GLACIAL LAKE SHORELINES FROM THE VASHON GLACIATION FOUND IN THE BEAR CREEK VALLEY NEAR REDMOND, WASHINGTON & THEIR IMPLICATION IN SALMON MIGRATION AND DISTRIBUTION IN THE PUGET SOUND REGION
In "Glaciation of the Puget Sound Region", published by the Washington Geologic Survey in 1913, J Harlen Bretz documented 10 major proglacial lakes in the Puget Sound Basin mainly based on the lakes' outlets and elevations. The Bear Creek valley walls document a large number of terraces and shorelines indicating that the lake that occupied the Bear Creek valley (and other lowlands in the Puget Sound Basin) drained slowly over an extended period of time. Major lake shorelines for Lake Puyallup (550' & 400'), Lake Sammamish (300'), and Lake Russell (160') can be located in the Bear Creek Basin; however, numerous minor lake levels and a major new proglacial lake at 185-190 ft elevation have been identified.
During the Lake Sammamish glacial lake stage, Bear Creek was under water with only the northern-most segment of the creek & tributaries near the King-Snohomish County line available for spawning. As the lake slowly drained, the Bear Creek channel became exposed in the lakebed and was incised into the lake's retreating shorelines. Over time, the distance that salmon needed to swim to reach their spawning habitat grew from a few hundred feet to over 10 miles.
Furthermore, the draining of the region's proglacial lakes at the end of the last ice age and the replacement of fresh water with salt water in the Puget Sound Basin provided an evolutionary mechanism for the transition of Kokanee (freshwater salmon - Oncorhynchus nerka) into Sockeye (anadromous salmon - also O. nerka.)
Additionally, isostasy, marine shoreline accretion, continental plate movement as the Pacific Plate moved north relative to the North American Plate, and plate subduction are also contributing factors in the lengthening of salmon migration routes.