Watershed temperature over the month of June was 1 degree F above average.
June precipitation was 135% of average, but average natural streamflow at Island Park for the month of June was only 92% of average.
As of July 6, watershed-wide natural flow has dropped to 89% of average.
Delivery of Island Park Reservoir storage water began on July 3.
Watershed temperature over the April-June period was 2 degrees F above average, continuing the 40-year trend of increasing springtime temperature.
April-June precipitation was 134% of average.

Precipitation 135% of average in June

After a warm May, very little snowpack remained across the Henry’s Fork watershed, leaving rain as the only mechanism to keep streamflow high. Fortunately, that happened, as watershed-total precipitation was 135% of average during June. Monthly climate summary for June, 2018 is given in this table.

Moreover, precipitation in the agricultural areas of the watershed was 183% of average in June. In fact, the valleys received more precipitation during June than the high-elevation areas of the Teton subwatershed. As a result, irrigation demand was lower than average over the month.

Total diversion in the Henry’s Fork watershed so far in irrigation year 2018, compared with average and 2017.

Streamflow Close to Average in June

Despite well above-average precipitation in June, streamflow was average at best around the watershed. Streamflow was higher than average during May because of rapid melt of a snowpack that peaked at 117% of average. However, once that snow was gone, even above-average rain wasn’t enough to keep streamflow above average for the month. This illustrates just how important snowpack is to streamflow in our watershed. Much of the rain that falls during June is taken up by plants, reducing the net effect of rain on streamflow. On the other hand, most snowmelt ends up as streamflow.

Graph of watershed-total natural flow so far in water-year 2018, compared with average and 2017.

Despite above-average precipitation over the watershed as a whole, precipitation did not fall uniformly across the watershed. June precipitation favored valley areas, the Fall River subwatershed, and the Centennial Range, all of which received at least 150% of average precipitation. On the other hand, June precipitation in Island Park was actually below average–84% to be precise. As a result, natural streamflow in the Henry’s Fork at Island Park was 93% of average for the month of June, ranking only 30th out of the 47 years since 1972. Note, however, that June natural streamflow in 2018 was the highest at Island Park since 2011 and the 5th highest since 1999. Thus, compared to recent years, streamflow in 2018 was high, even though it was below average compared to the past 47 years.

Mean natural flow in the Henry’s Fork at Island Park over the month of June, water years 1972-2018.

Why was Outflow from Island Park Reservoir so Variable this spring?

Island Park Reservoir was filled near the end of May in anticipation of need for irrigation delivery, which can happen as early as the first week of June. For example, irrigation delivery in 2016 began on June 3. HFF’s research on effects of Island Park Reservoir on water quality and the fishery downstream shows that keeping the reservoir as full as possible for as long as possible results in cooler summer-time water temperatures below the dam, lower turbidity, lower suspended sediment export from the reservoir, and higher winter flow. Thus, mid-summer fishing experience and long-term habitat quality and trout recruitment all benefit from a full reservoir.

Island Park Reservoir volume so far in water year 2018, compared with average and 2017.

However, to keep the reservoir full this spring as inflow changed rapidly due to snowmelt and rain, outflow also had to change rapidly. Given perfect weather forecasting and the ability to instantaneously adjust reservoir outflow as inflow changes, outflow from the reservoir could be managed more smoothly than actually occurred. However, neither of these things are possible, so to prioritize keeping the reservoir full during periods of rapidly changing inflow, some of the outflow peaks were higher than inflow peaks, and most flow changes occurred more abruptly than would happen in a natural system.

Outflow from Island Park Reservoir so far in water-year 2018, compared with average and 2017.

However, if you look carefully at the graphs of watershed natural flow and Island Park Reservoir outflow, you will see essentially the same pattern of flow increases and decreases. In the natural-flow graph, note that there were four distinct peaks this spring, plus one small “shoulder” in early June. The Island Park outflow graph also has four distinct peaks during April, May and June, each corresponding to one of the natural-flow peaks. A fifth peak in the outflow graph–the one that is lowest in magnitude and shortest in duration–occurred shortly after the little “shoulder” apparent in the natural-flow graph.

Irrigation Delivery Started July 3 With Full Reservoir

May and June rain kept irrigation demand below average, while streamflow remained at or above average. As a result, natural streamflow was able to meet irrigation demand until just a few days ago. The primary indicator of need for irrigation delivery from Island Park Reservoir is streamflow in the Henry’s Fork at St. Anthony. The irrigation-season target there is 1,000 cfs, so once flow drops close to that target, delivery is needed.

Graph of streamflow in Henry’s Fork at St. Anthony so far in water-year 2018, compared with average and 2017.

The graph above shows that streamflow at St. Anthony dropped rapidly once the mid-June rainy spell ended. Without delivery of water from Island Park Reservoir, flow at St. Anthony would have dropped below 1,000 cfs early this morning (Friday, July 6). Increases in Island Park outflow of 200 cfs on Tuesday morning (July 3) and 75 cfs on Thursday morning (July 5) kept streamflow at St. Anthony from dropping below 1,000 cfs today and will most likely keep flow there at or above 1,000 cfs until July 9 or 10. Planned release of 50 cfs from Grassy Lake next week will partially offset continued drop in Fall River flow and need for additional Island Park storage delivery.

The first date of storage delivery from Island Park Reservoir was July 3 this year, compared with June 3 in 2016 and with July 12 in 2017. Average reservoir content on July 3 is 94% of average, meaning that on average, 6% of the reservoir’s capacity (around 8,000 acre-feet) has already been used by the time storage was first delivered this year. Although keeping the reservoir completely full this spring came at a cost of frequent outflow adjustments during May and June, the extra 8,000 acre-feet equates to 40 cfs of additional outflow next winter, all other things being equal from here on out. This above-average reservoir content also means cooler outflow temperature and lower turbidity longer into the summer.

Spring of 2018 Continues Long-Term Warming Trend

On the whole, the month of June reflected the entire three-month April-June springtime period. Watershed temperature was 2 degrees F above average and precipitation was 134% of average for the three-month period, as shown in the table below. As during June itself, springtime precipitation as a percent of average favored valley areas and the Fall River subwatershed.

Spring-time temperature ended up 2 degrees warmer than average because of an extended period of warm temperatures from mid-April to mid-June, as shown in the graph below. Early April and late June were generally cooler than average.

Graph of watershed temperature relative to average so far in 2018, compared with that in 2017.

The strongest predictor of snowmelt timing in the Henry’s Fork watershed is mean April-June temperature at the nine SnoTel sites listed in the climate tables above. This measure of springtime temperature has been steadily increasing over the past 30-40 years at a little over 1 degree Fahrenheit per decade. At the beginning of the spring, I used this long-term trend to predict mean April-June temperature. The predicted value was 44.9 degrees F, and the observed value was 44.6 degrees F, well within the statistical uncertainty associated with the prediction. The graph below illustrates this. The colored dots and lines represent the data and trend, respectively, from each of the nine SnoTel sites. The solid black line is the watershed-averaged trend. The black diamond is the 2018 prediction, surrounded by a 95% confidence interval. The red diamond is the observed temperature. Thus, to within statistical precision, this year’s springtime temperature continued the long-term warming trend.