Montana and YNP Summer Streamflow and Fishing Forecast for Early May 2021

Posted on May 2nd, 2021 in Season Forecasts, Weather & Water Conditions


Winter snowpack and how this snow melts from April through June are the primary drivers of summer water conditions: whether the water’s high or low, whether it’s warm or cold, etc. This in turn drives the fishing. In general, we like to see slightly above-normal snowpack since this leads to cool water and aggressive fish.

The lower the snowpack and hotter and drier the summer weather, the more likely we will have tough or limited fishing on late summer afternoons due to warm water. In exceptionally hot/dry years, we may even have stream closures in core watersheds in our operations area. Some watersheds are now always closed from 2:00PM to midnight in late July and August, though none of these are important fisheries at this time.

Winter and Early Spring Weather Summary

Winter started with a bang in early October, with over a foot of snow and below-zero temperatures even in Livingston. Things then got dry by early November and stayed that way through January. The early part of winter was so dry that Bridger Bowl Ski Area had to delay its opening until almost Christmas.

February really saved our bacon. The whole month was very cold and very wet. In fact the ski area set its record for February snowfall. This heavy snowfall propelled Yellowstone and Montana snowpack to above normal levels in most river drainages by mid-February. Those that didn’t jump above average climbed over the 95th percentile, close enough.

March returned to slightly warm and slightly dry conditions for most of the month, with a few cold and wet outbreaks. Snowpack increased in a general sense, but declined against average.

April turned out to be highly variable, despite a forecast that predicted warm and dry. We’d go from a couple days of light snowfall that mostly melted by noon (here in the low country) to bright sun and 65 degrees a day later. The last few days of April were exceptionally warm, with temperatures in Livingston reaching the mid-80s on the 30th. This prompted an early start to the snowmelt.

Today things have cooled back down into the 50s at low elevation and are forecast to remain there through the middle of the week (today is Sunday). After a brief warmup to around 70 degrees, it’ll be back into the 50s again for most of the first half of May. The 6-10 and 8-14 day NOAA outlooks are both calling for a very high likelihood of below normal temps and above normal precip through May 15.

Current Yellowstone and Montana Snowpack

Current snowpack ranges from 71% to 91% of normal in the Yellowstone Country Fly Fishing areas of operation. This is a 10+ point drop for most basins just in the past 5 days, due to the sharp warmup. Most of April, snowpack remained where it had been in our last update, in the 85-97% range, though the latest round of snow around April 20 actually bumped things to 100% in one drainage.

Snowpack is 85% of normal in the  Upper Yellowstone Basin in Wyoming, basically meaning the Yellowstone River and all its tributaries upstream of Gardiner, Montana. This comprises all the core summer water within Yellowstone Park. The Upper Yellowstone Basin in Montana is at 87% of normal. This includes the Yellowstone Basin (including the Wyoming water) downstream to the Clark Fork confluence near Billings, Montana, at the eastern edge of YCFF’s operations area and beyond the Parks Fly Shop operations area.

Besides the above basins, which are the most important river drainages for both businesses, snowpack ranges from 71% of normal in the Jefferson Basin to  91% of normal in the Gallatin Basin. The worst snowpack is in the Jefferson Basin, which isn’t very important to us, but it’s not much better in the Madison Basin inside YNP, at 77% of normal.

Snowpack decline should level off through the first half of May provided we don’t get much warm rain at high elevations. In fact, it’s very possible snowpack numbers (as a percentage of average) will increase, particularly if the second week of May is as cool as the forecast says. It should be cool enough to snow at high elevations. I wouldn’t be surprised to see numbers increase 5-10% by mid-May except in those basins that are already under 80% of normal, since they’re lower elevation and already melting furiously enough they’re just going to blow.

Here’s a map showing Montana snowpack. The YCFF area of operations is circled in red. The Parks’ Fly Shop ops area is about 2/3 the size.

may 02 2021 snowpack in montana

General Expectations for Summer

Based on current Montana snowpack and predictions for spring weather, we expect the following for the core June-early September season. Conditions after mid-September depend on fall rain and snowfall:

  • An early beginning to the spring snowmelt, with a pause in mid-May that should bring an extended early “runoff window” on all area rivers
  • An early end to the spring snowmelt
  • Below-normal stream levels after July 1
  • Above-normal water temperatures in all river basins between July 15-20 and August 15-20
  • More 2:00PM fishing closures than usual between the above dates.
  • Potential for poor fishing conditions after 2:00-3:00 during hot spells on many fisheries that do not meet closure criteria (note: Montana closes trout fisheries from 2:00-midnight when they touch 73 degrees three consecutive days to avoid stressing trout; above 70 is poor fishing anyway)
  • Potential for round-the-clock fishing closures in a few low-elevation fisheries (note: these are unimportant fisheries to most anglers in midsummer)
  • Spookier than normal fish in late July, August, and September

Water-Specific Predictions

This is the meat and potatoes for most visiting anglers. Here’s when we expect various important fisheries to blow out with runoff, clear from runoff, and how we expect they’ll fish through the summer. Within each jurisdiction, waters are discussed in approximate order of when they’ll leave spring runoff.

