The Leschi Basin along the shore of Lake Washington includes eleven hydraulically connected NPDES Basins, numbered 26 through 36. This combined system carries storm water and waste water to a King County pump station. Nearly all these outfalls can be retrofitted by making minor adjustments to weir heights, and relining and replacing pipes. Following these retrofit improvements, SPU will reduce the number of times that polluted runoff and raw sewage overflow into Lake Washington.

The first sewer improvement project was completed in December 2014 with the replacement of an aging outfall pipe near Leschi Marina. The pipe was built in 1931 and had been damaged by tree roots and the installation of the Leschi Marina breakwater system.