Water year 2017 was above average in both precipitation and streamflow. Averaged over the whole watershed, precipitation was 138% of average, led by the Fall River watershed, at 144% of average. Due to the cumulative effect of four years of drought, especially in the groundwater-dominated subwatersheds, natural streamflow was only 109% of average, led by the Teton River at 134% of average. Streamflow in the upper Henry's Fork, which is dominated by groundwater, was only 91% of average.
Water Year 2017 Climate: Wet but Warm
The photo above, taken on October 1, 2016, shows how the water year started out, with widespread showers over the whole watershed. Precipitation continued throughout October, setting records for the month over most of Idaho. As the graph below shows, precipitation was around 140-150% of average over the whole watershed until May, which was one of the drier months of the year. Through the summer, accumulated precipitation remained at around 140% of average in Fall River and Teton River subwatersheds but fell to around 125% of average in the upper Henry's Fork subwatershed and in the valleys. Heavy precipitation from September 13 through the end of the water year substantially boosted water-year totals to between 133% and 144% of average by October 1.
While 2017 was definitely on the wet side, it was also warm, continuing a 40-year trend of steadily increasing temperature. Although not as warm as 2016, 2017 was over one degree warmer than the 1988-2016 average, and the July-September period was very warm, even after averaging in the last two weeks of September, which were much colder than average.
The photo below, taken on September 30, 2017 a few hundred yards downstream of where the other photo was taken, shows how the water year ended--much like it began with widespread showers.
Water Year 2017 Streamflow 9% Above Average
While precipitation was well above average, streamflow was only slightly above average due to the accumulated effects of four years of drought. Despite heavy rain in October, streamflow was below average most of the fall and winter, as shown in the graph below. Early melt of low-elevation snow in March increased streamflow to above-average values for most of March and April. Melt of the ample high-elevation snowpack kept watershed-total natural flow above average until early July, after which water supply was close to average for the remainder of the water year.
As shown in the table below, the Teton subwatershed had the highest natural flow as a percentage of normal, producing 134% of average water supply. Water-year total natural flow in Fall River was 120% of average and that in the upper Henry's Fork was only 91% of average due to the cumulative effect of drought on the large, deep aquifers that feed Big Springs, Buffalo River, and Warm River. Watershed-total natural flow was 109% of average, far better than last year's 73%. Several more years of average to above-average precipitation will be required to bring flow in the upper Henry's Fork back up to average or better, as the deep Yellowstone Platuea aquifers require 3-4 years to fully respond to long-term precipitation amounts.
The longest record of natural flow available in the Henry's Fork watershed is the 88-year record of natural watershed inflow between Henry's Lake and Ashton, which is the vast majority of natural flow in the upper watershed and includes the contributions of Big Springs, Buffalo River, and Warm River. This record is shown in the graph below. Note that 2017 was below the 88-year average. Also note the very long period of average to above-average water years between the late 1960s and the late 1980s, a string of 21 years with only one below-average year, which occurred in 1977. This is the period during which the fishery between Island Park Dam and Riverside Campground became world famous. Since then, periods of extended drought occurred from 1988 to 1994, from 2001 to 2005 and from 2013 to 2017.
2017 Diversion Near Average but Storage Delivery Below Average
Irrigation diversion followed the average very closely until the extended dry spell in August and early September. However, once heavy precipitation started falling in mid-September, diversion rapidly fell to below-average values by the end of the water year. Total diversion was much higher than in 2016 because supply was so much better in 2017.
Natural flow provided over 80% of the total water diverted during 2017, so very little delivery of storage water was required from Island Park Reservoir. Outflow from the reservoir was below average most of the summer because of low need for storage delivery. Natural flow in Fall River and Teton River met most of the irrigation demand all summer. Except for missing the timing of need for storage delivery in late June and early July, my April-1 predictive model performed extremely well in predicting magnitude of outflow throughout the summer.
Island Park Reservoir volume remained well above average all summer. Delivery of storage begin in mid-July, several weeks later than both average and the model prediction. However, need for storage delivery persisted later into the summer than expected due to hot, dry weather in August and early September. Water-year ending volume of 109,489 acre-feet (81% of capacity) was within one-half of one percent of the April-1 prediction.
The table below puts the irrigation-season ending content of Island Park Reservoir in context. Not only was this year's ending content much higher than last year but was also well above the long-term average.
Looking Ahead
With Island Park Reservoir already very near its average April-1 volume, outflow from the reservoir over the remainder of the fall and winter will be very close to inflow. Projected gain in the river between Henry's Lake and Island Park will average 415 cfs over the fall and winter. Currently, Henry's Lake is delivering about 50 cfs to Island Park Reservoir, but that will probably decrease to somewhere between 0 and 20 cfs later in the fall. Thus, projected outflow from Island Park Reservoir during the middle part of the winter is between 400 cfs and 450 cfs, which will be the highest outflows since the winter of 2011-2012.