Montana Fisheries

Private Lakes: The private lakes are now ice-free and fishable. As usual, they will be best from late April through mid-June (Story Lakes) or mid-July (Burns Lake). We are looking to add the Sitz Ranch Lakes to our repertoire. They are best before early July.

Missouri River – Land of Giants: Holter Reservoir is ice-free so jet boat trips are now available through Thanksgiving. The only limiting factor from here out is wind (N winds of 20+ mph are extremely dangerous). Due to low flows from the dam upstream, we expect things to get weedy by early July and be pretty tough in August and early September. Otherwise, this is a dependable fishery regardless of weather. Very low flows have thus far made things so-so at “LoG.”

Area Reservoirs: All low-elevation reservoirs are now ice-free. Wind permitting, we’ll be fishing some of those around White Sulphur Springs (there are four) this week. As usual, the best fishing is from ice-out through June on the low-elevation reservoirs near us.

Madison River (Lower): The Lower Madison is a perfectly reasonable fishery through the winter, though it’s best in May and early June. Due to expected low flows, the Lower Maddy may get questionable by June 20-25 this year. It now always has “hoot owl” 2:00PM to midnight restrictions from July 15 through August 15. These may be expanded to 24-hour closures this year if summer is hot.

Jefferson River: The “Jeff” is now blown with spring runoff, maybe through mid-June. The fishable window thereafter will depend on how fast it gets hot. Expect 2:00 closures after July 4, possibly expanding to a 24-hour closure from mid-July through August.

Boulder River: The Boulder has gone from 200cfs, less than half “floatable” flow, to 1200cfs in three days. 1200 is ideal floating level, but with a rise that fast it will probably be muddy. Once flows drop a hair early this week due to the cooldown, the Boulder will be an excellent choice until at least May 15, and probably more like May 20 before blowing again. Thereafter it will be muddy most of the time until about June 20. Note the “most of the time.” Unique among float rivers that ever get too muddy (the Lower Madison and Missouri never get too muddy to fish), the Boulder often drops into fishable shape for 2-3 days at a time even during the middle of the spring runoff. All it takes is a couple cold days. When these “runoff windows” occur, fishing is usually fantastic. The Boulder will drop from runoff for good around June 20. It will get too low to float by July 20 this year, and maybe as soon as the 15th. The headwaters will be fine to wade fish all summer, though the lower river may get low and warm enough that fishing after 2:00PM is not a good idea. This will be most likely the last week of July and first half of August.

Stillwater River: Upstream of the Rosebud confluence, the Stillwater tracks similarly to the Boulder in terms of entering and leaving runoff, as well as its floatable levels and late July and early August water temps. Downstream of the Rosebud, it’ll clear up around July 1 or perhaps a few days later. It should remain high enough to float until August 25. After that will depend on summer precip. High water temps suggesting a 2:00 quitting time are likely on hot days in August downstream of the Rosebud, though the Stillwater almost never gets statutory closures.

Yellowstone River: The Yellowstone in Montana has entered the early stages of spring runoff fast and hard, rising from under 2000cfs on April 27 to 6360cfs right now. It will drop into fishable shape sometime this week and stay there until May 15-20 if the forecast cool weather comes past. Thereafter, the river will remain in runoff for about a month except for the possibility of one or two runoff windows as described above for the Boulder. The Yellowstone will begin leaving runoff between June 20 and 25. The stretch from Gardiner to Yankee Jim Canyon and Carbella to Mallard’s Rest drops into shape first. Yankee Jim Canyon and points through and east of Livingston need another 10 days to two weeks to drop to safe levels (for us, 6000cfs on the Corwin Springs Gauge). The most consistent streamflows will be in July. The last week of July and the first three weeks of August may be too warm in late afternoon on a day-to-day basis from Gardiner to Livingston between July 20 and August 20, but 2:00 closures are unlikely unless runoff starts exceptionally early AND summer is very hot. East of Livingston, mandatory 2:00 closures are possible in the same timeframe and a lot of afternoons will be too warm on a day-to-day basis. Basically if it’s hot, bright, and dry, expect to take out early east of Livingston.

