Projected Winter Flow Below Island Park Dam

Rainbow over Henry's Fork River.

Island Park Reservoir update, 10/3/2016

In this blog, I provide:

  1. an update on current conditions at Island Park Reservoir (20% full, filling at 400 ac-ft per day, with 7,500 ac-ft filled already this fall),
  2. a review of management objectives (lower flows to 60-70 cfs by mid-October, while keeping flow at St. Anthony around 850 cfs),
  3. my best estimate of fall and winter operations and winter flows at the dam (120 cfs from the dam + 180 cfs from the Buffalo River after November 20),
  4. analysis of USGS streamflow measurement at Island Park (consistently lower than outflow measured through dam gates as plants have decayed), and
  5. some statistics on how water year 2016 stacked up (only slightly better than 2015 and second lowest natural streamflow since 1979).

Current conditions at Island Park Reservoir

  • Reservoir contents: 27,897 acre-feet (20.7% of capacity)
  • Outflow: 210 cfs by USBR gates; 199 cfs by USGS gage (see explanation below)
  • Inflow: 60 cfs from Henrys Lake + 450 cfs reach gain from Henrys Lake to IP
  • Mean storage rate since last flow adjustment, made September 28: 398 ac-ft per day
  • Total volume of water stored since storage began on September 13: 7,417 ac-ft

Island Park operations since 8/9/2016

Since the Drought Management Planning (DMP) Committee met on August 9, Island Park operations have proceeded as planned. Outflow from Island Park Reservoir has been reduced every week or so to meet three objectives:

  1. Keep reservoir contents above 20,000 ac-ft,
  2. Maintain streamflow in the Henry’s Fork at St. Anthony at or above 850 cfs, and
  3. Continue to reduce outflow every week until outflow reaches 60 cfs by mid-October. The actual flow will depend on the reservoir elevation at the time the gate is set at its minimum operational opening and is likely to be closer to 70 cfs rather than 60 cfs.  

Objective 1 was met, and we are well on our way to meeting objective 3. Objective 2 has been more difficult to meet, especially because prior to recent rating adjustments of the USGS gage stations in the watershed, the St. Anthony gage was reading about 100 cfs higher than actual flow. Although instantaneous readings have frequently fallen below 850 cfs due to natural daily cycles caused by aquatic vegetation in the Last Chance-Pinehaven reach, daily means have remained at or above 850 cfs except for a few days around September 20 (see Figure 1). However, diversion into the four canals downstream of the St. Anthony gage has steadily decreased since mid-August, leaving sufficient water in the river downstream of St. Anthony.

Graph of streamflow in Henrys Fork at St. Anthony during August and September 2016.

Figure 1. Streamflow in the Henrys Fork at St. Anthony, August 9 – October 3, 2016.

Anticipated fall and winter operation

Because of the aggressive fill strategy set by the DMP Committee, over 7,500 ac-ft has been stored in Island Park since September 13. Recent rain has increased reservoir inflow over what it would have otherwise been, resulting in about 1,500 ac-ft of storage that we would not have realized had the last month been as dry as June, July and August. Henrys Lake will continue to supply around 60-70 cfs through early November, providing additional inflow to Island Park Reservoir until then. Outflow will continue to be gradually reduced over the next few weeks until the minimum of 60-70 cfs is attained. The river’s total flow through Box Canyon, Last Chance and Harriman State Park will also include the Buffalo River, which is currently flowing at 190 cfs. I anticipate total flow no lower than 260 cfs through this reach all fall and winter.

Outflow will be increased when extremely cold weather arrives. In modeling potential winter scenarios, I assume this will happen around November 20. Based on a statistical model that has predicted winter inflows to within a few percent over the past several years, I predict inflow to be 79% of the 1979-2015 average, which is not great but is about 20 cfs better than last year. Under these predictions, outflow could be increased to 120 cfs on November 20, and the reservoir will reach content equal to last year’s on April 1. This would mean that we can equal last year’s outflow and fill, which I think would be a great success for the river, given where we started. This outflow from the dam will produce a little over 300 cfs in the river from the Buffalo River confluence through Harriman State Park. The three reasons we are likely to achieve this, given the low starting volume, are: 1) much more aggressive fill management this fall compared with last fall, 2) more precise implementation of the autumn fill strategy this year (see next section), and 3) better inflows than those last year as a result of better snowpack in 2016. Current weather predictions call for above-average precipitation for the month of October. If that happens and more water is stored in October, then winter outflow greater than 120 cfs is possible.  

