Eric C. Carson

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.

Holocene tephrochronology of the Cold Bay area, southwest Alaska Peninsula ☆

Abstract The major-element glass geochemistry of 92 tephra samples from the southwest Alaska Peninsula provides the basis for establishing a Holocene tephrochronology for the region. Electron microprobe analysis has been combined with field descriptions of samples, stratigraphic relationships between tephra samples and sample localities, and glass shard micro-morphology to correlate these sampled distal tephra units throughout the area of Cold Bay and adjacent Morzhovoi Bay. Radiocarbon dating provides age constraints on correlated horizons. Previous research had clearly delineated only one horizon in the region, the so-called ‘Funk/Fisher’ ash, dating to between 8425±350 and 9130±140 14 C yr BP. In addition to constraining the bimodal andesitic and dacitic glass chemistry of that horizon, this study has recognized six additional tephra layers in the area. Two horizons pre-date the Funk/Fisher ash and four are younger than it. A tephra containing dacitic and andesitic components was identified in the vicinity of Morzhovoi Bay, with a minimum age of 9300±80 14 C yr BP and a maximum age of 10,200±75 14 C yr BP. A rhyolitic horizon composed of cm-sized, rounded pumice clasts was identified in the vicinity of Cold Bay; it has been correlated to the ca 9500 BP eruption of Roundtop volcano on Unimak Island. The four younger tephra beds date to between 6070±340 and 3600±140 14 C yr BP. The oldest of the four is rhyodacitic, followed by a mixed rhyodacitic–andesitic horizon, another rhyodacitic horizon, and finally an andesitic layer. Comparison of all the correlated horizons to proximal samples collected on Unimak Island provides conclusive geochemical evidence that the ca 9100 BP Caldera-forming eruption of Fisher volcano is the source of the Funk/Fisher ash. Correlation between the rhyodacitic tephra horizons and proximal samples from Fisher volcano suggests that Fisher Caldera is the source of one of the rhyodacitic tephra horizons that post-dates the Funk/Fisher ash. Additional tephra samples from the southwest Alaska Peninsula and Unimak Island that were collected prior to this study correlate to the tephra horizons identified in the Cold Bay area and identify one additional horizon.

Temporal and Seasonal Trends in Streamflow in the Uinta Mountains, Northeastern Utah, and Relation to Climatic Fluctuations

ABSTRACT Over the past half century there has been a trend in numerous regions of the western United States toward a declining portion of the total annual streamflow occurring during the snowmelt season (April through July). Changes in the seasonality of discharge primarily reflect a shift in the seasonal distribution of precipitation. Years with an unusually large fraction of annual streamflow occurring during the spring snowmelt coincide with winters with unusually high precipitation. Years with an unusually small fraction of annual streamflow occurring during the spring snowmelt coincide with unusually low precipitation during the winter and unusually warm temperatures during the spring. The magnitude of the peak annual flood is sensitive to amount of winter precipitation and variations in spring and early summer temperatures. Large peak annual floods occur in years with high precipitation and low temperatures in the late winter and spring, which preserve the maximum snowpack. Small peak annual floods occur in years with low precipitation throughout the winter and high temperatures in the spring. During the period of gage record, peak annual flood magnitudes exhibit temporal variability. Extreme events in the peak annual flood record, particularly years of large snowmelt floods, have increased in frequency since 1960. Large floods during this time period occur in years with atypically cool temperatures in April and May, which would likely preserve the winter snowpack through the spring.