Reconciling Alluvial Records and Stream Behaviors on the Colorado Plateau
Location
Eccles Conference Center
Event Website
http://water.usu.edu/
Start Date
4-2-2009 10:20 AM
End Date
4-2-2009 10:25 AM
Description
Streams in semiarid settings have undergone cycles of arroyo cutting and filling over the Holocene in response to climatic forcing. Many have specifically cited variability in the frequency of large floods as a causal factor. Reconstructions of the changing grade of these streams are usually based on alluvial records found in broad alluvial valleys. In contrast, paleoflood hydrologists reconstruct paleoflood records by studying deposits preserved in constricted bedrock canyons, typically assuming a constant stream grade over millennial timescales. These contrasting stream behaviors must be reconciled in order to test the hypothesis that paleofloods are associated with arroyo cutting and to determine what characteristics determine what type of alluvial record is preserved in a given stream reach. This OSL/radiocarbon-based chronostratigraphic investigation has the goal of linking these two alluvial record types in Buckskin Wash, a major tributary of the Paria River, south-central Utah, where a broad alluvial reach drains into a severely constricted slot canyon. We are testing the hypothesis that the same floods that cut arroyos in the alluvial reach are preserved as depositional packages in the constricted reach downstream. Geochronology is not yet complete, but it appears that the temporal relationship is not so simple. The most recent alluviation in the broad valleys of the plateau occurred between 1300 and 1880 AD. The bulk of the deposits in the slot canyon were deposited in the latter half of that period. Older deposits in the slot canyon have been identified, separated from the younger alluvium by a significant hiatus in paleoflood deposition. Observations from the modern wash suggest that the grade of the constricted reach is more variable than is typically assumed by paleoflood hydrologists. Ongoing work includes further geochronologic work and hydraulic modeling to characterize the transition zone between the two end-member geomorphic reaches.
Reconciling Alluvial Records and Stream Behaviors on the Colorado Plateau
Eccles Conference Center
Streams in semiarid settings have undergone cycles of arroyo cutting and filling over the Holocene in response to climatic forcing. Many have specifically cited variability in the frequency of large floods as a causal factor. Reconstructions of the changing grade of these streams are usually based on alluvial records found in broad alluvial valleys. In contrast, paleoflood hydrologists reconstruct paleoflood records by studying deposits preserved in constricted bedrock canyons, typically assuming a constant stream grade over millennial timescales. These contrasting stream behaviors must be reconciled in order to test the hypothesis that paleofloods are associated with arroyo cutting and to determine what characteristics determine what type of alluvial record is preserved in a given stream reach. This OSL/radiocarbon-based chronostratigraphic investigation has the goal of linking these two alluvial record types in Buckskin Wash, a major tributary of the Paria River, south-central Utah, where a broad alluvial reach drains into a severely constricted slot canyon. We are testing the hypothesis that the same floods that cut arroyos in the alluvial reach are preserved as depositional packages in the constricted reach downstream. Geochronology is not yet complete, but it appears that the temporal relationship is not so simple. The most recent alluviation in the broad valleys of the plateau occurred between 1300 and 1880 AD. The bulk of the deposits in the slot canyon were deposited in the latter half of that period. Older deposits in the slot canyon have been identified, separated from the younger alluvium by a significant hiatus in paleoflood deposition. Observations from the modern wash suggest that the grade of the constricted reach is more variable than is typically assumed by paleoflood hydrologists. Ongoing work includes further geochronologic work and hydraulic modeling to characterize the transition zone between the two end-member geomorphic reaches.
https://digitalcommons.usu.edu/runoff/2009/AllPosters/9