LAST-GLACIAL FRESHWATER FLUXES TO THE NORTHEASTERN PACIFIC FROM THE PACIFIC NORTHWEST, A VIEW FROM THE LAND (Invited Presentation)
Last-glacial runoff is estimated for the Columbia River basin from refilling rates of ~25-40 km3 per year for the 18-15 ka duration of glacial Lake Missoula, as judged by counts of varves deposited between jökulhlaups from the cyclically refilling lake. This filling rate exceeds the modern Clark Fork River flow by a factor of 1.3–2.1. Similarly, pluvial Lake Bonneville, south of the modern Columbia River, gained an average of ~1.3 km3 per year during 22–18 ka, about 1.5 times the modern inflow rate into Great Salt Lake. This Bonneville basin surplus entered the Pacific Ocean during the 18–15 ka period when pluvial Lake Bonneville spilled into the Columbia River basin. Extrapolated to the entire Columbia River basin, the 1.3–2.1 ratio of enhanced runoff equates to 250–400 km3/yr for the colder and wetter conditions of 22-15 ka.
Annual fluxes were also affected by ice-sheet growth and wastage, quantified by North American ice-sheet models. Rapid wastage between 16 and 14 ka produced an additional 100–150 km3/yr per year of runoff in the Columbia basin, increasing late last-glacial runoff by 25–100%. For the Fraser basin, such augmentation from ice-sheet decay was likely greater, nearly 400 km3/yr at about 14 ka, likely doubling Fraser River flow. Meltwater augmentation was likely even greater further north.
The largest Missoula floods, probably during the period 18–16 ka, provided week-long pulses of as much as 2500 km3 at a maximum rate of ~107 m3/s. The months-long Bonneville flood (~106 m3/s) provided a one-time pulse of about 5000 km3 from the Great Basin at about 18 ka, 10 to 20 times the average annual Columbia River flux. Megafloods provided significant pulses equivalent to several years of runoff, but the largest ones likely preceded the periods of most voluminous runoff.