Summer Streamflow and Fishing Forecast Update

This post is about likely summer and early fall water and fishing conditions, based on current snowpack and current predictions for late winter and early spring weather. Please click the “Season Outlooks, Weather, and Water Conditions Updates” link in the Topic menu to view previous updates to learn how weather and water have tracked since mid-February. I’ll be posting updates on about a biweekly basis until about the middle of April, then weekly updates thereafter.

If you are unfamiliar with how streamflows work in the Rockies, here’s a quick rundown. To a large degree, streamflows are based on winter snowpack in the mountains and how this snow melts (when it starts and how fast it runs downhill). Summer rains have only minor effects and mostly impact day-to-day clarity rather than overall flow. Only from about the beginning of September onward are rain events likely to dramatically increase flow. Before this, the rivers simply have enough snowmelt in them and are running high enough that rain barely causes most streamflows to tick upward except on small rivers prone to thunderstorms, like Soda Butte Creek. In sum, snowpack is vital for determining the duration of spring runoff and the overall summer streamflows and water temperatures. Generally speaking, all rivers save those immediately downstream of large dams (such as the Missouri in its chain of lakes and the Madison below Ennis Lake) endure a spring runoff period of four to eight weeks of high and unfishable water levels, with the overall length and severity of the runoff depending on the body of water in question, the amount of snow in the hills, and when this snow starts melting. Therefore, knowledge of the amount of snow in the mountains and the way we expect this snow to melt is critically important for trip planning and predicting what we expect the summer and early fall fishing to be like. This crystal ball gets clearer the later into the spring we get.

We’re now starting to get a far clearer picture of what to expect for summer. Since the previous update, a lot has changed, so pay close attention to the following info if you’re still in the process of planning your trip.

Late March was exceptionally warm and moderately wet. This has led to decreases in snowpack across all river basins. Some snow sensors saw increased snow depth and moisture, but not enough to keep up with the average increases in late March, while other snow sensors (especially those at low elevations) saw significant decreases even in “real” terms. Barring heavy snow at high elevation in late April, which is not forecast, it now seems likely that ALL snow sensors have seen snowpack for the year and the snow has begun to melt. This peak snowpack has occurred between three weeks and five weeks ahead of schedule. If you need proof of global warming, here it is. Since I (Walter) began guiding back in 2001, peak snowpack has been occurring earlier and earlier.

Overall snowpack numbers are still strong. The most important basin in the PFS operations area, the Upper Yellowstone Basin in Yellowstone Park and northwest Wyoming, is at 137%. This is a relatively small decline of seven points since the last update, and recent cool weather has kept things stable for about a week. Other basins have seen bigger declines, generally around ten points with a continuing slow downward trend despite the cooler weather. Other basins in our operations area now range from 91% to 118% of normal, with the lowest percentages to our NW, in the Missouri and Gallatin River drainages.

Compared to last season, snowpack in the Yellowstone’s drainage basin as well as the Madison’s drainage basin are running substantially higher than they were at this point last year, in the case of the Yellowstone 40 POINTS HIGHER. The Missouri River and Gallatin River basins are now several points LOWER than they were last year, a very drastic change from my most recent update (March 21). Based on the current 6-10 and 8-14 day NOAA outlooks, I expect all basins to increase in comparison to last year through the middle of April, since last year early April was exceptionally warm and the past few days and the long-range forecast are calling for normal-to-cool and wet weather.

So what’s all of this mean for summer fishing???

Here it is in a nutshell…

We anticipate above normal streamflows in the Yellowstone drainage throughout the summer season, particularly within Yellowstone Park and the upper end of Paradise Valley. Above normal streamflow is also likely in the Madison drainage, particularly within Yellowstone Park. Streamflows in the Missouri River should be near-normal, with below normal snowpack in the Missouri basin itself offset by above normal snowpack in the basins of its headwaters (except the Gallatin).

