Cold, Wet June Dramatically Improved Summer 2020 Water Supply

Photo of rainbow trout
  • Weather during the April-June spring period is a major factor determining summer water supply.
  • Although April-June temperature was average and 1.6 degrees below expectation, high variability resulted in three very warm periods that melted this year's average snowpack 10 days earlier than average.
  • The second half of June was very cold and wet, compensating for the early snowmelt and delaying need for Island Park Reservoir draft by one week relative to average.
  • As of July 14, 10 days into the draft period, Island Park Reservoir is still 91% full, compared with 83% full on average, setting up a fourth consecutive year of above-average carryover and high winter flows.


Spring 2020 Summary

The three-month April-June period is the most critical in determining summertime streamflow and need for reservoir draft, as it encompasses the period of peak snow accumulation and melt of that snowpack. Snowmelt is determined by two factors: 1) April-June temperature and 2) how much snow there is to melt. Once the snowpack has melted, rain during mid- to late June can have a large effect on early-summer streamflow, irrigation demand, and need for Island Park Reservoir draft. From a fishing standpoint, springtime weather can have a large effect on water quality and on timing and quality of aquatic insect hatches.

This blog contains information current through July 14, 2020.


The spring of 2020 can best be characterized by high variability in both temperature and precipitation. Averages do not tell the whole story. Mean temperature for the spring ended right up at average. At the nine SnoTel stations, mean April-June temperature was 42.6 degrees F, compared with 42.6 degrees on average and with 42.4 degrees F last year. The past two springs have provided relief from a four-decade trend of increasing springtime temperatures at these nine stations. Based on that trend, about 1.1 degrees F per decade, last spring’s mean temperature was predicted to be 44.1 degrees F and this spring’s was predicted at 44.2 degrees (see graphic). So, in both springs, temperatures were about 1.6 degrees below expectations. Anglers have noticed the positive effects these springtime temperatures have had on quality and timing of insect hatches compared with recent years.

Graph of April-June temperature

However, daily variability in springtime temperatures was much greater this year than last, which is readily apparent in the temperature graph below. A useful measure of variability is the coefficient of variation, defined as standard deviation divided by the mean. Daily coefficient of variation in temperature this spring was 24%, compared with 19% last year and 21% on average. Starting with the week before Memorial Day, we had four episodes during which temperatures dropped from 5-10 degrees above average to 5-20 degrees below average over the course of 24-36 hours. These very strong and unseasonable cold fronts were generally accompanied by high winds and brought heavy precipitation in three of the four cases—those being the Memorial Day weekend, June 16-18 and June 28-30.

Graph of temperature departure from average

Precipitation was below average in April and May but well above average in June. The two big precipitation events in June pushed the springtime total to 104% of average for the three-month period. Springtime precipitation was 99% of average in the Teton subwatershed, which received much more winter precipitation than the other headwater areas. Springtime precipitation was 101% of average in the valleys, which also received above-average winter precipitation. Springtime precipitation was 104% of average in Fall River and 110% of average in the upper Henry’s Fork. This helped distribute water-year total precipitation more uniformly across the watershed, although the upper Henry’s Fork still lags the other regions. Os of July 14, water-year precipitaiton to date is 94% of average.

Graph of precipitation in HF watershed

The three-year running-average watershed precipitation was 1.6 inches above the long-term average in early April, but that has increased to 2.3 inches above average since then.

Graph of three-year average precipitation

In the primary irrigated regions of the watershed—between Ashton and Rexburg—springtime precipitation was 115% of average, boosting moisture availability there from just a hair below average in mid-May to 2.5 inches above average today.

Graph of moisture availability in HF agricultural regions


After spending most of the winter around 90% of average, snow water equivalent (SWE) started the month of April at 97% of average, greatly favoring the Teton subwatershed. A ninth inning rally brought SWE up to average during mid-April. SWE peaked on April 17, within a few days of average peak timing. During the first extended period of warm weather at the end of April and beginning of May, all of the low-elevation snow melted rapidly, and SWE remained below average the rest of the spring except for a day or two around the Memorial Day weekend. Other than that brief excursion to average, snowmelt was running about 10 days earlier than average. Despite average snowpack and average temperatures, the early melt was a result of three extended periods of well above average temperatures. The first of these melted the low-elevation snow, the second melted the mid-elevation snow, and the third melted the rest of the high-elevation snow. Thus, the high variability in temperatures accelerated snowmelt over what would be expected strictly by looking at averages.

Graph of snow water equivalent

Snowmelt this spring was about two weeks earlier than last year. This was due in part to the high temperature variability mentioned above but was also due to a much lower snowpack this year than last. More snow takes longer to melt, regardless of temperature. This year’s snowpack peaked at the same time as last year’s, but peak SWE was 4.6 inches (14%) lower than last year. As can easily be seen from the SWE graphic, that difference was maintained throughout the spring. During the mid-April to late-June melt period, the difference between last year’s SWE and this year’s ranged between 2.9 inches and 7.5 inches, averaging 5 inches, pretty close to the initial difference in peak SWE between the two years.

