While natural streamflow clearly reflected long-term hydrologic conditions and short-term precipitation and temperatures, patterns in irrigation demand and management in 2022 largely reflected poor water supply across the whole upper Snake River basin, with secondary dependence on water supply and weather within the Henry’s Fork watershed.
Administrative water availability
The 1935- and 1936-priority storage rights in Island Park Reservoir and Grassy Lake, which belong solely to Fremont-Madison Irrigation District (FMID), did not accrue any water during irrigation year 2022, leaving FMID with only around 64% of its storage right. This was a consequence of low priority of FMID’s storage rights within basin-wide administration, low upper Snake River reservoir carryover from 2021, and poor water supply across the basin. Irrigators within FMID knew ahead of irrigation season that storage would be limited, so they made crop choices and management decisions appropriate to the water supply. Those decisions put irrigation demand in the Henry’s Fork watershed on a trajectory for relatively low diversion right from the beginning of the spring. In addition, the cold wet spring delayed need for irrigation, and peak diversion in 2022 occurred on July 14, compared with July 9 on average and June 30 in 2021. The wet spring also maintained natural-flow priority near average until early July, much better than would have happened without the spring rains. Natural-flow rights in the Henry’s Fork first dropped to 11/5/1895 priority on July 11, vs. July 15 on average and June 23 in 2021. This priority date is a good indicator of administrative water availability in the Henry’s Fork watershed, as many large canals on the Henry’s Fork and lower Fall River have priority dates on or slightly earlier than 11/5/1895. Natural-flow rights remained poor until late September, except for a few days in early August when widespread rain fell across the whole basin.
Irrigation diversion
The combination of low storage availability, cool wet weather in the spring, and poor natural-flow rights in the middle of the summer resulted in very low diversion during water year 2022. Based on preliminary numbers, total diversion was the lowest on record, at only 88% of the modern-period 2001-2021. Daily diversion in 2022 was above-average for a day or two in mid-July and again during a period of record high temperatures and low precipitation in October. Even though physical water supply was low at that time, many canals elsewhere in the basin had already shut down operations for the season by then, allowing some more junior natural-flow users to divert water during the early fall. Diversion during 2022 added yet one more data point in the long-term trend in diversion that has resulted from increased use of sprinkler irrigation vs. historic flood, furrow and border-ditch irrigation. Diversion decreased steadily during the 1980s and 1990s, the period of most rapid conversion in irrigation methods, dropped abruptly during the very dry year of 2001, and has stayed relatively constant since then, with year-to-year variability determined by the same set of factors discussed above. Although increased irrigation efficiency has reduced total annual diversion by about 240,000 ac-ft since the late 1970s, it has also reduced return flows and groundwater inputs to the river by about the same amount. These inputs are called “river reach gains” and travel primarily through shallow aquifers, which cool water temperature and act as a short-term storage mechanism. Given that diversion in 2022 was the lowest in the 1978-2022 record, it is not a surprise that reach gain was also among the lowest on record, ranking 39th out of 45 years. Total reach gain to the lower Teton River and Henry’s Fork in 2022 was negative, meaning that over the irrigation year as a whole, the net movement of water between the river and the aquifer was away from the river and into the aquifer.
Teton River administration
Streamflow shortage in the Henry’s Fork watershed during years of low snowpack is greatest in the Teton River, which necessitates greater need for augmentation of streamflow there to meet demand. Augmentation is supplied by two sources: 1) delivery of water diverted from the Henry’s Fork to the Teton River through the Crosscut Canal and 2) water injected into the Teton River from so-called “exchange wells.” These wells pump groundwater into the river and were drilled following the Teton Dam failure as a mechanism by which to offset pumping from the river by irrigators along the rim of Teton Canyon. These irrigators held storage rights in Teton Reservoir and would have pumped this water directly from the reservoir. In most years, the administrative aspect of this diversion can be met with FMID storage and other sources such as Palisades storage and rental water. The physical water is provided by Crosscut Canal injection as needed. In dry years such as 2021 and 2022, the exchange wells provide a mechanism for both administrative and physical delivery of water to the Teton River.
The total amount of injection into the Teton River was lower in 2022 than in 2021, reflecting overall lower diversion rates in 2022. Because exchange pumping is used only during dry years, it is not surprising that both 2021 and 2022 ranked among the highest years of exchange-well pumping. In both years, Crosscut Canal diversion was lower than average, in part because exchange pumping offset some of the need for Crosscut delivery and in part because
overall diversion was lower.
