Fluvial responses to Holocene environmental change, Uinta Mountains, northeastern Utah

This research has shown that stream channels and streamflow in the Uinta Mountains of northeastern Utah are sensitive to climatic change, local geomorphology, and land use over timescales ranging from decadal to millennial. Analysis of U.S.G.S. annual duration flood series shows that magnitudes for annual flood peaks, when averaged for the Uinta Mountains gage sites, increased by 0.39 standard deviations for the period 1962 to 1998 compared to 1918 to 1961. This statistically significant break in the annual duration series agrees closely with a documented shift in the frequency and seasonality of El Niño-Southern Oscillation (ENSO) events recorded in hydrologic activity for other localities in the western United States. Evaluation and application of previously collected dendrochronologic data for the southern Uinta Mountains supports the idea that climate variations have been responsible for large temporal variation of streamflow over the past ∼350 years. For the reconstructed period A.D. 1637 to 1970, mean annual discharges on Ashley Creek exhibited statistically significant non-random clustering of values lower than the 10th percentile and greater than the 90 th percentile of discharges. Quantitative reconstruction of former bankfull flood magnitudes based on morphologic dimensions of relict cut-off channels preserved in floodplains of the northern Uinta Mountains also indicate that shifts in climatic conditions have led to significant past episodic variations in magnitudes of high-frequency floods. Bankfull floods were 10 to 20% larger than modern from 1000 to 2800 cal yr BP and prior to 4600 cal yr BP, but were 10 to 15% smaller than modern from near present to 1000 cal yr BP and from 2800 to 4600 cal yr BP.; Human-related activities have strongly influenced hydrologic processes since the late-19th century. For example, in the West Fork Black's Fork valley, overbank alluvial sedimentation since clear-cut logging in the late 19th and early 20th Centuries has contributed several centimeters of floodplain sedimentation whose specific local thickness is sensitive to local valley cross section morphometry. HEC-RAS modeling of floods of variable magnitudes indicates that local variations in valley cross section morphology strongly influence flood conveyance and floodplain sedimentation characteristics today and probably during past. Individual channels that are as old as 7080 ± 40 14C yr BP also exhibit evidence of colluvial and localized fluvial sedimentation. Radiocarbon ages from the Yellowstone River basin in the southern Uinta Mountains indicate that the deglaciation in the lower portion of the basin had begun by 9980 ± 100 14C yr BP, and was potentially occurring by ∼11,200 14C yr BP. Limiting radiocarbon ages from the headwaters of Yellowstone River within 1 km of the cirque headwall indicate deglaciation occurred by 7860 ± 60 14C yr BP (8550 to 8740 cal yr BP), and probably significantly earlier.