In general, we expect waters to enter runoff early and drop out of runoff at an average (Madison, Missouri basins) to slightly late (Yellowstone Basin, including the Lamar) date. This is a big change from previous updates, as the warm weather in late March has primed all basins for runoff. It is likely that runoff will begin on all rivers with the first big warmup, probably sometime between April 20 (we hope not) to May 5. If we do have an April 20 (or so) start to runoff, it is STILL possible the Yellowstone will clear from runoff in late June and we will see low water in August. A month ago I would have never predicted this, but the snowpack warmed up A LOT in late March.

Here are predicted clearing dates for the important waters in our operations area, as well as some general fishing notes. I have included a lot more info in this update, particularly for Yellowstone Basin fisheries within and just outside Yellowstone Park.

The Yellowstone River Outside YNP: I expect the Yellowstone to enter runoff early this year, no later than May 10 and probably by May 5. Even late April is possible, though we hope not. I would not expect good Mother’s Day Caddis hatch fishing this year unless late April and early May are exceptionally cool and the remaining mid-elevation snow can therefore melt slowly (this is not in the monthly and seasonal NOAA long-term outlooks). The Yellowstone outside the park will LIKELY drop into floatable shape sometime during the first week of July, though if it enters runoff in the last week of April rather than the first week of May, the last few days of June are still possible. Here’s a called shot: I predict we will run our first post-runoff drift boat trip on the Yellowstone on July 3-5. It is now 95% certain the Yellowstone will be floatable by July 8. I expect decent to good conditions for the Salmonfly hatch this year, which should begin just as the river is coming into shape. Water conditions should be terrible/unfishable in late May and June, excellent in July. and good in August and early September. Last year saw excellent conditions in late June and early July, fair conditions in late July and early August, and poor conditions (including closures) in late August and early September. Late September and October conditions are always dependent on short-term weather. Provided runoff begins later than April 25, we will be safe from any water temperature-related stream closures in August. If runoff holds off until May 5, we will have excellent conditions in August as well as July. I expect Yankee Jim Canyon to become floatable around August 1 and remain floatable for about 5 weeks.

The Yellowstone Inside YNP: Generally similar to the Yellowstone outside the park, though the Grand Canyon should be fishable with nymphs and streamers by June 25 at the absolute latest. It will still be very high, however. The Black Canyon’s Salmonfly hatch will continue in spotty fashion at least through July 25 provided runoff holds off until April 25 and I would not be surprised to find the fish willing to eat a Salmonfly or two near cold tributaries until August 1. The Yellowstone between Yellowstone Lake and Sulphur Cauldron should offer better fishing this season than any year since the late 1990s. The lake trout populations are down, cutthroat numbers in the lake are rebounding in response, and we’ve seen much stronger cutthroat numbers even during the past couple drought years than any other year since the late 90s, and flows from Yellowstone Lake will be higher than any other stretch of water in our entire operations area, which will push the cutthroats towards the banks where they will be easier to find. We WILL guide here this season, for the first time in more than a decade.

The Lamar Drainage: Slough Creek should become fishable in the first week of July, the Lamar and Soda Butte around July 7 to 10. We expect excellent water conditions on these streams this year, with some relief from the low water, crowded conditions, and overstressed fish of the past two August/September periods. I predict the most consistent (though not necessarily “best”) fishing on Slough Creek will occur in the second half of July, and on the Lamar and Soda Butte during the last ten days of July and first ten days of August. Slough Creek is sometimes fishable right at the beginning of the park season, for a few days, before the onset of heavy runoff. This is very unlikely this year due to the warm snowpack and likelihood of a heavy and early runoff.

The Firehole, Gibbon, and Madison: The Firehole will be fishable on the park opener (May 27), though likely still high and tea-stained. It will be the only river that will fish well on this date except POSSIBLY the Gardner, if the weather is cool. The Madison and Gibbon below Norris will drop into shape around June 1-5. The Gibbon above Norris will not be ready before June 10 and will be much better after the 20th. Good fishing on the Firehole will continue until June 25 and will become day-to-day between June 25 and July 4, with cool temps necessary for acceptable fishing. July 4 through Labor Day will see water temps too high for the Firehole to be fishable. The Madison and Gibbon below Norris Geyser Basin should fish well through July 1 and in the mornings through at least July 4, though cool weather will extend the general fishing through July 4 and the morning fishing through July 15. They will be too warm from July 15 through Labor Day in a general sense, though the Madison from the Barns Pools down to Hebgen Lake can be worth it in the mornings after August 20 provided nights are cool.

