I'm not actually 100% certain that this is C. erronea; my larval ID skills are rudimentary. But definitely Cordulegaster, and captured just a few meters downstream from where the 2018 meeting of the Dragonfly Society of the Americas, Northeast Regional chapter (NEDSA), saw an adult erronea patrolling, and Valinn Ranelli collected a larva. This remains the only known breeding site for the species in MA (one was discovered in VT the following summer).