Mercer Island’s stormwater system serves a complex network of 88 drainage basins. The system relies heavily on “natural” conveyances. There are more than 15 miles of ravine watercourses that carry stormwater, and 26 miles of open drainage ditches. 40 percent of the ravine watercourses are privately owned, while roughly 70 percent of the drainage ditches are on public property. See Figure 2 – Stormwater Drainage Basins.
The artificial components of the system include 58 miles of public storm drains, 59 miles of private storm drains, and more than 4,500 catch basins.
The public portion of the system is maintained by the City’s Maintenance Department as part of the Stormwater Utility, with funding generated through a Stormwater Utility rate itemized on bimonthly City utility bills.
Mercer Island has no known locations where stormwater recharges an aquifer or feeds any other source used for drinking water.