Author: Bruce_Staples

Henry’s Fork, March 30th, 2024

Major run-off has yet to reach the river as flows throughout are about normal for this time of the year. Based on Dr. Van Kirk’s information below, a greater than normal run-off could result from the high accumulation of snow in drainage streams originating in the Teton and Big Hole ranges and the southwest corner of Yellowstone Park. Look for run-off to begin as we warm in April and trout will hunker closer the bottom to pick off the abundance of drifting food forms. Right now on the lower river, BWOs and March brown mayflies and midges provide good top water fishing with best success during overcast days with little ( Is that possible?) wind. Presenting streamer, woolly bugger, and rubber leg patterns is also effective especially under these atmospheric conditions. Smaller nymph patterns drifted into holding water will also bring responses from resident salmonids. All this will change with the arrival of serious run-off. We will keep track of its progress to help with planning your fishing strategy.

Headlines from Dr. Rob Van Kirk’s SWE Report for the Henry’s Fork Drainage Filed March 29th

• Heavy precipitation yesterday increased the water-year total and SWE each two percentage points to 104% and 95% of average, respectively.
• Generally wet, cool weather is expected to continue for the foreseeable future.
• At an outflow of 449 cfs, Island Park Reservoir gained 354 ac-ft yesterday from precipitation and is 92% full, compared with 83% full on average.
• The upper Snake River reservoir system is 90% full and continuing to fill, despite 3500 cfs of diversion for managed aquifer recharge and 2500 cfs spill at Milner Dam.

Details

Mean temperature yesterday was right at average, and precipitation averaged 0.58 inch across the watershed, the highest one-day total since March 2. Precipitation heavily favored the upper Henry’s Fork sub-watershed and Yellowstone Plateau, where all five SnoTel stations in that area received at least 0.8 inch of water equivalent. White Elephant led all stations yesterday at 1.6 inches. Totals elsewhere in the watershed ranged from a few one-hundredths of an inch to 0.5 inch. This was enough to move the water-year total up two percentage points to 104% of average, ranking 12th out of the last 36 years (57th percentile).

New snow water equivalent (SWE) accumulations ranged from 0.2 inch at Grand Targhee and Pine Creek Pass to 1.7 inches at White Elephant. All stations in the Island Park/Yellowstone Plateau area received 0.9 inch or better. The day’s totals moved watershed-mean SWE up two percentage points to 95% of average, ranking 21st out of the last 36 years (43rd percentile). Current SWE is at its seasonal maximum at all nine stations. This ensures that annual peak SWE at Island Park will occur later than its statistical average, which is March 27. Average date of peak SWE at Pine Creek pass is March 30, and given current conditions and weather forecasts, this year’s peak SWE there will likely also occur later than average.

Henry’s Fork, March 16th, 2024

Limited access prevails along the lower river. BWOs are out and with an extra hour of daylight more time is available to enjoy trout responding to them. Expect the river to cloud a bit as the warm weather will enhance bank side and nearby melting.

Fun Farm Bridge

• Water-year total precipitation stayed at 106% of average, while SWE dropped a percentage point to 95% of average.
• Dry and warm weather is expected for the next six days.
• At an outflow of 416 cfs, Island Park Reservoir dropped by 92 ac-ft yesterday and is 91% full, compared with 80% full on average.
• The upper Snake River reservoir system is 88% full, compared with 68% full on average.
• Total reservoir storage plus existing snowpack across the basin is high enough that water will be spilled out of the system at Milner Dam starting on March 25.

Details

New snow water equivalent (SWE) over the past four days ranged from 0.2 inch at Pine Creek Pass to 1.7 inch at White Elephant. Current SWE is 95 of average: 92% in upper Henry’s Fork, 97% in Fall River, and 99% in Teton River. Current SWE ranks 17th out of the 36 years (54th percentile), just ahead of 1998 and just behind 2002. After the upcoming six days of dry weather in the forecast, SWE will fall to around 91% of average and drop several places in the rankings.

Dry conditions with above-average temperatures are expected for the next six days, before precipitation returns late next week. Valley snowmelt is expected, and that will increase streamflow a little. However, below-freezing nights will keep mid- and high-elevation snowpack intact before more mountain snow and valley rain is expected late next week. Long-term outlooks are leaning toward above-average precipitation, with gradually cooling temperatures 1–2 weeks out.

 

Rob Van Kirk, Ph.D.

Senior Scientist

Henry’s Fork Foundation

P.O. Box 550

Ashton, ID 83420