Before the construction of the canal, Lake Union emptied into Salmon Bay via a creek which followed roughly the same course as the Fremont Cut does today.

[citation needed] Because of the connection via the Hiram M. Chittenden Locks to the salt water of Puget Sound, the water is brackish to a certain extent,[2] which increases in the summer as the inflow rate from Lake Washington decreases and the locks open more frequently for pleasure craft.

[5] In Chinook Jargon, the trade language primarily used in the 19th century, it is called Tenas Chuck ("small water").

"[7] Lake Union was formed by the recession of the Vashon Glacier approximately 14,000 years before present.

[4] Several Seattle neighborhoods take their name from the lake: Eastlake, Westlake, Northlake, and South Lake Union; and three major streets are named in relation to it: Westlake Avenue, which runs along its western shore from Downtown to the Fremont Bridge; Eastlake Avenue, which runs along its eastern shore from Cascade to the University District, and Northlake Way, which runs along its northern shore from the University District past Gas Works Park to the edge of Fremont.

[9] Lake Union's proximity to and scenic views of the central Seattle and University District skylines make it a popular recreational spot.

The world-famous Duck Dodge sailboat races are run on Lake Union each Tuesday during the summer.

It is the venue for summer concerts and Seattle's major Fourth of July fireworks show.