Natural streamflow

Patterns in streamflow reflected large temperature swings through mid-June and precipitation at the end of June. Cool weather in March and early April delayed low-elevation snowmelt, which ended up occurring all in a very short period at the end of April and early May, producing the first of six peaks in natural flow. Subsequent shorter, less extreme warm periods produced a few more small peaks before very warm weather in late May and early June melted most of the mid-elevation snow over the span of two weeks. The season’s peak flow occurred on June 3, at 9,758 cfs. This peak was close to average in timing and 97% of the average 1-day maximum natural flow. A small peak associated with melt of remaining high-elevation snow occurred shortly thereafter. The last peak for the season, which occurred on June 30 at 5,854 cfs, was completely rain-driven. Total natural flow for the watershed to date is 94% of average, commensurate with total water-year precipitation to date but lower than my April-1 prediction of 97% of average.

Graph of natural streamflow

Diversion and irrigation demand. Cold weather kept diversion well below average until warm weather arrived at the end of April. Diversion reached average during the four periods of warm, dry weather we have had this spring but fell well below average in between, again reflecting high variability in this spring’s weather. Relatively low diversion during late June and early July was due to heavy precipitation over the agricultural areas during that time and lower need for irrigation. Diversion climbed rapidly to seasonal averages during the week of July 6 and has stayed there since then. Total diversion so far this irrigation year is 88% of average.

Graph of diversion in HF watershed

Water management. Island Park Reservoir started the spring at 120,000 ac-ft (89% full), where it must be maintained as long as ice cover remains. This level keeps ice below spillway infrastructure. Ice still covered the reservoir during the short-duration, high-magnitude peak in inflow that occurred around May 1. This required passage of the natural springtime freshet through the reservoir to maintain reservoir level, resulting in scour and removal of around 500 tons of sediment out of the Harriman reach of the river. This scour accomplished 25% of the annual sediment transport out of that reach in just a few days. The reservoir was then filled during the month of May and remained full--with only minor adjustments in outflow, until draft began on July 4.

Graph of Island Park Reservoir content

The net storage graph shows that the reservoir (along with a little remaining space in Henry’s Lake) was filled this year during a 7-week period in the fall and a 4-week period in the spring. Outside of those two short windows, streamflow downstream of the dam was very close to the river’s natural flow prior to the onset of reservoir draft on July 4. This management regime benefits water users by filling the reservoir as full as possible as far ahead of irrigation season as possible and benefits anglers and aquatic ecosystems by maintaining the natural hydrograph during the winter, the spring freshet period, and the month of June.  

Graph of natural and regulated flow at Island Park

Summer 2020 outlook

As we learned again this June, timely rain can erase some of the water-supply deficit associated with early snowmelt. Twice during June, streamflow in the lower watershed was dropping toward levels that indicated need for draft of Island Park Reservoir. Prior to the June 16-18 rain, conditions indicated reservoir draft around June 20-22. That rain moved need for draft out a week or so. By June 28, conditions indicated need for draft around July 1. The rain of June 28-30 increased water supply back to where it was in early June, offsetting early snowmelt and pushing need for Island Park draft out another few days. A small increase in outflow from Island Park Reservoir started reservoir draft on July 4, just to create a little buffer in case diversion increased over the holiday weekend. This onset of draft was 7 days later than the long-term median, saving around 3,000 ac-ft, worth 14 cfs of winter flow next year. However, the need for a large amount of draft didn't occur until July 7, as low-flow indicators were reached on the lower Henry's Fork.

Graph of streamflow in HF at St. Anthony

My "600-cfs rule" indicated need for Island Park Draft on July 9, so it performed well yet again. The rule states that total streamflow (natural flow plus reservoir draft) needs to exceed total diversion by roughly 600 cfs. This is the amount needed to leave a total of around 400 cfs in the lower Henry’s Fork and South Fork Teton River downstream of all diversions and compensate for seepage loss (~200 cfs) from stream channels in the lower watershed. Thus, when natural flow drops to within 600 cfs of demand, reservoir draft is needed to maintain total streamflow above the 600-cfs threshold.

Graph of water supply versus demand


Peak irrigation diversion occurs around July 10, and current data suggest we may have already reached that. Even if the rest of the summer is warm and dry, draft of Island Park Reservoir will be below average as a result of delayed onset of draft due to June rain and continued good streamflow on Teton River, the only of the three subwatersheds where snowpack was above average. That will set us up for a fourth consecutive year of good reservoir carryover.