Montana Fish Wildlife and Parks describe the Gallatin River as about twelve miles long from its origin at the confluence of the West and East Gallatin rivers to Three Forks, Montana, where it joins the Jefferson and Madison rivers to form the Missouri River. The river flows through a narrow valley consisting of agricultural and grazing lands at elevations less than 5,000 feet. The banks are primarily undercut, and long, deep pools provide much of the fish cover. Except for the East and West Gallatin rivers, tributaries to the Gallatin River are limited to a few spring creeks. Water can be slightly turbid year-round due to the sediment input from the East Gallatin. The Gallatin River below the confluence of its forks suffers from sedimentation, warm water temperatures, dewatering, and the presence of M. cerebralis, the causative agent of whirling disease. Trout populations decline in the lower river due to these factors and a variety of other cumulative impacts.