LAST-GLACIAL FRESHWATER FLUXES TO THE NORTHEASTERN PACIFIC FROM THE PACIFIC NORTHWEST, A VIEW FROM THE LAND (Invited Presentation)

Present-day terrestrial freshwater input to the NE Pacific Ocean is ~700 km³/yr, dominated by the Columbia (~200 km³/yr) and Fraser (100 km³/yr) rivers. What were the inputs during the last glacial maximum, 25–14 ka? The terrestrial geologic record suggests several factors: (1) greater precipitation and runoff; (2) changes in contributing area; (3) growth and decay of the Cordilleran ice sheet; (4) megafloods from ice-dammed and pluvial lakes sending pulses of accumulated runoff into the northeast Pacific.

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.