Montana may be an eviromental backwater but our state D.E.Q. would have our butts in a sling for an illegal chlorine discharge.{fines and possible loss of our operators licenses}It's a shame somebody didn't get a sample to test the chlorine level. Based on the chlorine level the flowrate of the creek and the time from spill to testing you could get a fairly close estimate of how much chlorine was spilled.Here in Mt. to be discharged into a waterway the water has to be dechlorinated prior to discharge,if I remember right the most common agent for dechlorination is sodium thiosulfate.Above 1-MG/L chlorine is hazardous to aquatic life.
chlorine also dissipates{more effective disinfectant} quicker in warm water.For the chlorine to reach the stream either there was an extreme operator error, major mechanical failure or lack of backflow/antisiphon protection so hopefully your state regulatory agency will be on the water system like a bad haircut on Don King.