Portland, Oregon completed a project in 2011 that successfully eliminated almost all of the combined sewer overflows into the Willamette river and Columbia Slough (a swampy area on the south side of the Columbia river near the airport).

There was a ton of work done to reduce the amount of water ending up in the sewer during storms followed by some large infrastructure improvements to improve the carrying capacity of the sewer itself.

https://www.portland.gov/bes/about-big-pipe