Assuming it's none of the lakes mentioned in the article "15 Best Lakes of Montana?"
https://www.thecrazytourist.com/15-best-lakes-montana/
And what about these private air strips?
https://glaciermt.com/airstrips
Possible places to start digging, see if there's any overlap. Maybe one of these private airstrips is located near the lake in question. In any case, praying for good to conquer evil in Montana.