EDMONTON -- Canadian National Railway has been fined for a discharge of diesel fuel into the North Saskatchewan River in Edmonton.

Alberta Environment says CN must pay $125,000 after pleading guilty to two charges under the province's environment laws.

Provincial court documents say CN discharged a quantity of diesel on April 9, 2015, into a storm sewer that drains into the river.

Alberta Environment began investigating after people reported a hydrocarbon sheen on the water up to two kilometres long.

CN (TSX:CNR) did not report the discharge until three months later.

CN said in a statement it is committed to operating safely and in an environmentally responsible way.

"We regret this unfortunate incident," said spokeswoman Kate Fenske. "We have already made changes to our operations to avoid a reoccurrence."

An agreed statement of facts says the amount of diesel that was discharged isn't known.

"However, the diesel that reached the North Saskatchewan River as a result of the discharge impaired the quality of water in the river by rendering a portion of it unfit for consumption by humans or animals for a short but unknown period of time."

Alberta Environment said Monday most of the fine is to go toward a trust fund for river and river bank conservation.

CN still faces federal charges over the spill. The company declined comment on that matter.