RESPONSE OF BANKFULL FLOOD MAGNITUDES TO HOLOCENE CLIMATIC VARIATION, UINTA MOUNTAINS, NORTHEASTERN UTAH

Long-term variations in Holocene flood magnitudes were quantified from bankfull dimensions of abandoned channels preserved on floodplains in the Uinta Mountains. In this sub-alpine setting, the magnitude of the modal (most commonly occurring) flood is intimately associated with the snowpack accumulated during the winter and released in the spring and early summer. Temporal variations in climate that impact the timing and amount of precipitation in the region are therefore reflected in bankfull flood magnitudes.

Cross-sectional capacities of relict, abandoned channels in the northern Uinta Mountains were reconstructed by coring across relict channels perpendicular to flow direction. Relationships derived from modern gage data were used to convert bankfull cross-section areas to bankfull discharges. Age control for the relict channels was provided by radiocarbon dating of basal channel fill sediments and application of an empirical equation relating thickness of overbank sediments on point bars to the age of the associated channels. The results indicate systematic (non-random) variations of bankfull floods in the northern Uinta Mountains. Large floods--10 to 20 % greater than modern--dominated from 8500 to 5000 BP, and again from 3000 to 1000 BP. Small floods--10 to 15 % less than modern--characterize the periods from 5000 to 3000 BP, and from 1000 BP to near present.

The record of Holocene bankfull flood magnitudes compares well with independent evidence of climatic variation in the area during the middle to late Holocene. The fossil pollen record indicates that warm, dry conditions prevailed through the middle Holocene, corresponding with small floods during that time. Lichenometric evidence for a late Neoglacial re-advance corresponds with large floods at that time. Dendrohydrologic and lichenometric data indicate persistent dry conditions during the last millenium, during a period of small floods. The early Holocene record indicates large floods prevailed during what has been interpreted as a warm, dry period through much of the central Rockies. I hypothesize that the increased seasonal variation in solar radiation during the early Holocene allowed accumulation of significant snowpacks in the alpine and sub-alpine areas of the Uinta Mountains, producing large floods despite warmer mean annual temperatures during this period.