The Jefferson is born near Twin Bridges, Montana, where the Big Hole River and the Beaverhead River merge together. 80 miles in length, the Jefferson makes its way through private farm and ranch land, to the confluence with the Madison and the Gallatin Rivers, forming the Missouri River. The late summer months into the fall can be productive with terrestrials such as grasshoppers, beetles or ants. Fall fishing on the "Jeff" is the recommended time to target big browns on the fly, since they are more vulnerable during the pre-spawn. If you're looking for a scenic float away from some of the higher pressure rivers consider a day on the Jefferson.