The material known as fly ash was in some train cars that derailed Friday into 40 Mile Creek near the resort community.
Parks Canada spokesman Bill Hunt says CPR crews have hauled out all but one of the cars containing fly ash from the water.
He says the challenge now is to ensure that spilled material isn't disturbed before it can be removed from the icy creek.
Hunt says workers have built a makeshift dam across the creek to prevent the fly ash from moving downstream.
He says the fly ash could make it harder for fish to breathe.
"We are working with experts to understand what the trace elements are in the ash and at what levels they might be a hazard for fish," Hunt said Monday.
Fly ash is a byproduct of burning coal that is found in the stacks of coal-burning furnaces. It is sold to companies that use it to make concrete.
Hunt said the rail bridge has been repaired and two train cars remain in the creek, one with fly ash and the other carrying soybeans.
The Transportation Safety Board has said that fly ash is not classified as dangerous under the Dangerous Goods Act.