The Salmonfly hatch on the Yellowstone will almost certainly be fishable this year. Expect them beginning as the river drops out in late June through July 4, but probably not much or any thereafter. Ants will likely be better terrestrials this year. Terrestrial season is likely to start July 20 this year (early).

Most Small Streams: Except for a couple low-elevation odds and ends that fish well in late June, like the Musselshell River and upper Smith River, most small streams will begin coming into shape in the second week of July and be best between mid-July and Labor Day. Most run ice-cold, which makes them excellent bets in late July and August on afternoons when big rivers may be too warm.

Yellowstone Park Fisheries

Note: The Yellowstone Park season opens May 29 this year, as late as it possibly can. The season always opens at sunrise the Saturday of Memorial Day Weekend. Because of the late opener, we expect all of the early season fisheries to be ready on the opener, which is unusual. On the other hand, the late opener means that the Yellowstone from the falls to the Lamar (Grand Canyon), Slough Creek, and the Gardner River are almost certainly going to be muddy. In years with early openers, these waters can be fishable for opening weekend before they enter runoff.

Firehole River: The Firehole will be clear enough and fishable on the opener and be best before June 15. It may be too warm on bright afternoons thereafter and will almost certainly be too warm after noon no later than June 25, and possibly by June 20. The park has apparently stopped closing geothermally-heated waters like the Firehole when water temps get too high, as they always do in July and August, but the Firehole should be closed to all fishing in July and August downstream of Old Faithful this year, even if it isn’t. Above Old Faithful is a fine mountain creek in high summer, since it has little geyser water.

Madison River in YNP: The Madison will be clear enough and fishable on the opener and best before June 25. It will be too warm on bright afternoons thereafter and too warm period between July 4 and late August. Closures should but probably won’t be instituted from July 4 through August 15-20.

Gibbon River in YNP: The Lower Gibbon below Gibbon Meadows will be low and clear enough on the opener and best from June 5 through June 20. It will gradually get too warm thereafter and be too warm in July and August. From Norris Geyser Basin through Gibbon Meadows will drop into shape June 5-10 and be best through June and too warm between July 4 and Labor Day. From Virginia Cascade to Norris will drop into shape around June 20 and be best in July and August. Upstream of Virginia Cascades is still undergoing grayling and westslope cutthroat restoration so is not recommended this year.

YNP Lakes: Yellowstone Lake will be fishable out of the gate more than likely. Cascade, Grebe, and other small lakes in the central part of the park may be reachable on the opener but will almost certainly be ready by June 5. These lakes will be best in June and early July. Note that Blacktail Ponds opens in early July. It will likely be too warm already by then this year.

YNP Central Plateau Streams: Streams like Nez Perce will drop into shape around June 15-20 and be best before early August.

Gardner River: Occasional runoff breaks are possible before June 20 downstream of Boiling River, but it will drop into shape for real downstream of the High Bridge June 20-25 with the Salmonfly hatch thereafter downstream of Boiling River. The Salmonflies will last until July 20 in Sheepeater Canyon. The river below Boiling River will be too warm after 2:00 and maybe in general between July 20 and early September. Osprey Falls to Boiling River will be fine but likely crowded in easy-access areas all summer. The upper river (brookie water) will come into shape around July 1 and be best before August 15.

Yellowstone River: The Yellowstone River above the falls always opens July 15. From the falls to the Lamar (Grand Canyon) will begin coming into shape around June 15 and be best from the point it’s barely fishable until about July 20 and again after September 15. It will be cool enough all summer, but can see heavy pressure in easy-access areas in late July and August. In the Black Canyon from the Lamar to Gardiner, it will track similarly to the Yellowstone outside the park as described above, except the Salmonflies will be present primarily in the first half of July and there’s less likelihood it’ll get too warm on late summer afternoons.

Lamar, Soda Butte, and Slough Creek: Rough stretches of Slough and the Lamar will start coming in around June 25. Meadow stretches will come in around July 1. The fishing will be best before mid-August with low but not warm water conditions thereafter. We anticipate OVERWHELMING crowds on roadside easy-access portions of these streams this year. We’re talking people every 25 yards. This is due to the pent-up demand from people who didn’t travel last year combined with people who still can’t travel to more exotic locations overseas.

Rough Small Streams: As usual, small streams draining areas other than the park’s central plateau are steeper and rougher than others. They will come in around July 1 and be best from July 15 through August.