Question: Which one of the two landmarks between which Rocky Mountain Trench stretches that is a large natural freshwater lake west of the Mississippi River?
Context: The Omineca Mountains, also known as "the Ominecas", are a group of remote mountain ranges in north-central British Columbia, Canada.  They are bounded by the Finlay River on the north, the Rocky Mountain Trench (here filled by Lake Williston) on the east, the Nation Lakes on the south, and the upper reaches of the Omineca River on the west.  They form a section of the Continental Divide, that, in this region, separates water drainage between the Arctic and Pacific Oceans.  The lower course of the Omineca River flows through the heart of the range.  To the south of the Ominecas is the Nechako Plateau, to the west the Skeena Mountains and Hazelton Mountains, to the north the Spatsizi Plateau and the Stikine Ranges, while east across the Rocky Mountain Trench are the Muskwa Ranges. Fort Ware, is the former name of the community of Kwadacha.  Referred to by locals simply as Ware it is an aboriginal community in northern British Columbia, Canada, located in the Rocky Mountain Trench at the confluence of the Finlay, Kwadacha and Fox Rivers, in the Rocky Mountain Trench upstream from the end of the Finlay Reach (north arm) of Williston Lake.  According to the Natural Resources Canada website, the population of Fort Ware is 350.  Fort Ware is in the federal electoral riding of Prince George-Peace River. Lake Eloise is a large natural freshwater lake on the south side of Winter Haven, Florida.  It is part of the southern Chain of Lakes.  With a 1,174 acre surface area, it is one of the larger lakes in Polk County, Florida.  Lake Eloise has a somewhat oval shape and has a large cove on its south side.  It borders Lake Summit on its northeast and it almost borders Lake Lulu on its northwest.  Lake Eloise is mostly surrounded by residential areas.  However, the northern half of its eastern shore is occupied by Legoland Florida, which was once Cypress Gardens, a large area theme park.  Some citrus groves border the lake on its west side.  Eloise Loop Road passes by the south end of the cove on the lake's south side. The Twin Lakes of Lake Wales, Florida, are two small natural freshwater lakes on the south side of the city.  Sometimes they are simply called Twin Lakes, but sometimes they are individually called Twin Lake West and Twin Lake East.  All shores of Twin Lake West are swampy, as is the area between the two lakes.  The west shore of Twin Lake East is swampy.  The west lake, the larger of the two, has a water area of 18.76 acre .  The water area of the east lake is 15.19 acre . Clear Lake is a natural freshwater lake located in Lake County in the U.S. state of California, north of Napa County and San Francisco.  It is the largest natural freshwater lake wholly within the state, with 68 mi2 of surface area.  Lakes have existed at the site for at least 2,500,000 years, possibly making it the oldest lake in North America. Lake Okeechobee ( ), also known as Florida's Inland Sea, is the largest freshwater lake in the state of Florida.  It is the eighth largest natural freshwater lake in the United States and the second largest natural freshwater lake (the largest being Lake Michigan) contained entirely within the contiguous 48 states.  Okeechobee covers 730 sqmi , approximately half the size of the state of Rhode Island, and is exceptionally shallow for a lake of its size, with an average depth of only 9 ft .  The lake is divided between Glades, Okeechobee, Martin, Palm Beach, and Hendry counties.  All five counties meet at one point near the center of the lake. The Rocky Mountain Trench, also known as The Valley of a Thousand Peaks or simply the Trench, is a large valley in the northern part of the Rocky Mountains.  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.  The trench bottom is 3-16 km (2–10 miles) wide and ranges from 600-900 metres (2,000–3,000 feet) above sea level.  The general orientation of the Trench is an almost uniform 150/330 degree geographic north vector and has become convenient for north/south aviators. Tulare Lake, named Laguna de Tache by the Spanish, is a freshwater dry lake with residual wetlands and marshes in the southern San Joaquin Valley, California, United States.  After Lake Cahuilla disappeared in the 17th century, Tulare Lake was the largest freshwater lake west of the Mississippi River and the second-largest freshwater lake entirely in the United States (excluding the Great Lakes which share area with Canada), based upon surface area.  A remnant of Pleistocene-era Lake Corcoran, Tulare Lake dried up after its tributary rivers were diverted for agricultural irrigation and municipal water uses. 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. Bear Lake is a natural freshwater lake on the Utah-Idaho border in the Western United States.  About 109 sqmi in size, it is split about equally between the two states; its Utah portion comprises the second-largest natural freshwater lake in Utah, after Utah Lake.  The lake has been called the "Caribbean of the Rockies" for its unique turquoise-blue color, which is due to the reflection of calcium carbonate (limestone) deposits suspended in the lake.  Its water properties have led to the evolution of several unique species of fauna that occur only within the lake.  Bear Lake is over 250,000 years old.  It was formed by fault subsidence that continues today, slowly deepening the lake along the eastern side.
Answer: Flathead Lake