| Summary: | Drainage patterns established in the Tertiary in the North Cascades were reorganized to accommodate southern drainage of Cordilleran Ice Sheet meltwater. Repeated continental glaciation rendered the Skagit an interconnected valley, with meltwater routes opening it to the Fraser and Okanogan watersheds, and linking it to a drainage system around the east margin of the Puget lobe of the ice sheet. Alpine glaciers from two major tributaries blocked Skagit valley during the late Wisconsin Evans Creek stade, creating glacial lakes Concrete and Skymo. Organic material from lake sediments provides the first radiometric constraint on the beginning of the Evans Creek stade in the Cascades about 25,040 14C yr BP. Sediments and macrofossils at the Cedar Grove section define two advances of Baker alpine glaciers during this stade, separated by a period of warmer and wetter climate at 20,310 14C yr BP. During colder parts of the Evans Creek stade macrofossils indicate treeline was as much as 1200 150 m lower than present, which corresponds to a mean July temperature depression of approximately 7 1˚C. Glacier equilibrium line altitudes (ELA) during the cold periods were depressed 730-970 m below the modern glaciation threshold. Skagit valley alpine glaciers advanced several times to positions 5-10 km below valley heads between 12, 200 and 9,975 14C yr BP. ELA depression during these advances vary from 340 ± 100 m to 590 ±75 m, with greater depression in maritime western tributaries. Skagit ELA depression values are about 200 m less than reported for the southern North Cascades during the Sumas stade. The effect of the Cordilleran Ice Sheet on precipitation likely caused ELAs to be higher in the Skagit valley than in the southern North Cascades.
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