Response of bankfull flood magnitudes to Holocene climate change, Uinta Mountains, northeastern Utah

Long-term variations in Holocene fl ood magnitude were quantifi ed from the bankfull dimensions of abandoned channels preserved on flsurfaces in the northern Uinta Mountains of northeastern Utah. Cross-sectional areas of abandoned channels were reconstructed, and relationships derived from the modern gage records were used to estimate bankfull discharges from bankfull cross-section areas. The results indicate systematic (nonrandom) variations of bankfull fl oods in the northern Uinta Mountains. Large fl oods, as much as 10%‐15% greater than modern, dominated from 8500 to 5000 calendar yr B.P., and again from 2800 to 1000 cal yr B.P. Small fl oods, as much as 15%‐20% less than modern, characterize the periods from 5000 to 2800 cal yr B.P., and from 1000 cal yr B.P. to near present. The middle and late Holocene record of bankfull fl ood magnitude compares well with independent evidence for climatic variation in the area. The early Holocene record indicates that larger than modern bankfull fl oods coincide with warmer than modern mean annual temperature. We hypothesize that an increased range of magnitude for seasonal solar radiation during the early Holocene favored the accumulation and rapid melting of deep snowpacks in the high Uinta Mountains, thus producing large fl oods despite warmer mean annual temperatures. The episode of smaller than modern bankfull fl oods between 5000 and 2800 cal yr B.P. coincides with records of increased forest fi re frequency in the northern Uintas. Larger than modern fl oods from 2800 to 1000 cal yr B.P. coincide with a local decrease in forest fi re frequency and evidence for minor local glacial readvances. The decrease in fl magnitudes following 1000 cal yr B.P. corresponds to numerous local and regional records of warming during the Medieval Climatic Anomaly.