Event Title

Fluvial Architecture of Episodic Arroyo Entrenchment and Aggradation in Kanab Creek, Southern Utah

Location

USU Eccles Conference Center

Event Website

http://water.usu.edu

Start Date

4-5-2016 5:12 PM

End Date

4-5-2016 5:15 PM

Description

Many alluvial valleys in the American Southwest are entrenched with arroyos, and stratigraphic evidence exposed in modern arroyo walls indicates that these fluvial systems experienced repeated periods of entrenchment and re-aggradation during the mid- to late-Holocene. Previous research suggests arroyo dynamics were regionally synchronous, implying that climate fluctuations are the dominant drivers. However, many of these interpretations rely on records with limited and problematic age control, as well as broad correlations across the southwest. While hydroclimatic fluctuations must play a role, intrinsic reach- or catchment-specific geomorphic thresholds to entrenchment are hypothesized to partially control the timing of arroyo processes, suggesting that episodes of arroyo cutting and filling need not necessarily be regionally contemporaneous. This study focuses on Kanab Creek, southern Utah. Kanab Creek is entrenched within two arroyos that are separated by a bedrock knickzone, indicating that arroyo headscarp retreat cannot propagate from the lower reach into the upper and suggesting potentially different alluvial histories. Episodes of prehistoric arroyo cutting and filling are reconstructed from 29 sites by recognition of buttress unconformable contacts in the arroyo-wall stratigraphy and age control derived from optically stimulated luminescence (OSL) and accelerator mass spectrometry (AMS) radiocarbon dating. Results indicate at least five periods of aggradation occurred since ~6.0 ka in the lower basin, each interrupted by an episode of arroyo entrenchment. Although the upper basin record is not yet well-constrained, stratigraphic and geochronologic evidence suggests that the earliest period of entrenchment in this reach occurred within dating resolution of the lower reach downstream, suggesting a common response to climate forcing. Comparison of these records to recently completed chronologies from arroyo systems in adjacent catchments indicates near-synchronous arroyo processes over the last ~1.5 ka; however, beyond 1.5 ka correlations are less clear. Broadly contemporaneous alluviation suggests a climatic driver, and comparison to paleoclimate records suggests that arroyo entrenchment events may be driven by transitions from periods of multi-year drought to wetter periods. However, not all such transitions are associated with arroyo entrenchment, suggesting the importance of geomorphic thresholds in modulating the timing of climate-driven arroyo processes. Variations in the longitudinal profile concavity of an alluvial stream may represent equilibrium adjustments of channel gradient to a changing balance between sediment load and hydrologic competence. Here we calculate changes in reach-concavity attendant with aggradation and incision through time and apply it for the first time to Holocene arroyo dynamics. We find that during aggradation, channel concavity is quantifiably decreased along the study reach of Kanab Creek, making the longitudinal profile straighter and channel slope more evenly distributed. Theoretically, this is consistent with both sediment loading/coarsening and a decrease in effective discharge or competence. Following the same approach in neighboring or other analogous drainages would provide insight into spatial variations in the degree of adjustment, and perhaps help identify more specific controls and drivers on aggradation and incision and the threshold between those conditions.

Comments

A poster by Kirk Townsend, who is with Utah State University, Luminescence Laboratory

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Apr 5th, 5:12 PM Apr 5th, 5:15 PM

Fluvial Architecture of Episodic Arroyo Entrenchment and Aggradation in Kanab Creek, Southern Utah

USU Eccles Conference Center

Many alluvial valleys in the American Southwest are entrenched with arroyos, and stratigraphic evidence exposed in modern arroyo walls indicates that these fluvial systems experienced repeated periods of entrenchment and re-aggradation during the mid- to late-Holocene. Previous research suggests arroyo dynamics were regionally synchronous, implying that climate fluctuations are the dominant drivers. However, many of these interpretations rely on records with limited and problematic age control, as well as broad correlations across the southwest. While hydroclimatic fluctuations must play a role, intrinsic reach- or catchment-specific geomorphic thresholds to entrenchment are hypothesized to partially control the timing of arroyo processes, suggesting that episodes of arroyo cutting and filling need not necessarily be regionally contemporaneous. This study focuses on Kanab Creek, southern Utah. Kanab Creek is entrenched within two arroyos that are separated by a bedrock knickzone, indicating that arroyo headscarp retreat cannot propagate from the lower reach into the upper and suggesting potentially different alluvial histories. Episodes of prehistoric arroyo cutting and filling are reconstructed from 29 sites by recognition of buttress unconformable contacts in the arroyo-wall stratigraphy and age control derived from optically stimulated luminescence (OSL) and accelerator mass spectrometry (AMS) radiocarbon dating. Results indicate at least five periods of aggradation occurred since ~6.0 ka in the lower basin, each interrupted by an episode of arroyo entrenchment. Although the upper basin record is not yet well-constrained, stratigraphic and geochronologic evidence suggests that the earliest period of entrenchment in this reach occurred within dating resolution of the lower reach downstream, suggesting a common response to climate forcing. Comparison of these records to recently completed chronologies from arroyo systems in adjacent catchments indicates near-synchronous arroyo processes over the last ~1.5 ka; however, beyond 1.5 ka correlations are less clear. Broadly contemporaneous alluviation suggests a climatic driver, and comparison to paleoclimate records suggests that arroyo entrenchment events may be driven by transitions from periods of multi-year drought to wetter periods. However, not all such transitions are associated with arroyo entrenchment, suggesting the importance of geomorphic thresholds in modulating the timing of climate-driven arroyo processes. Variations in the longitudinal profile concavity of an alluvial stream may represent equilibrium adjustments of channel gradient to a changing balance between sediment load and hydrologic competence. Here we calculate changes in reach-concavity attendant with aggradation and incision through time and apply it for the first time to Holocene arroyo dynamics. We find that during aggradation, channel concavity is quantifiably decreased along the study reach of Kanab Creek, making the longitudinal profile straighter and channel slope more evenly distributed. Theoretically, this is consistent with both sediment loading/coarsening and a decrease in effective discharge or competence. Following the same approach in neighboring or other analogous drainages would provide insight into spatial variations in the degree of adjustment, and perhaps help identify more specific controls and drivers on aggradation and incision and the threshold between those conditions.

https://digitalcommons.usu.edu/runoff/2016/2016Posters/15