The Night Season by Chelsea Cain

He captured the Beauty Killer, one of the most deranged serial killers in the country. Now, Portland police detective Archie Sheridan faces a different kind of killer - a brutal rain season that has flooded the Willamette River, claiming several lives. As water levels rise, so does the fear. Because some of the victims didn't drown;they were murdered.

The first body contains a rare poison. Three others prove to be murders as well. And with each gruesome discovery the medical examiner uncovers, Archie begins to realize he has not escaped his nightmares, even with his deadliest enemy behind bars. The flood has washed up old skeletons from the past. And a ruthless new serial killer rules the night...