Storm drains are underground pipes that carry water from impervious surfaces (streets, sidewalks, parking lots and rooftops) to the nearest public waterway (creeks, ponds, lakes, bays, the ocean etc.).
They are designed to prevent flooding and to carry away excess water.
But they also carry anything that flows with water, e.g. trash, soap oil etc.
As of March 2010, only about 320 acres of watershed drained through filters before entering the Lake (not counting curb inlet filters).
Urban runoff from the remaining 4,330 acres flows to the Lake without any type of treatment, but additional filters are being planned by the City of Oakland.
Sixty two storm drain outlets empty into Lake Merritt. They range in size from several inches across to ten foot wide culverts large enough to walk through. The vast majority of storm drain flows are untreated.
But as of March 2010, five large, continuous deflection separation (cds) filters have been installed by the City of Oakland in the Lake Merritt watershed and more are planned.