In the long-running Clean Water Act citizens' suit against the Los Angeles County Flood Control District, NRDC v. Los Angeles County Flood Control District, 9th Cir. October 31, 2016, a unanimous panel of the Ninth Circuit Court of Appeals ruled that injunctive relief against violations of a Clean Water Act permit can be based on past permit violations even though the permit that is the subject of the litigation has been amended with more lenient provisions that make continuing violations unlikely. The case addresses NRDC's efforts to impose strict stormwater controls in Los Angeles County. It first made its way to the U.S. Supreme Court in 2012, when the Court ruled that transfers of water from the storm sewer to a channelized portion of the Los Angeles River were not "discharges" to surface waters under the CWA.

As that decision was coming down, the County's permit was amended to include a safe harbor provision that allows the County to escape penalties for high pollutant levels in some instances. The County argued that the new permit mooted NRDC's claims for violations of the old permit. The Ninth Circuit disagreed, because the County is still subject to effluent limitations for its discharges, so that it is not "absolutely clear" that the County's violations would not recur. The Court stated that it is not the law that relaxations of permit standards, no matter how de minimis, would moot claims of violation of the previous permit. Mootness only occurs where it is "absolutely clear that the allegedly wrongful behavior could not reasonably be expected to recur".

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