, have created novel environmental conditions which challenge the adaptive capacities of many contemporary species. . . . . . . . . 14 .