Gardner River and Tributaries: The mainstem Gardner downstream of Boiling River is always fishable from the beginning of the park season for fit anglers willing to handle very rough and physical walking and wading FOLLOWING A DAY OR TWO OF COOL, DRY WEATHER, WHILE BEING TOO HIGH AND DIRTY AT OTHER TIMES. This pattern will continue through June 15-20 this year, with the river fishable almost every day thereafter. The river will be fishable on a day-to-day basis from Osprey Falls to Boiling River after June 20 and for good after July 1. Above Osprey Falls, consistent fishing will not begin until after July 4 and maybe as late as July 10. The tributary creeks will come into play the last ten days of June or possibly as late as July 4. The lakes will be ready by the opener, though Fawn will be difficult to access until June 20 due to the river and stream crossings required. Joffe Lake MAY be the only body of water across the north end of the park to be easily accessible, open to fishing, and fishing well on the opener. It depends on the day-to-day weather.

Small Streams: With the exception of those in the Firehole drainage (ready in mid-June and good by July 1) and those in the Gardner Basin (see notes in the Gardner entry above), most small streams will not be fishable before mid-July and will be best in August and early September. This will be a good late summer for the small streams, which is when we most love to fish them.

Lakes in Yellowstone Park: Yellowstone Lake will be best before July 1 and should be fishable from the opener. Lewis Lake will ice-out by the end of the first week of June and will fish best in the two weeks thereafter. Shoshone Lake will ice-out at the same time but will be difficult to access accept by boat until late June, due to snow on the trails. Smaller lakes except Blacktail (which opens July 3) will be ready to fish and accessible by June 10 and will fish best from this point through early July. See notes in the Gardner River entry for lakes in its drainage.

The Lower Madison below Ennis Lake: Recent reductions in the snowpack of the upper Madison River Basun outside YNP mean that the lower Madison below Ennis Lake will see near-normal water conditions this year, maybe just SLIGHTLY high. The river is floatable now and should remain so through June 20 at least. Here’s hoping for cool weather in late June to keep this water cool enough to fish well until the Yellowstone clears. Otherwise, we might have a week or ten days when the Yellowstone is still too high and dirty and this water is too warm, and therefore not have anywhere to float…

The Missouri: Recent reductions in snowpack in most of its tributary basins as well as the mainstem Missouri basin itself mean that near-normal water conditions are likely. This is a bummer as this water could use a good flush to reduce weed growth. We still expect good fishing on my (Walter’s) jet boat guide trips here from late this month through early June, but this water will likely be low and full of weeds and therefore not worth the trip from late July through September, as has been the case the past two seasons.

Private Lakes: The lakes (particularly the Story Lakes) are already in shape and fishing well. They should see normal conditions this year, with Story and Merrell fishing well into late June and Burns fishing through July.

Paradise Valley Spring Creeks: The creeks are not impacted by runoff. Fishing is best before mid-May and again from June 20 through July, then gets steadily better once more after September 15.

Overall, we expect solid average to somewhat above average streamflows this season, based on current snowpack numbers and predicted forecasts/outlooks through April. We will have higher water than 2016, 2015, 2013, and 2012, with lower water than 2014 and 2011. For the moment, we anticipate conditions comparable to 2010.

3 comments on “Summer Streamflow and Fishing Forecast Update

  1. Thank you for the very informative and useful update! Question… will the Missouri be worthwhile to fish the first 10-days of July? Thanks, Rick

    • Yes, early July is a great time on the Missouri. Since it’s protected by dams, runoff is ALMOST a non-factor there (a few days of muddy water are possible downstream of the Dearborn River confluence in May).

  2. Many thanks! This will help select exact dates for our visit. We’ll be in touch regarding a float, or maybe a trip on the Yellowstone in the park.

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