This is the reference I found...

"As the ice sheet retreated from its maximum position, meltwater drained into the axis of the Lowland but could not follow what would become its modern drainage path north and west out the Strait of Juan de Fuca, because the strait was still filled with many hundreds or even thousands of meters of ice. Instead, meltwater was diverted south along the margins of the retreating ice sheet, coalescing into ever-broader rivers. Channels and locally broad plains of the Vashon recessional outwash now form much of the landscape in these ice-marginal locales and recessional river valleys. These landforms can be traced downstream to their glacial-age spillway out of the Puget Lowland, south through the valley of Black Lake near Olympia and then along the valley of the modern Chehalis River west to the Pacific Ocean."