The Big Lost River originates just east of the Sun Valley area over Trail Creek summit in the Copper Basin area. It flows as a freestone river until it reaches Mackay Reservoir. Below the reservoir it becomes a quality tailwater fishery. Virtually undeveloped, this upper river has some of the most breathtaking scenery in all of Idaho. A predominantly Rainbow fishery, the Big Lost River offers a variety of fly-fishing opportunities. From small freestone streams in the headwaters to the larger waters below the reservoir, this system has it all. The river is also blessed with numerous insect hatches throughout its season. It is best fished from July through October with some winter fishing in March.