SEDIMENT YIELDS OF GRAND CANYON TRIBUTARIES CALCULATED OVER MILLENNIA AND MILLIONS OF YEARS

Understanding of the process that cut Grand Canyon and its history has advanced in the last decade through the use of new chronologic tools. However, the understanding of sediment yields from tributary canyons is still poorly known over millennial to million-year timescales. We measured ¹⁰Be in river sediment collected from 23 Grand Canyon tributaries ranging in area from 1 to 258 km² (mean = 49 km², median = 12 km²) to determine basin-average erosion rates and thus millennial-scale sediment yields. We also calculated the volume of those basins and used the broadly accepted age of 6.0 Ma to estimate the average sediment yield over the “age” of Grand Canyon. Based on ¹⁰Be, we find that Grand Canyon tributaries have a bimodal distribution of millennial-scale sediment yields. In eastern Grand Canyon (EGC), from Lee’s Ferry (river mile 0) downstream to river mile 99, tributaries deliver an area-weighted average of 279 ± 70 t/km² *y of sediment (n = 19; uncertainties for average sediment yields and erosion rates represent the standard error of the mean). In western Grand Canyon (WGC), from river mile 171 to river mile 205, the tributaries deliver an area-weighted average of 106 ± 61 t/km² *y of sediment (n = 4). Volumetric sediment yields for EGC averaged over 6 Ma are 270 ± 20 t/km² *y of sediment, similar to the millennial-scale ¹⁰Be estimates. Volumetric estimates averaged over 6 Ma in WGC are 262 ± 15 t/km² *y, ~ 2.5 fold higher than the ¹⁰Be-derived millennial sediment yield. However, sediment yields averaged over 17 Ma for WGC are 102 ± 5 t/km² *y, similar to the millennial-scale estimates. Taken at face value the sediment yields measured over millennial-scales for EGC and WGC are similar to those calculated over 6 Ma for EGC and 17 Ma for WGC and support a multi-aged Grand Canyon.