PEEKING UNDER BOULDERS IN PURSUIT OF DEBRIS FLOW GEOCHRONOLOGY AT WHISKEYTOWN NATIONAL RECREATION AREA

We collected samples for optically stimulated luminescence (OSL) and radiocarbon analysis to establish a history of debris flows in Whiskeytown National Recreation Area, while simultaneously working on identifying possible mechanisms that trigger the flows (i.e., wildfire, intense precipitation events that cause flooding, or normal spring melt on denuded areas). Using LiDAR, we identified several landforms suggestive of past debris flows. We sampled terraces and areas adjacent to debris flow levees, and in some cases obtained sediment from under boulders making up the debris flow levees.

We sampled five debris flow transported boulders on Brandy Creek after ascertaining that the fine-grained matrix around the boulders was potentially mixed by modern human agents, or modified by other high discharge events in the top 10-20 cm. The boulders are composed of granodiorite and appeared to be part of a sparce debris flow levee. A small Bobcat® was employed to carefully lift the 1 to 1.5 m intermediate axis boulders to access the fine-grained matrix material they were socketed in. This surface is ideal for sampling sediment to discern the timing of the boulder emplacement.

Of the boulders we sampled, ages are in the range of 880-865, 740-710, 600, and 410-355 calendar years. In one case, the OSL and radiocarbon on charcoal fragments are quite consistent (950-810 years quartz OSL and 885-845 yrs BP radiocarbon), while in the second case they are not (790-630 years quartz OSL and 385-325 years BP radiocarbon). We also sampled a cobble layer adjacent to Paige-Boulder Creek near the confluence with Clear Creek which returned OSL ages of 820 and 1060 years along with a radiocarbon age of 800 BP years. These ages closely match some of our boulder ages in upper Brandy Creek. Sediment sampled from Clear Creek terraces downstream of the dam and the confluence of Paige-Boulder Creek returned OSL ages of 250-310 years which also match some of the boulder ages. Ages in the lower most part of the trenches were in the range of 2480-2720 years, 5010-5590 years, and 5780-6480 years (radiocarbon was 5310-5390 BP years from same layer). The boulder fields, Paige-Boulder Creek, and Clear Creek terrace cobble layer all support a debris flow sequence of the last ~1,000 years that appears to conform to a recent frequency between 125-150 years.