THE BUMPY BARCODE GOES TO HELL(S CANYON, IDAHO): LARGE-N DETRITAL ZIRCON MIXTURE MODELING OF THE SNAKE RIVER DRAINAGE

Ken Pierce was an early advocate of the use of detrital zircons (DZ) to evaluate modern and Neogene drainage patterns of the Snake River and tributaries, as modified by the northeastward migration of the Snake River-Yellowstone hotspot. Early DZ studies of a few grains used simple presence and absence to show the drainage reorganization caused by the hotspots passage.

This study used 27 large-n (n ≈ 300 grains) samples analyzed and recently developed inverse Monte Carlo mixture modeling across the Snake River Basin in Idaho and Oregon. Results are as follows.

1) Samples from the Snake River at Farewell Bend, immediately upstream of the Hells Canyon Complex (HCC) of Dams, showed the relative zircon contributions of an area of over 180,000 km2 square kilometers to be: Payette River 50%, Snake River at Marsing 30%, Boise River 10% and Owyhee, Malheur, and Weiser Rivers less than 5% each.

2) Samples taken from the Snake River and adjacent historic terraces within Hells Canyon below the HCC showed that that the modern mainstem relative zircon contribution is more different from the historic terrace samples closer to the dam and becomes more similar downstream due to the relative importance of local sources.

3) Similarly, a terrace and mainstem pair below the confluence of the Snake and Salmon Rivers showed an increase in relative zircon contributions of the Salmon through time. The mainstem sample showed the Salmon provides 90% of the detrital zircons while the Snake only provides 10%. At the time that the terrace was deposited (Holocene, less than 3 ka), the mix was Salmon 67% and Snake 33%.

We interpret these results to suggest sediment trapping in the Snake River by upstream dams, such that the Snake at the confluence with the Salmon River today provides fewer zircons than it did before dam installation. Results have further implications for relative sediment yield through consideration of outcrop area, zircon fertility, and topographic relief and comparisons of those prediction to other sediment yield results.