Natural History and Geology
Picture five Space Needles stacked on top of each other. This was the size of the Vashon Glacier, a 3,000-foot tall ice sheet that gouged out Lake Union and Lake Washington and sculpted Puget Sound during its retreat over 13,000 years ago.
When the first people arrived to Lake Union over 5,000 years ago, the lake had a natural outlet to Puget Sound, where the Fremont Bridge is now located. Four streams flowed into the main body of the lake and two streams flowed into Portage Bay. Lake Union was separated from Lake Washington by a marshy isthmus about 1/2 - mile wide. The Duwamish carried their canoes on a portage trail between the two bodies of water - hence the name Portage Bay.