Nestled in the northeast corner of the park, the Lamar provides good fly fishing from about late July on. Since it drains the largest and most rugged mountains in Yellowstone, the fishing is influenced by runoff until late in July.
Summer storms can muddy it in a hurry, so have a backup plan. With good access off of the road from Cook City (northeast entrance), a good trail system, and a variety of water types, this is a great destination for the fly fisher. Here you'll find everything from riffles to runs to raging pocketwater. Some sections of the Lamar have fairly good access while others are a hike in affair. Just upstream from the Lamar River Bridge is a steep canyon. This is fast paced water with large boulders, cascades and pocketwater. Access here is difficult but possible for an athletic angler.
Just upstream from the canyon the river spreads out and is a popular place for fly fishermen. In most places hiking into this meadow is easy from the road. You can access the upper Lamar from the Soda Butte Creek footbridge. How far you want to go is up to you, but remember, the backcountry campsites must be reserved in advance.
There are a variety of hatches which occur throughout the season including BWOs, PMDs, Callibaetis, Gray Drakes, caddis, Golden Stones and the giant Salmonfly. The Green Drake hatch, in particular, is often a very strong one and it occurs in late August and September.
Be sure to have a good selection of hoppers as this river is one of the best for hoppers in the park.
Although this is a smaller river, the wind makes a 9’ 5 weight rod a necessity. The key to success on the Lamar is to hit the hot spots and cover a lot of water. Ignore the shallow riffles and work the log jams and deeper holes. Put on a fly you have confidence in and work an area with a few casts. Don’t spend the day switching flies, if a good pattern doesn’t show any interest then move on, there may not be any fish in that spot.