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.
During the first part of the month of July, you were not able to fish all the streams in the park because some we still closed to fishing. During the last part of July, the Madison inside the park, most of the Firehole and the lower Gibbons River will be too warm to fish. Most of the streams in lower elevations will be too warm to fish. The northeast section of the park including the Lamar, Soda Butte and Slough Creek are usually in good shape but you always have to check the stream flow data to know for sure because thunderstorms can foul the water up. The upper section of the Yellowstone River will be open to fish by the 16th of July and will be a prime destination as well as lower sections of the Yellowstone River. The southwestern section of the park will be in good shape and that includes the Belcher and Fall Rivers.
The last part of July is a good as any time and maybe even the best time to fish the park. Just make sure you plan the trip well in advance so you can choose where you want to stay without concerns about motels, cottages and campgrounds being filled. You can always call or write us at Perfect Fly to help you with planning your trip including such things as what you need in the way of what flies to have with you, as well as other gear, clothing, equipment, places to stay, streams to fish, etc.