During the last part of the month of August, the number of anglers fishing slow down some. The season so far has seen a lot of fishing pressure on the easily accessible, famous rivers. Even so, there are plenty of streams and portions of streams that have not been fished hard at all. Hatches fall off some but are still the most important thing even though terrestrial insects such as grasshoppers, beetles, and ants become important as well. Bright sunshine and low water will sometimes drive the fish down deep, so don’t forget about streamers, especially sculpin and leech patterns.
The Yellowstone River, both the Grand and Black Canyon sections, are still very good choices in the last part of August. For one important reason, the fast water canyon waters rarely warm up to the point warm water will hurt the fishing. Another reason is there is little to no road access to either canyon and the pressure is always less than many other streams. Much of it can require a little hiking.
The upper Gardner River is one of our favorite locations in August. The lower part of the river will get too warm but not the sections in the higher elevations. From Boiling River upstream to Osprey Falls is a good area with some fast, pocket water. This will take a little hiking though. Obsidian, Winter, Indian and Panther creeks at the top of the Gardner River drainage are all small streams that are good choices.
Streams in the Lamar Valley, the Lamar, Slough Creek and Soda Butte Creek are all still excellent choices, especially if you get away from easy road access where there has been pressure. They all contain stretches that are tougher to reach if you are willing to do a little hiking.
The Snake and Lewis rivers in the upper sections require a little hiking but are still very good choice. The Bechler River and Fall River in the Southwestern section of the park are very good choices although they also require a little hiking to get to the best parts. They are rarely crowded.
The canyon section of the Snake river is a good choice in late august. Upper sections of the Firehole river is a good choice in late August.
The following is why it is important to decide prior to making your trip. Since Yellowstone National Park is a huge park, it can involve traveling across the park in order to fish a stream you want to fish if you wait until you get there to make the decision as to where you are going to fish. You need to be located in a town or campground near the stream or otherwise you could spend almost half of the day traveling back and forth across the park at relatively slow speeds. In other words, you need to plan your trip in advance of traveling to the park. There is always some general variation in the weather from year to year as well as variations in the snow pack which controls the amount of water in most of the streams. I am basing the following information on the typical or average weather conditions we have experienced during the years we have been fishing in the park.