• Rob Van Kirk

Water Year 2019: 3rd Year of High Reservoir Carryover

  • Climate: Record-breaking February snow, springtime temperature 2 degrees below average, and heavy precipitation in September. Mean temperature for water year 2019 was equal to the 1989-2018 average, but mean April-June temperature was 2 degrees F below average.

  • Natural streamflow: 98% of average: 97% in upper Henry’s, 103% in Fall River, and 96% in Teton River. Snowmelt in Fall and Teton rivers lasted into July. Upper Henry’s Fork water supply was above the 1930-2019 average for the first time since 2012.

  • Irrigation Management: Draft of Island Park Reservoir started 9 days later than average and ended 4 days earlier than average. Over the period of draft, streamflow in the Henry’s Fork at St. Anthony averaged 1,070 cfs, compared with this year’s target of 1,000 cfs.

  • Island Park Reservoir: Ended the water year 73% full, compared with 46% full on average. Outflow during the upcoming winter is expected to be 500-550 cfs for the third consecutive year.

  • Predictive Model Performance: Based only on April 1 conditions, my predictions for most key hydrologic parameters were within 10% of actual values except those directly affected by spring and summer weather. The model over-predicted natural flow by 8.1% and Island Park Reservoir carryover by 6.8%.


Midway through the winter, the probability of a 3rd consecutive year of good snowpack looked slim. In the middle of January, accumulated precipitation and snow water equivalent (SWE) were both 78% of average. In the upper Henry’s Fork, SWE was only 63% of average, although SWE was somewhat better in the Teton subwatershed. A couple of storms late in January favored the northern part of the watershed, putting SWE in both Teton and upper Henry’s at around 85% of average at the beginning of February. Precipitation then fell on all but three days in February, moving water-year precipitation and SWE up to 120% of average. February’s precipitation heavily favored the Island Park and Ashton areas, which hadn’t seen that much snow since the late 1990s. Cold, wet weather late in September produced snow down to valley floors at the end of the month. The three highest-elevation SnoTel sites ended water year 2019 with snow on the ground, ranging from 0.6 inch SWE at Phillips Bench (Teton Pass) to 2.1 inches at Grand Targhee.

March was dry but cold, setting the stage for a long, slow melt of that above-average snowpack. Peak snow accumulation occurred on April 16, 8 days later than average, and equal to last year’s peak date. Peak SWE was 118% of average in both years, and this year’s snowmelt started out like last year’s—at a much higher rate than average. SWE had dropped down to average by mid-May, when an extended period of cool weather set in. Sub-freezing mornings persisted even in the valleys into the last week of June. Mean April-June temperature was 2 degrees below the 30-year average and typical of springtime temperatures last experienced in the late 1990s. As a result, snowmelt rate dropped substantially, and snow persisted into early July, about equal to average and a good three weeks later than in 2018.

Mid-summer turned out to be average in temperature and a little on the dry side. Accumulated precipitation dropped from 110% of average on June 1 to 106% of average in early September. Afternoon showers and thunderstorms provided a little moisture in the mountains, but the valleys were very dry, especially during August. This kept irrigation demand high until September 6, when fall rains started. September precipitation ranged from 3 inches at Crab Creek to 8.6 inches at Grand Targhee, averaging 4.6 inches over the whole watershed. The 30-year average for the month is 1.77 inches. The valleys received 3.5 inches, moving the one-year agricultural moisture availability from nearly 3 inches below average to 3 inches above average in one month.

Three-year precipitation over the whole watershed ended the water year almost 7 inches above average, reflecting good precipitation since September of 2016, when the slow climb out of the 2013-2016 drought began.

Water-year precipitation ended up at 114% of average over the whole watershed: 124% of average in the valleys, 119% of average in upper Henry’s Fork, 109% of average in Teton headwaters, and 108% of average in Fall River headwaters. Ashton was the clear winner among all stations at 133% of average precipitation. Rexburg, White Elephant, and Island Park came in at 129%, 128%, and 125% of average, respectively.

