Water Year 2019: 3rd Year of High Reservoir Carryover

Updated: Jan 13

  • 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 several weeks. During that time, moisture availability (precipitation minus evapotranspiration) fell to around three inches below average. Autumn rains began on September 6, and diversion dropped over 1,000 cfs within a week, ending need for storage draft by September 9. Diversion remained below average over the remainder of September, and the three-inch moisture deficit was quickly erased by the end of the month.

Draft of Island Park Reservoir started on July 12, 9 days later than average and ended on September 9, 4 days earlier than average. My “600-cfs” rule of thumb performed well again; start and end of reservoir draft occurred within a few days of when natural equaled 600 cfs of total diversion. The 600 cfs figure is roughly what is needed in excess of diversions to account for stream channel losses and to leave enough flow in the South Fork Teton River and the lower Henry’s Fork to provide enough hydraulic head at the downstream-most canal diversions. As expected, need for storage draft coincided well with need for delivery of water to the Teton River through the Crosscut Canal. Crosscut delivery first increased to 100 cfs on July 18 and averaged over 200 cfs through September 9, when delivery was no longer needed to meet demand on the Teton River.

In general, the “600-cfs” rule coincides with streamflow in the Henry’s Fork at St. Anthony reaching 1,000 cfs. Over the past few years, the Henry’s Fork Drought Management Planning Committee has set a target flow of 1,000 cfs at the St. Anthony gage to meet irrigation demand and streamflow needs in the lower watershed with a minimum amount of Island Park Reservoir draft. During the period of reservoir draft, streamflow in the Henry’s Fork at St. Anthony averaged 1,070 cfs, compared with 1,085 cfs in 2018. Statistical analysis of factors affecting draft of Island Park Reservoir show that on average, reservoir carryover increases by 4,300 ac-ft for each 100 cfs of streamflow at St. Anthony below the long-term average of 1,411 cfs. Thus, maintaining St. Anthony streamflow at 1,070 cfs in 2019 increased Island Park Reservoir carryover by an expected 14,663 ac-ft over the long-term average, all other factors being equal. This additional carryover is equivalent to a 75-cfs increase in winter outflow from Island Park Reservoir.

Island Park Reservoir Management

Because of good reservoir carryover in 2018, a modest decrease in outflow during October and November was sufficient to fill the reservoir to 119,000 ac-ft by December 1. The maximum desirable reservoir content on April 1 is around 120,000 ac-ft, to ensure that ice remains below the rubber collar attached to the top of the concrete spillway. Thus, outflow from Island Park Dam was equal to inflow over the winter, resulting in a December-February mean outflow of 560 cfs, the highest since the winter of 2012. Winter outflow in 2018 averaged 504 cfs, making the two-year average of 532 cfs the highest since 1999-2000. Winter outflow from Island Park Dam is the single most important variable predicting recruitment of Rainbow Trout into the population in Box Canyon, so it was not a surprise that recruitment in the spring of 2019 (affected by winter flow in 2018) was the highest since 2013 (affected by winter flow in 2012). The Box Canyon Rainbow Trout population in spring of 2019 was the fourth highest on record.

During March and April, some additional room was made in the reservoir to accommodate above-average runoff following February’s record-setting precipitation. A three-day springtime freshet flow of 1,800 cfs was delivered at the end of April, designed to remove fine sediment from the river between Last Chance and Pinehaven while aquatic vegetation was it its annual minimum. Inflow to the reservoir during May was only slightly above average, as discussed in a previous installment, so outflow was lowered in the middle of the month to fill the reservoir. The reservoir filled on June 3 and remained full until July 12, when irrigation-season draft began. Outflow during that time period ranged from 1,440 cfs during a rainy period during early June to 622 cfs on July 11. Outflow during irrigation season was generally below average except for a few days in late August and early September. Peak outflow from the reservoir was 1,230 cfs on July 25 and in the neighborhood of 1,200 cfs for four days in late July. Reservoir draft ended on September 9, and the reservoir filled a little between then and the end of the water year.

One interesting note about reservoir content during 2019 was the large contribution of direct precipitation on the reservoir surface. Because the reservoir was near its maximum surface area for the whole water year and because precipitation in Island Park was 125% of average, direct precipitation on the reservoir surface added a net volume of 12,356 ac-ft over the water year, twice the average figure of 6,072 ac-ft. This is water added to the reservoir over and above inflow from streams. Reservoir draft this year was 36,017 ac-ft, so direct precipitation accounted for over one-third of the drafted volume. This figure emphasizes two things the additional benefit of keeping the reservoir as full as possible so it has a large surface area.

