Virtually all fly fishing guides and outfitters in Montana watch streamflow data and streamflow forecasts like hawks, especially during runoff season (that is to say: right now) and when summer thunderstorms are rolling around. This is no different than farmers watching the weather forecasts. Here are the important sites to allow YOU to check streamflows, both right now and expected flows for the days ahead.
Montana Streamflow Data: This site returns data from all USGS gauging stations in Montana. The site is organized by river drainage, then from upstream gauging stations to downstream stations. In my area, the Yellowstone Basin graphs from the Lamar River in Yellowstone Park down to the graph at Springdale are the graphs I use most often, with the Stillwater graph secondary. By far the most important graphs for general streamflow are the Corwin Springs and Livingston graphs on the Yellowstone, while the Lamar and Gardner graphs are important for telling me about sudden rises in water level (which are almost always accompanied by mud) due to storms.
Advanced Hydrologic Prediction Service – Billings: Here’s the streamflow prediction site for eastern Montana. This site shows flow graphs noted in the previous link, but also shows predicted flows for the next few days for most gauging stations. The basic graph pictured below is most useful during the spring runoff season when we are trying to plan for future trips based on weather forecast. If you’re looking at a big predicted bump coming up, it’s best to get fishing beforehand, because that bump means mud.
This site also includes an option to view “Probability Information.” This is a longer-range forecast of predicted flows, but it isn’t updated very often and I often find it inaccurate. Here’s a sample graph of probability information:
Select the above graph by clicking the dropdown menu off the lower right corner of the graph, then selecting “Flow – Weekly Chance of Exceeding Levels.” This is most useful to anglers, as flow rather than gauge height determines fishability. Too much water and things are too rough, and probably muddy to boot.