Which one of the two landmarks between which Rocky Mountain Trench stretches that is a large natural freshwater lake west of the Mississippi River?
Rocky Mountain Trench:  It is both visually and cartographically a striking physiographic feature extending approximately 1600 km from Flathead Lake, Montana to the Liard River, just south of the British Columbia-Yukon border near Watson Lake, Yukon. Flathead Lake: Flathead Lake (Salish: člq̓etkʷ ) is the largest natural freshwater lake by surface area that is west of the Mississippi River in the contiguous United States, if one takes Red Lake and Lake of the Woods to be north of the Mississippi River, rather than west of the river.
Flathead Lake