The reservoir ended the water year at 99,188 ac-ft (73% full), compared with the 1978-2018 average of 62,675 ac-ft 46% (full). This high carryover followed end-of-September volumes of 109,489 ac-ft in 2017 and 98,508 ac-ft in 2018. Reservoir carryover has not been this high three years in a row since 1997-1999. During those three water years, watershed-wide natural flow was 153%, 130%, and 124% of average, respectively. Watershed-wide natural flow in 2017, 2018, and 2019 was 111%, 104%, and 98% of average, respectively. These are remarkable figures showing how improved reservoir management over the past few years has increased carryover in Island Park Reservoir to the benefit of water users, water quality, and the fisheries. Fremont-Madison Irrigation District and U.S. Bureau of Reclamation deserve a lot of credit for managing the Henry’s Fork irrigation system so carefully to keep the reservoir as full as possible.

Outflow from the reservoir during the upcoming winter is expected to exceed 500 cfs for the third consecutive year, due to high reservoir carryover and improved baseflows in the upper Henry’s Fork subwatershed. Natural watershed inflow between Henry’s Lake and Island Park is predicted to be 462 cfs, versus 472 cfs in 2019. Inflow from Henry’s Lake will add an additional 20-30 cfs, and direct precipitation is expected to add another 20-30 cfs. Thus, I expect total inflow to average 500-520 cfs over the upcoming winter.

Predictive Model Performance

In the spring of 2017, I developed the first version of a numerical simulation model to predict water supply, irrigation-system management, and Island Park Reservoir carryover for the upcoming irrigation season. The model uses hydrologic and climatic data available on April 1 of each year to predict conditions from the April 1 through September 30. Model outputs include an expected value (the average of 5,000 different random simulations) and a 90% prediction interval around the expected value. The prediction interval encloses the middle 90% of outcomes of the 5,000 different simulations. By definition, the probability of an outcome outside of the interval is less than 10%, given the data available on April 1. I have made incremental improvements to the model since its initial construction and will continue to refine it each year based on performance. Today I will compare the model’s predictions with actual conditions during 2019. You can see the original 2019 predictions in my blog post of April 10, 2019.

The model got one thing wrong right off the bat. The headline of the April 10 blog post was “Water Supply Above Average for the Third Consecutive Year.” Natural flow ended up at 98% of average for the whole water year and 96% of average for April through September. The model predicted 105% of average natural flow for April through September. Although this difference may seem large—the model’s actual error was only 8.1% relative to the observed value, a prediction of 1.74 million acre-feet versus 1.61 million acre-feet. For context, the difference of 130,000 ac-ft between the prediction and the observed value is around half of the storage capacity of Henry’s Lake, Island Park Reservoir and Grassy Lake combined.

I have summarized the most important irrigation-season parameters in the table below. You can see from that table that the model predicted all but one of the flow parameters to within 11%. The largest discrepancies between model predictions and observed conditions—including peak late-summer outflow from Island Park Reservoir—all resulted from the weather we experienced from April 1 through early September. Of course, the weather over a 5-6 month period cannot be predicted ahead of time, so the model simulations use the full range of possible climatic conditions over that period, and the predicted value is the average of all of those conditions.

The only climatic variable predicted ahead of time was mean April-June temperature at the nine SnoTel sites in the watershed, which has been following a statistically significant increasing trend of 1.1 degrees F per decade for over 30 years. Based on this trend, the predicted April-June temperature was 44.5 degrees, but the actual temperature turned out to be 42.4 degrees. The probability of a temperature this low given the long-term trend was less than 5%. Thus, less than 5% of the model runs would have incorporated temperatures as cold as this. Based on the expected temperature, the model predicted that draft of Island Park Reservoir would begin on July 3 and that Crosscut Canal delivery to the Teton River would first exceed 100 cfs on July 7. The actual dates turned out to be July 12 and July 18, respectively, due to cool springtime temperatures that delayed snowmelt in the Fall River and Teton watersheds.

At the other end of the irrigation season, the model predicted that Crosscut Canal delivery would fall below 100 cfs on August 9, whereas the actual value turned out to be September 8. This resulted from the extended warm, dry period of weather from mid-August to the onset of the September rains. The additional need for Crosscut Canal delivery necessitated relatively high outflows from Island Park Reservoir during August and September. The model predicted a peak late-summer outflow from Island Park Reservoir of 792 cfs, whereas the observed late-summer peak was 1,020 cfs, reached on August 23 and again on August 30.

The single most important output of the model is Island Park Reservoir carryover, which I define as the content of the reservoir on September 30. The observed value was 99,188 ac-ft. The model predicted a value of 115,040 ac-ft, an over-prediction of 16.0%. However, reservoir carryover is a decreasing function of irrigation-season streamflow in the Henry’s Fork at the St. Anthony gage. The model prediction is based on perfect attainment of the target flow at St. Anthony, which was set at 1,000 cfs. Given the observed streamflow of 1,070 cfs at St. Anthony, the model predicted reservoir carryover at 105,886 ac-ft, an over-prediction of only 6.8%. The over-prediction was due largely to lower-than-expected streamflow during late August and early September. The model has now been applied in water years 2017, 2018 and 2019. Errors in predicting end-of-season content in Island Park Reservoir over those years were <1%, 7.4%, and 6.8%, respectively.