A cool, rainy sunset at Vernon Bridge in May.
Climate. Peak snow water equivalent (SWE) was 71% of average and 3rd lowest in the 1989-2022 record, but SWE peaked on April 24, 12 days later than average. Temperatures were well below expected values in the spring but set numerous record highs in July, September, and October. For the water year, mean temperature was 1 degree F above average. Water-year total precipitation was 91% of average—much higher than peak SWE—thanks to heavy precipitation in June and again in August. Short-term drought indicators improved modestly over the water year, while medium- and long-term indicators worsened. Most of the watershed ended the water year in “moderate drought,” where it started.
Natural flow. Natural streamflow was 72% of average, ranking as the driest year in the 1978-2022 record. By subwatershed, natural flow was 73% of the 1978-2021 average in upper Henry’s Fork, 73% in Fall River, and 70% in Teton River. In the long upper Henry’s Fork record, natural flow was 77% of average, ranking 87th out of the last 93 years, just ahead of 2016. April-September natural flow was 69% of the 1978-2021 average, better than my April-1 prediction of 60% of average, due to spring rain. Center-of-mass, a measure of runoff timing, was 6 days later than average due to the cold spring. For the watershed as a whole, natural flow has been above the 1978-2022 average in 8 of the past 23 years, compared with 12 of the 22 years prior.
Irrigation management. Because of dry conditions across the entire upper Snake River Basin, Fremont-Madison Irrigation District accrued only 64% of its storage right. Due to poor administrative water availability, the cold spring, and low natural flow in mid- to late-summer, diversion during the 2022 irrigation season was 88% of the 2001-2021 average and the lowest since modern record-keeping began in 1978. Basin-wide water demand required release from Island Park Reservoir in excess of what was needed to meet needs within the Henry’s Fork watershed. Between August 24 and September 19, about 16,140 ac-ft of water was sent from Henry’s Lake and Island Park Reservoir to American Falls.
Island Park Reservoir management. Winter inflow to Island Park Reservoir was the second lowest in the 1934-2022 record, but the outflow of 220 cfs was higher than that in 14 other years since 1978. Island Park Reservoir filled physically in mid-May, and subsequent runoff events due to heavy rain required commensurate outflow increases to keep the reservoir full until needed to meet irrigation demand. Reservoir draft started on June 28 and ended on September 21—both around 5 days later than average. The reservoir reached its minimum for the year on September 20 at 45% full, compared with 45% full on average and 41% full last year. Over the past five years, a variety of water-management and conservation measures have resulted in an average annual increase in reservoir carryover of 25,000 ac-ft (48%) and in winter outflow from Island Park Dam of 116 cfs (38% improvement through the 2022 winter).
Island Park stream gaging. HFF made 14 flow measurements during the summer and fall of 2022. Mean absolute error of HFF measurements compared to USGS adjusted flow was 4.9%, the smallest since we started measuring streamflow there in 2019. Thus, our measurements provided accurate real-time estimates of streamflow to river and water users during time periods between USGS measurements.
Accuracy of predictive models.
Natural streamflow for April-September was predicted to be 60% of average; actual outcome was 69% of average and well within the statistical prediction interval.
Predictive models gave a slightly better than 50% chance that April-September streamflow would be the lowest in the 1978-2021 record; the actual outcome was 4th lowest.
Runoff timing was predicted to be average; the actual outcome was 6 days later than average due to the cool spring.
Mid-summer outflow from Island Park Reservoir was expected to be similar to that in 2021. That was the outcome for magnitude, but timing of peak outflow was delayed by three weeks in 2022 due to the cool spring.
Predicted Island Park Reservoir carryover was 44% full; the actual outcome was 45% full.
Water quality. Due to cold, wet weather, water temperature was below average from early April through late June. Well above-average water temperatures in late summer and fall did not outweigh the earlier cold temperatures, delaying hatch timing and suppressing aquatic vegetation (macrophtye) growth for the whole summer. The latter resulted in much lower-than-average dissolved oxygen concentrations in river reaches dominated by macrophyte photosynthesis. However, dissolved oxygen was well within suitable ranges for trout in all river reaches all summer. Turbidity at Island Park Dam was above average for all of June and July, and this persisted throughout the Harriman Reach to Pinehaven, as macrophytes were not abundant enough early in the summer to trap suspended sediment. Over the year, net sediment export out of this reach was lower than in 2021 and much lower than in 2017 and 2018, when springtime flows were higher and irrigation-season flows were lower. Sediment data suggest that sediment from the 1992 event likely remains in the reach, but aquatic insects show substantial improvement since then.
Fishing experience. Our data corroborate angler experience of poor fishing on the Harriman Ranch reach in 2022: low fish population, delayed hatch timing, continuously cold weather in the spring, continuously hot weather in the late summer and fall, low macrophyte abundance, low insect abundance, high streamflow, and high turbidity combined to produce poor fishing. The two key factors affecting fishing experience there are water management and sediment export out of Island Park Reservoir. Collaborative improvements in water management and conservation have quantifiably reduced negative effects of the 2020-2022 drought on fishing experience. More detailed investigation of mechanisms that relate sediment mobilization in Island Park Reservoir to insect abundance will be needed to develop additional management actions to maintain the quality of hatches anglers desire.
The full report is linked here. Warning: reading this is not for the faint of heart! The report is 30 pages of single-spaced text and tables plus 61 figures.