Despite some very hot weather during the first week of the month, September ended up 1 degree F below average in temperature. Warm temperatures at the beginning of the water year and late in the summer were offset by cool temperatures during late winter through mid-summer, and temperature over the whole water year ended up right at average.

Natural Streamflow (water supply)

With 118% of average peak SWE (115% of average April-1 SWE) and 114% of average water-year precipitation—we expected to see above-average streamflow as well. Predictions for spring runoff across the upper Snake River basin were well above average, prompting springtime flood-control operations to make room in the reservoir system. However, the anticipated runoff did not materialize, and natural flow ended up just about average basin-wide. In the Henrys Fork watershed, natural flow was 98% of average, ranking 21st out of the past 42 water years.

The apparent discrepancy between precipitation amounts and natural streamflow (“watershed yield” as hydrologists call it) in the Henry’s Fork watershed was due to three factors. First, the highest precipitation amounts (116% of average SWE and 119% of average precipitation) in the watershed occurred in the upper Henry’s Fork, where streamflow is dominated by groundwater. Response to precipitation in groundwater springs is attenuated over several years, meaning that the full effect of this year’s precipitation in the upper Henry’s Fork will be realized over the next few years, not in this water year alone.

Second, 11% of total water-year precipitation occurred in the last 25 days of the water year, so this precipitation had negligible effect on total water-year streamflow. Without the September precipitation, the water-year total would have been around 104% of average instead of 114% of average, but total natural flow would have dropped only a percentage point to 97% of average.

Lastly and most importantly, soil moisture was very low at the beginning of the water year due to very dry conditions last July through October. Furthermore, last year’s snowpack began accumulating before the ground froze, allowing much of this spring’s snowmelt to replenish soil moisture rather than run off into streams. In the moisture availability graph, note that moisture (precipitation minus evapotranspiration) was well below average until February’s record-setting precipitation started, fell back below average in June, and didn’t recover again until the middle of September. Although the data in that graphic come from the valley areas, they represent conditions across the whole watershed.

Despite below-average natural flow over the whole water year, the cool spring played a key role in limiting the need for draft of Island Park Reservoir to meet irrigation demand. In the natural flow graphic, you will see a big “hole” in the hydrograph during May and another smaller one in late June. These were extended periods of time during which natural flow was well below average, indicating below-average contribution from snowmelt. The snow that would have melted during those periods melted in late June and early July instead, maintaining average to slightly above-average from June 30 to July 13, which, on average, is the period of greatest irrigation diversion.

From the standpoint of Island Park Reservoir management and fishing conditions, the biggest story of the year was continued improvement in natural flow in the upper Henry’s Fork subwatershed. Natural flow there improved for the third year in a row, reflecting ongoing recovery from the 2013-2016 drought. In general, the groundwater springs in the upper Henry’s Fork respond to precipitation over a time frame of 3-5 years. The upward trend in the three-year average precipitation graph illustrates nearly continuous recovery from the drought in terms of precipitation, and this has been realized in continuous, albeit incremental, improvement in water supply in the upper Henry’s Fork. Although still below the 1978-2019 average, natural flow in the upper Henry’s Fork rose above the 1930-2019 average for the first time since 2012. Improved streamflow in the upper Henry’s Fork balanced water supply across the three subwatersheds much better than it has been in many years—97% of average in upper Henry’s, 103% of average in Fall River, and 96% of average in Teton River.

Irrigation Management

Irrigation diversion followed typical patterns for most of the spring and summer. During periods of cool, wet weather during the springtime, diversion dropped below average, increasing again during warm, dry periods. This kept total diversion close to average prior to July. Because the summer was dry overall, and soil moisture decreased steadily from mid-June until early September. This resulted in above-average diversion over most of July and then again from mid-August until early September. However, cool weather preserved snowpack in the Teton Range well into July, delaying need for draft of the reservoir system to meet irrigation demand.

Following the usual decrease that occurs in late July and early August when hay is being cut and grain is done, diversion increased again in the middle of August. Due to a three-week period of warm temperatures and little precipitation, diversion stayed at around 125% of average for sever