Henry’s Fork, May 6th, 2023
Ground water flooding impacts access at the Vernon Bridge.
Headlines from Dr. Rob Van Kirk’s SWE Report Filed Yesterday for the Henry’s Fork Drainage
- The month of April was cooler and slightly drier than average.
- Watershed SWE peaked last Monday, at 119% of the average annual peak; 94% of this year’s peak SWE is still on the ground, compared with 87% on average.
- Natural flow is increasing in response to snowmelt; moderate rainfall is expected later in the week.
- Last week’s freshet exported 321 tons of sediment out of the Island Park to Pinehaven reach, compared with about 30 tons had the freshet not occurred.
Here is some interesting info from Dr. Van Kirk on the effects of the freshet performed for four days in late April.
Over the four days of the freshet operation, 321 tons of fine sediment were exported from the river reach between Island Park Dam and Pinehaven. Based on pre- and post-freshet numbers, we expect that about 30 tons would have been exported over those four days had the freshet not been done. For reference, the average annual net export of fine sediment from the reach over irrigation years 2016-2022 is 650 tons. So, the freshet moved half of the average annual sediment budget in three days. Freshet-related turbidity has dropped to near pre-freshet levels, but now increased stream flow from natural snow melt is increasing turbidity again, especially in the lower watershed.
Meanwhile, warmer water temperatures are accelerating aquatic insect development, although hatch timing remains later than average and even later than last year over the lower watershed. We expect rapidly warming conditions over the next few days, but the cooler forecast for late in the week could slow down progress toward average hatch timing ( His info on aquatic insect hatches helps explain the relatively slow recent fishing on the lower river).
Rob Van Kirk, Ph.D.
Senior Scientist
P.O. Box 550
Ashton, ID 83420
208-881-3407 CELLG