Fennon Slough

Flathead Lake is one of the 300 largest natural lakes in the world and is the largest natural freshwater lake in the western United States. Of those large lakes, Flathead is one of the cleanest. Studies at the Flathead Lake Biological Station show that water quality in Flathead Lake is among the best in the world. Due to its massive volume and normally active winds over the surface, Flathead Lake does not freeze over most winters, although the bays often have winter ice cover. The southern half of the lake is on the Confederated Salish and Kootenai Tribes' Flathead Indian Reservation. The tribes manage water quality on the reservation, and jointly manage fisheries in the lake with the state of Montana's Department of Fish, Wildlife and Parks. Anglers must purchase a tribal recreation permit to fish the lower half of the lake. Kerr Dam is located on the southern end of Flathead and controls the upper 10 feet of the lake’s water. Flood control and recreation require the lake level to be at low pool elevation (2883 feet) by April 15, refilled to 2890 feet by May 30, raised to full pool (2893 feet) by June 15, and maintained at full pool through Labor Day.

Lake whitefish and Mysis shrimp are the primary forage species of Flathead’s lake trout. Lake whitefish are the most important forage of large lake trout, while Mysis shrimp are primarily eaten by juveniles. Lake trout are opportunistic feeders, however, and also feed on redside shiner, peamouth chub, northern squawfish, largescale sucker, longnose sucker and sculpin, as well as juvenile trout and perch when available.

Spawning migrations of several different species are evident in Flathead Lake. Lake whitefish make a spawning at the Flathead River outlet near Polson. Lake trout spawn on steep, rocky points during November and December. Catch rates are the highest of the year for lake trout during this time. Cutthroat trout make a spawning run up the lake’s tributaries in late spring. Bull Trout migrate up the Flathead River to spawn in Autumn.