Tens of thousands of water pollution sources must get a permit to discharge into US waterways. Many of them violate their permit, and some are caught.
EPA is making it a little easier to track down some of the known perpetrators. On April 29, 2010, the agency announced that it is providing a Web tool  that summarizes by state the number of smaller dischargers whose actions have been reviewed, the number that had a serious violation of their permit, and the number against whom a state took enforcement action. From that point, you can go to EPA's Enforcement and Compliance History (ECHO) database  and get more details.
- "Clean Water Act Annual Noncompliance Report (ANCR) - 2008 Data for Non-Major Permittees." 
- Press release, April 29, 2010, "EPA Launches New Web Tools to Inform the Public About Clean Water Enforcement." 
The number of violators in any given state can be substantial, and there are thousands in the country. There are significant differences among states in the percentage of dischargers reviewed; for those cases that were reviewed, the percentage of violators tends to be substantial. There also are significant differences among states in the percentage of cases in which enforcement action was taken; many states acted against only a small fraction of known violators.
The EPA site is limited in that it doesn't include information for years other than 2008, it doesn't track large dischargers, and it doesn't address the issue of the huge variability among states in their willingness and ability to address this issue and cooperate with EPA. Some of these shortcomings can be addressed by doing your own legwork on the ECHO site.