TY - JOUR
T1 - Drainage Area, Bedrock Fracture Spacing, and Weathering Controls on Landscape-Scale Patterns in Surface Sediment Grain Size
AU - Neely, Alexander B.
AU - DiBiase, Roman A.
N1 - Funding Information:
This project was supported by National Science Foundation Grant EAR-1608014 to R. DiBiase. We acknowledge Angeles National Forest, San Bernardino National Forest, and Mount San Jacinto State Park for continued trail maintenance enabling access to field sites. We thank J. Carr, J. Del Vecchio, E. Greenberg, and P. Silverhart for field assistance, E. Loucks for assistance in constructing structure-from-motion models, and McLanahan's downtown market for providing a comfortable workspace during the preparation of this manuscript. We thank Mikael Attal, Claire Lukens, and one anonymous reviewer for their detailed reviews and suggestions that improved the clarity of this manuscript.
Funding Information:
This project was supported by National Science Foundation Grant EAR‐1608014 to R. DiBiase. We acknowledge Angeles National Forest, San Bernardino National Forest, and Mount San Jacinto State Park for continued trail maintenance enabling access to field sites. We thank J. Carr, J. Del Vecchio, E. Greenberg, and P. Silverhart for field assistance, E. Loucks for assistance in constructing structure‐from‐motion models, and McLanahan's downtown market for providing a comfortable workspace during the preparation of this manuscript. We thank Mikael Attal, Claire Lukens, and one anonymous reviewer for their detailed reviews and suggestions that improved the clarity of this manuscript.
Publisher Copyright:
©2020. American Geophysical Union. All Rights Reserved.
PY - 2020/10/1
Y1 - 2020/10/1
N2 - Sediment grain size links sediment production, weathering, and fining from fractured bedrock on hillslopes to river incision and landscape relief. Yet models of sediment grain size delivery to rivers remain unconstrained due to a scarcity of field data. We analyzed how bedrock fracture spacing and hillslope weathering influence landscape-scale patterns in surface sediment grain size across gradients of erosion rate and hillslope bedrock exposure in the San Gabriel Mountains (SGM) and northern San Jacinto Mountains (NSJM) of California, USA. Using ground-based structure-from-motion photogrammetry models of 50 bedrock cliffs, we showed that fracture density is ~5 times higher in the SGM than the NSJM. 274 point-count-surveys of surface sediment grain size measured in the field and from imagery show a drainage area control on sediment grain size, with systematic downslope coarsening on hillslopes and in headwater-colluvial channels transitioning to downstream fining in fluvial channels. In contrast to prior work and predictions from a hillslope weathering model, grain size does not increase smoothly with increasing erosion rate. For soil-mantled landscapes, sediment grain size increases with increasing erosion rates; however, once bare bedrock emerges on hillslopes, sediment grain size in both the NSJM and SGM becomes insensitive to further increases in erosion rate and hillslope bedrock exposure, and instead reflects fracture spacing contrasts between the NSJM and SGM. We interpret this threshold behavior to emerge in steep landscapes due to efficient delivery of coarse sediment from bedrock hillslopes to channels and the relative immobility of coarse sediment in fluvial channels.
AB - Sediment grain size links sediment production, weathering, and fining from fractured bedrock on hillslopes to river incision and landscape relief. Yet models of sediment grain size delivery to rivers remain unconstrained due to a scarcity of field data. We analyzed how bedrock fracture spacing and hillslope weathering influence landscape-scale patterns in surface sediment grain size across gradients of erosion rate and hillslope bedrock exposure in the San Gabriel Mountains (SGM) and northern San Jacinto Mountains (NSJM) of California, USA. Using ground-based structure-from-motion photogrammetry models of 50 bedrock cliffs, we showed that fracture density is ~5 times higher in the SGM than the NSJM. 274 point-count-surveys of surface sediment grain size measured in the field and from imagery show a drainage area control on sediment grain size, with systematic downslope coarsening on hillslopes and in headwater-colluvial channels transitioning to downstream fining in fluvial channels. In contrast to prior work and predictions from a hillslope weathering model, grain size does not increase smoothly with increasing erosion rate. For soil-mantled landscapes, sediment grain size increases with increasing erosion rates; however, once bare bedrock emerges on hillslopes, sediment grain size in both the NSJM and SGM becomes insensitive to further increases in erosion rate and hillslope bedrock exposure, and instead reflects fracture spacing contrasts between the NSJM and SGM. We interpret this threshold behavior to emerge in steep landscapes due to efficient delivery of coarse sediment from bedrock hillslopes to channels and the relative immobility of coarse sediment in fluvial channels.
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U2 - 10.1029/2020JF005560
DO - 10.1029/2020JF005560
M3 - Article
AN - SCOPUS:85093932088
SN - 2169-9003
VL - 125
JO - Journal of Geophysical Research: Earth Surface
JF - Journal of Geophysical Research: Earth Surface
IS - 10
M1 - e2020JF005560
ER -