Lower watershed streamflow
In most years the Henry’s Fork irrigation system can be managed to meet physical demand only within the watershed, subject of course to the basin-wide administrative constraints mentioned above. In this case, the amount of water added to the lower-watershed irrigation system from the exchange wells and the watershed’s three storage reservoirs (Grassy Lake, Henry’s Lake, and Island Park Reservoir) must meet within-watershed diversion and leave enough in the river to provide adequate river stage (“depth”) at the lowest points of diversion and maintain basic aquatic ecosystem function in the lower Henry’s Fork. Surface water can leave the Henry’s Fork irrigation system through three pathways: the South Fork Teton River, North Fork Teton River, and mainstem Henry’s Fork. Higher streamflow in any one of these stream channels increases the amount available locally for aquatic ecosystem function, but comes at the costs of higher exchange well pumping and higher draft of the reservoir system. The latter, in turn, has negative consequences for fisheries and aquatic ecosystem function upstream of, in, and immediately downstream of Island Park Reservoir. On the other hand, too little streamflow at the bottom of the irrigation system can lead to lower fish habitat and a shortage of water available for the downstream-most diversions, which are Rexburg Irrigation on the South Fork Teton, Teton Island Feeder on the North Fork Teton, and Consolidated Farmers on the Henry’s Fork.
The general management strategy to balance these factors within administrative constraints is to:
set flow in the North Fork Teton to 0 downstream of Teton Island Feeder when administrative storage is being used,
maintain flow in the South Fork Teton just high enough to absorb daily fluctuation in diversion at Rexburg Irrigation (~50-150 cfs),
fix a streamflow target flow in the lower Henry’s Fork,
minimize flow variability in the South Fork Teton and lower Henry’s Fork.
The first of these components ensures that only the amount of water needed by the Teton Island Feeder water users is delivered to the North Fork Teton during the period when storage is being used. Any water in excess of this amount is charged to storage users, even if they can’t or don’t divert it all. There are several small diversions on the lower North Fork Teton, but they are entitled to only natural flow that emerges as groundwater inputs and return flows to the river downstream of Teton Island Feeder. Once the North Fork Teton constraint applies, a set of headgates at the North Fork-South Fork split (called the “splitter) is operated to send the appropriate amount of water down the North Fork, with the remainder flowing down the South Fork.
The total flow reaching the splitter is controlled by how much Henry’s Fork water is diverted and delivered through the Crosscut Canal. On the Henry’s Fork, water is delivered from the reservoir system to meet the Crosscut Canal need and meet diversion on the Henry’s Fork downstream of the Crosscut plus the lower-Henry’s Fork streamflow target. Historically, that target was set at the St. Anthony streamflow gage and was around 1,000 cfs. However, that target does not consider variability in diversion downstream of St. Anthony and generally resulted in very low flows downstream of Consolidated Farmers Canal in July, higher releases than necessary from Island Park Reservoir later in the season, and generally higher variation in streamflow all summer. To remedy these shortcomings, the Henry’s Fork Drought Management Planning Committee set the target at 350 cfs immediately downstream of the Consolidated Farmers diversion in 2020 and 2021. This flow is calculated by subtracting diversion from the four canals downstream of St. Anthony from flow at the St. Anthony gage and nominally represents the flow in the river at the Parker-Salem Highway (aka Red Road) bridge. In 2020, this new “Parker” target saved over 1,200 ac-ft in Island Park, while lowering variability.
Implementing this strategy is much easier said than done, given daily changes in diversion at over 100 pumps and canals in the watershed, stream and canal losses to and gains from groundwater, and streamflow travel times of around 20 hours from Island Park Reservoir to the Crosscut Canal diversion (Chester Dam), several more hours to reach the Teton River, and another hour or two to pass the splitter and reach the Rexburg Irrigation and Teton Island Feeder diversions. Fortunately, remote-controlled headgates at the Crosscut diversion and splitter installed in 2020 now allow FMID managers to make small adjustments at any time of day, saving water and reducing travel and time costs. New stream and canal gages and calculations provide real-time data to inform operation of the new remote-controlled headgates.
In some years physical water from the Henry’s Fork watershed must be delivered to meet demand farther down on the Snake River (when physical water stored in Island Park belongs to American Falls Reservoir on paper), and 2022 turned out to be one of those years. Outflow from Island Park Reservoir was increased on August 24 above that needed to meet within-watershed needs as specified in the management strategy above and maintained at that level until September 19, two days prior to the end of reservoir draft. Streamflow at Parker during the complete period of Island Park Reservoir draft averaged 561 cfs, while streamflow during the period of reservoir draft but outside of the August 24-September 19 window averaged 466 cfs. The 2018-2021 average is 462 cfs, so within-watershed management in 2022 was consistent with that implemented over the past five years. Streamflow at Parker during the August 24-September 19 window averaged 780 cfs, and the difference between that and 466 cfs equates to about 16,140 ac-ft of water sent from the Henry’s Lake and Island Park Reservoir to American Falls.
Even with that, streamflow at Parker and St. Anthony during the period of reservoir draft in 2022 was lower than the 2001-2022 average, saving water in Island Park Reservoir. Further, despite the additional variability in flow associated with this 26-day period of higher streamflow, coefficient of variation in streamflow at Parker in 2022 was only slightly higher than average. Streamflow in the South Fork Teton River at Rexburg was the same as it was in 2021 but with lower variability. These observations show that more precise management of the Henry’s Fork irrigation system saves water in Island Park Reservoir and reduces day-to-day variability in lower-watershed streamflow even in years when additional water must be sent out of the watershed to meet basin-wide needs.
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