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Water Year 2018: Average Supply but Above-Average Reservoir Carryover

Updated: Jan 12, 2022

  • Water-year 2018 ended up close to average, at 102% of average precipitation and 105% of average streamflow.

  • Upper Henry's Fork subwatershed was below average in precipitation and streamflow, vs. above-average values in Fall and Teton rivers.

  • However, Upper Henry's Fork water supply improved from 70% of average in 2016 to 91% in 2017 and 94% in 2018, indicating recovery of deep aquifers from 2013-2016 drought.

  • May and June rain compensated for early snowmelt and resulted in below-average irrigation diversion.

  • Despite very dry conditions and below-average streamflow during July, August and September, Island Park Reservoir ended the water year at 73% full, compared with 43% full on average, thanks to careful and precise water management.

This blog is even longer and more detailed than my usual long, detailed blogs, but there was a lot to learn from water-year 2018. This blog is a slightly edited compilation of the four daily water reports I wrote during the first week of the new water year. The four reports appear in four different sections in this blog:

  1. Climate

  2. Natural flow and diversion

  3. Streamflow and water management in lower watershed

  4. Streamflow and reservoir management in upper watershed

Tables and graphs appear at the end of each section.

If you would like to receive my year-round daily reports, send an email to

Before diving into the content, I'm including photos from water-year transitions in each of the past three years, each taken at the same location. The weather conditions in the three photos are strikingly similar, moisture streaming into the watershed as autumn takes hold.

October 1, 2016

September 30, 2017

September 30, 2018


Overall, water year 2018 turned out to be pretty close to average, over a period of record that goes back about 30 years with temperature and 35-40 years with precipitation.

Watershed-mean temperature ended up 1 degree F above average, despite extended warm periods from mid-November through mid-February and mid-April through mid-June. These warm periods were more-or-less offset by extended cool periods in October, between mid-February and mid-April, and during late June. The summer turned out to be warmer than average, especially in July and September. August was a little cooler than average due in part to cool weather during the second half of the month and in part to smoke-filled skies, which kept afternoon high temperatures a few degrees cooler than they would have been otherwise.

The water year started out with a dry October, although moisture was good going into the water year due to cool, wet weather during the last two weeks of water year 2017. November was on the wet side, while December was fairly dry. Water-year precipitation was below average until mid-February, when a long period of cold, wet weather finally arrived, bringing precipitation back up to average in late March. Heavy rain around Memorial Day and again during the second half of June brought water-year precipitation up to 111% of average in late June. After that, precipitation was very light and widely scattered in space and time, except for a widespread rain event on August 27 that brought one-half inch to the whole watershed. After the otherwise dry summer, water-year total precipitation ended up at 102% of average.

By subwatershed, Fall River consistently received more precipitation than upper Henry’s Fork and Teton through most of the winter and spring. Among the high-elevation areas, the Teton subwatershed received more summertime precipitation, as southerly monsoonal moisture occasionally moved far enough north to hit the Big Hole Mountains, Snake River Range, and southern half of the Teton Range. The very weak and disappointing monsoon season left the upper Henry’s Fork very dry during the summer. By the end of the water year, Fall River and Teton River ended up with 102% and 103% of average precipitation, respectively, while upper Henry’s Fork came in at only 97% of average.

Snow-water-equivalent (SWE) accumulation generally reflected patterns of temperature and precipitation. The water year started out with quite a bit of snow already on the ground at higher elevations from September’s wet, cold weather. Accumulation was quite a bit above average in November and tracked average very closely from mid-December through mid-March. The biggest snowstorms of the year happened in late March and early April, leading to a snowpack that peaked at 117% of average: 120% of average in the Teton watershed, 121% of average in Fall River, and 111% of average in upper Henry’s Fork. Total SWE accumulation peaked on April 18, 8 days later than average. Unfortunately, very warm weather in May and early June, along with heavy, warm rain around Memorial Day melted this promising snowpack quickly. SWE at all nine SnoTel sites in the watershed zeroed out by June 25, 30 days earlier than average.

