The Kelsey Creek drainage basin is about 10,870 acres (4,400 ha),[2] including about 75% of the city of Bellevue in addition to a portion of Redmond.

[5] Originally the Kelsey Creek drainage basin included what is now Phantom Lake.

At the time, the slough was both wide enough and deep enough to accommodate log rafts, launches, tugs, small steamers, and stern-wheelers.

The Mercer Slough was partially dredged in the 1920s to make it navigable to small watercraft.

[4] The King County government and City of Bellevue began acquiring parcels in the Mercer Slough in 1957 for preservation of the wetlands and surrounding natural areas.