Forming a trio with Eastlake Avenue and Northlake Way, Westlake Avenue is so named for running along the western shore of Lake Union. Beginning today at Stewart Street between 5th Avenue and 6th Avenue, just north of McGraw Square, it runs 2½ miles north to 4th Avenue N between Nickerson Street and Florentia Street — the south end of the Fremont Bridge.
Westlake Avenue once started a couple of blocks to the south, at 4th Avenue and Pike Street, and based on the quarter section map, it appears that its former route through Westlake Park between Pike Street and Pine Street is still public right-of-way as opposed to park land. (The portion between Pine Street and Olive Way was vacated in 1986 to make way for the Westlake Center mall, which opened in 1988, and the portion between Olive Way and Stewart Street was closed in 2010 to allow for the expansion of McGraw Square.)
Westlake was extended south to 4th and Pike from Denny Way in 1902 (one former mayor has called for that extension to be closed to cars); the original Westlake Avenue (now, properly, Westlake Avenue N) was created in 1895 as part of the Great Renaming ordinance, Section 5 of which reads
That the names of Rollin Street, Lake Union Boulevard and Lake Avenue from Depot Street [changed by the same ordinance to Denny Way] to Florentia Street, be and the same are hereby changed to Westlake Avenue.
Rollin Street, the southernmost portion, was named for Rolland Herschel Denny (1851–1939), the youngest member of the Denny Party at just six weeks old. In its honor, an apartment complex that opened at the corner of Westlake and Denny in 2008 is named Rollin Street Flats.
Northlake, Eastlake, Westlake… why no Southlake?
Having covered Northlake, Eastlake, and Westlake so far, one might ask: why is there no Southlake?
There does appear to have been a Southlake Avenue for a time — 1909 to 1924 or so, based on the last mention of it I could find in Seattle newspapers, an article in the August 8, 1924, edition of The Seattle Times on a car crash that had taken place a number of weeks earlier. Now the northern section of Fairview Avenue N, it extended from the intersection of Valley Street northwest to E Galer Street and Eastlake Avenue E, “thus eliminating the present grade on Eastlake for University traffic” in the words of a real estate advertisement in the August 23, 1914, edition of the Seattle Post-Intelligencer. But why the Southlake name disappeared seems clear: once it was decided to extend the Fairview name along the shore lands, there was no other appropriate road to carry it. The northern and eastern shores of Lake Union are just shy of 2 miles long each, but since the lake is shaped like a ? (and, surprisingly, like a uterus if Portage Bay is included) there is hardly any southern shore to speak of — only about ¼ mile.
As for the neighborhood name, I’m not sure why South Lake Union came to be used instead of Southlake. Perhaps it’s as simple as the lack of a similarly named street to “anchor” the neighborhood.