Precipitation in the valley areas of the watershed turned out to be one of the more interesting stories of water year 2018 and compensated for the early snowmelt and dry summer in terms of irrigation need. Precipitation in April, May and June, while above average across the whole watershed, was above average by a much larger margin in the valley areas. At the end of June, water-year precipitation in the valleys was over 130% of average, and accumulated moisture availability in the watershed was 5 inches above average. This meant that irrigation demand was very low throughout the spring and early summer, as soil moisture remained good well into July. By the end of the water year, however, moisture availability had fallen back to average, meaning that a fully irrigated crop of alfalfa needed its average application of 37.5 inches by the time all was said and done. Despite the dry summer, water-year precipitation in the valleys still ended up at 112% of average for the water year; unfortunately for late-season irrigation need and soil moisture, the above-average precipitation was simply offset by above-average evapotranspiration, which is why net moisture (precipitation minus evapotranspiration) ended right at average.

Natural Flow and Irrigation Diversion

Consistent with average climatic conditions in 2018, streamflow in 2018 also turned out to be pretty close to average. All comparisons in this section are made to the 40-year period 1978-2017, which is the period of “modern,” daily, computerized water-rights accounting records in the upper Snake River basin.

As mentioned above, watershed-total precipitation was 102% of average, and peak snow-water-equivalent (SWE) was 117% of average. These factors, plus good moisture and baseflows inherited from water-year 2017, combined to produce natural streamflow of 105% of average during water-year 2018, despite a very dry summer. However, the distribution of water supply across space and time was not uniform.

On the spatial side, upper Henry’s Fork (watershed upstream of Ashton) received less precipitation and SWE than Fall River and Teton River. Upper Henry’s Fork received 97% of average precipitation, compared with 102%-103% in Fall and Teton rivers. Similarly, peak SWE was 111% of average in upper Henry’s Fork, versus 120-121% in Fall and Teton rivers. As a result, natural flow was only 94% of average in upper Henry’s Fork, versus 113%-114% in Fall and Teton rivers. Another interesting way to view these statistics is as a rank out of the 41 water years between 1978 and 2018. Natural flow in Fall River ranked 10th, and that in Teton River ranked 12th, decisively in the top third of water years experienced since 1978, whereas upper Henry’s Fork came in at only 24th, in the bottom half!

Low water supply in upper Henry’s Fork resulted not only from below-average precipitation in 2018 but also from low levels in the Yellowstone Plateau aquifers that provide the majority of streamflow in the upper Henry’s Fork. Outflow from these aquifers responds to precipitation at a time lag of 3-4 years, so outflow is still reflecting drought conditions in 2015 and 2016. Nonetheless, streamflow in the upper Henry’s Fork is gradually climbing out of hole dug by the 2013-2016 drought. This year’s flow, at 97% of average, was better than those of the last two years: 91% of average in 2017 and 70% of average in 2016.

Temporally, the water year was sharply divided between its first nine months and its last three. Streamflow was consistently above average between the beginning of the water year and the end of June, due to three factors: 1) good baseflows inherited from 2017, 2) early snowmelt, and 3) heavy precipitation in April, May, and June. I should emphasize the effect of the early snowmelt here. This kept streamflow very high during April and May, but that was a period of time when irrigation demand was low and reservoirs were close to full. Thus, the high streamflow did not contribute to meeting demands on the river, although high streamflow during the spring is good for moving accumulated sediment out of stream channels and maintaining long-term stream and riparian habitat conditions. Had that same snowpack lasted longer in the summer, natural flow would have ended up a little lower during April and May and a little higher in July and August. Only heavy rains in May and June saved 2018 from being a very dry year, with high demand on reservoir storage. Once those rains were over, streamflow dropped very rapidly and stayed around 85% of average for all of July, August, and September.

Speaking of irrigation, my daily reports emphasized all summer that the May and June rains resulted in good moisture in the agricultural regions of the watershed, reducing irrigation need by at least 100,000 ac-ft during the early part of the summer. Diversion reached average on only a few individual days between April 1 and July 9, which is the average date of peak irrigation demand. This year’s peak diversion occurred on July 11 and stayed well below average for most of July, as soil moisture remained good for several weeks after the end of the June rains. However, diversion increased again in August and remained at or above average for most of August and September, as the effects of very low precipitation and high evapotranspiration were fully realized throughout the valley. In September, much of the need for irrigation was simply to keep the dust down and soil workable during and after harvest. Through September 30, total diversion was 783,386 ac-ft, only 82% of the long-term average of 952,586 ac-ft. May and June rains were primarily responsible for the decreased irrigation demand, but I should point out that total di