Actually the older cities of the Northeast are the worst culprits. They have ancient combined storm drain and sewer systems. Essentially, when it rains the streets drain down the inlets you see on the road, directly into the sewage conveyance systems. In modern cities, these systems are seperate.
When there is a large amount of rain, the systems overload so it has to release excess water so the treatment plant doesn't flood out. What results is RAW sewage dumped into the discharge area. Wilmington and Philadelphia both locally have this type of system that I'm aware of.
But unfortunately our local surfrider would rather focus on the discharge of TREATED effluent in Rehoboth, than the millions and millions of gallons of untreated raw sewage surchage entering the Delaware river from Wilmington and Philadelphia.
And to quote:
source:
http://www.septicprotector.com/Delawaredanger.html
700 MILLION GALLONS from Wilmington which is what 1/10th the size of Philly upstream...not to mention New York, Boston, Dirty Jerz, ect.