After much legal wrangling over earlier rulemaking efforts, on May 19 EPA issued a new final rule pursuant to Section 316(b) of the Clean Water Act. You can find a helpful fact sheet about the rule here

EPA estimates that the rule will apply to approximately 1,065 existing facilities, and it expects that the industrial sectors most affected include chemical manufacturing plants, iron and steel manufacturing plants, and aluminum manufacturing plants. If any of your plants draw at least 25 percent of their water exclusively for cooling purposes from an adjacent body of water, and the facility is designed to take in more than 2 million gallons of water per day, you must verify whether you are in compliance with the new rule.

Affected facilities will need to implement the “best technology available” to reduce fish impingement.  In practice, this means deploying one of seven options pre-approved by EPA. Some affected facilities will also need to conduct site studies to see if any site-specific controls need to be implemented.  Finally, if an affected facility intends to add new units to add electrical generation capacity, those units will need to meet one of two defined standards.