LATE GLACIAL AND HOLOCENE HISTORY OF BAKER RIVER VALLEY, WASHINGTON

Baker River valley drains the southeast flank of Mount Baker, a 3,286 m stratovolcano, and the windward slope of the rugged Picket Range in North Cascades National Park. Alpine glaciers had retreated and the lower valley was open after 16,333 ¹⁴C BP when the Puget Lobe of the Cordilleran Ice sheet first advanced eastward up Skagit valley. During the early phase of its advance, the ice sheet built a large outwash fan over thick proglacial lacustrine deposits in lower Baker valley. Full ice sheet glaciation eventually carved a basin on the east edge of these deposits that later held Glacial Lake Baker (GLB). Collapse of the ice sheet over northern Puget Sound during deglaciation led to a reversal of ice flow in lower Skagit valley and frontal retreat to the east, opposite the direction of ice advance. During initial formation of GLB sometime around 12,200 ¹⁴C BP, a lobe of the ice sheet dammed Baker valley and held the lake level at a maximum surface elevation of 335 m asl. At this stage, the lake drained west around the ice margin and down Skagit River via the Lake Tyee channel and Grandy Creek. As the ice sheet retreated to the east up Skagit valley, a new outlet of the Baker valley was established near Concrete, Washington. Glacial deposits initially held the lake level at about 252 m and the lake covered about 58 km². Rapid down-cutting at this outlet slowed when it encountered bedrock at about 143 m elevation. Relatively slow erosion at the bedrock outlet allowed GLB to persist until after 3,205 ¹⁴C BP, at which time the lake surface elevation was at least 137 m. The Holocene remnant of GLB received extensive postglacial sedimentation from adjacent slopes and valleys, as well as deposits from the 10,800 ¹⁴C BP Carmello Crater and the 8,800 ¹⁴C BP Sulphur Creek eruptions.