ANNUALLY-LAMINATED LAKE SEDIMENT REVEALS LATE HOLOCENE ALLUVIAL CHANNEL HISTORY AND A RECORD FLOOD IN 1867, TOLT RIVER, WESTERN WASHINGTON

Quantifying flood hazard requires assessing long histories of streamflow as well as assessing changes in channel capacity and elevation. The Snoqualmie River valley of western Washington is subject to large floods associated with atmospheric river events; river channels are also aggrading and incising and carry high sediment load from tributaries flowing through thick, landsliding, glaciolacustrine sediments. In this study, a lacustrine sediment record from Langlois Lake is shown to record extreme floods on the Tolt River fan. Langlois Lake is uniquely situated to record floods, as a three-meter rise in river level is required to overtop a natural levee and reach the lake. Annual, mm-scale, biogenic laminations were counted on images from a freeze core and ages verified by Pb-210 dating and pollen stratigraphy. The 250-year varve chronology shows the most recent flood event into the lake (1 cm of river silt) was in ca. 1867 AD. While there is little recorded history from the eastern Puget Lowlands at this time, daily weather records from forts from Vancouver WA to San Juan Island show a four-day rainfall in December 1867 on par with, or exceeding, comparable 20th and 21st-century station data from the region. No other river-silt deposit exists in the record within the last 950 years, suggesting the 1867 event exceeded not only the major 1990 and 2009 floods but also floods for centuries before. However, changes in channel bed elevation also contribute to past flood stage. A 534-cm, 6100-year, sediment core from Langlois Lake shows rapidly accumulating river silt with beds of 1-cm or more marked 32% of the last 6000 years. Radiocarbon dating and regional paleoclimate studies suggest that the silt sections date to dry and/or highly variable periods when the Tolt fan likely aggraded sediment. Future work will address whether forest-fire history covaries with flood history. Organic (non-flooding) segments date to wetter times when the Tolt River may have incised and/or built higher levees. This study found that the Tolt fan has a highly non-stationary and climatically-linked history of aggrading and incising on decadal to millennial time scales. While these dynamics confound assessing paleoflood magnitude, ancillary evidence points to the 1867 flood as larger than any of the “recent” 100-year floods in western Washington.