Streamflow measurement at Island Park Dam

September and October are the worst months for discrepancy between the USGS gage reading and actual outflow, as aquatic vegetation rapidly decays and moves out of the channel. This rapid decrease in vegetation causes river depth to drop, even as flow stays the same. As a result, in between rating-curve adjustments, the apparent flow shown by the real-time USGS gage will be lower than actual flow. Over the past few weeks, we have had a good opportunity to compare actual outflow with apparent gaged outflow, because the Island Park hydroelectric plant has not been operating. When the power plant is operating, some or all outflow comes through the power plant, where flow cannot be directly measured, leading to reliance on the USGS gage station for estimation of outflow. However, recently, all outflow has passed through the USBR gates on the west side of the dam. Outflow from the gates is determined by a well-defined formula that depends on gate opening and reservoir elevation. This relationship does not involve river depth and so is not affected by plant growth and decay or other changes in the river channel.

Figure 2 compares outflow from Island Park Dam as measured by the USGS gage, both before and after rating adjustments, and as measured directly through the USBR gates. It is easy to see that when the rating curve is adjusted (usually the day following field measurements), the USGS gage and USBR gates show the same outflow. Between the rating curve adjustments on August 26 and September 21, the unadjusted USGS gage consistently gave flow estimates lower than what was actually being delivered out of the gates, with the greatest discrepancy occurring right before the September 22 rating adjustment, when plants had been moving out of the channel for several weeks since the last adjustment. USGS flow data after adjustment are much closer to flows measured at the USBR gates but still not exact for several reasons that I will attempt to explain in future communications. In any case, reliance on the USBR gates this fall to set outflows has resulted in a much higher degree of precision in management than last year and one reason the DMP Committee objectives have been met so far this fall.

Graph of outflow from Island Park Dam

Figure 2. Outflow from Island Park Dam as measured by the USGS gage and USBR gates. The “USGS unadjusted” values are those that appear as current data on the USGS web site in between rating adjustments. The “USGS adjusted” values are those that appear in the data archive after rating adjustments are made. The “USBR gates” values are calculated from the relationship among flow, gate opening, and reservoir elevation.

Some water-year 2016 teasers

Since water year 2016 is now over, I thought I would include some preliminary observations on 2016 water supply in the upper Henrys Fork watershed. These numbers will change slightly after I run the calculations again using approved USGS flow data. I will also calculate streamflow for other subwatersheds once the approved data become available.

Here I report natural basin yield (unregulated streamflow) in the Henrys Fork at Island Park Dam. This is the flow that would have passed the Island Park gaging station in absence of Henrys Lake and Island Park Reservoir (see Figure 3). While runoff during April and May of 2016 was substantially better than that in 2015, winter flow was lower in 2016, and mid-summer flow was no better in 2016 than it was in 2015. Both years were substantially below the modern average. Note the very early snowmelt peak in both years. The second peak in May of both years was due to rain.

Graph of water-year natural streamflow in Henrys Fork at Island Park.

Figure 3. Natural flow at Island Park Dam for water years 2015 and 2016, compared with the 1979-2016 average.

In terms of total water-year streamflow, 2016 was only slightly better than 2015, was the second driest year in the last 38 water years, and saw only 72% of the 1979-2016 average basin yield  (see Figure 4 and Table 1).

Graph of total water-year basin yield in Henrys Fork at Island Park, 1979-2016.

Figure 4. Annual water yield (unregulated streamflow) in the Henrys Fork at Island Park.

Table 1. Ten driest water years since 1979, as measured by natural basin yield (unregulated streamflow) in the Henrys Fork at Island Park. Time period for calculating average is 1979-2016. Note that 2014, 2015, and 2016 are all on this list.



Mean annual streamflow (cfs)

Annual basin yield (ac-ft)

Percent of average




















































This summer, many people, including some water-management professionals, have questioned my characterization of water year 2016 as very dry, because precipitation was not far below average. Total water-year precipitation at the relevant snow survey sites around the watershed ranged from 80% to 90% of the 1981-2010 average (Table 2). However, the more relevant measure of drought to us is water in the stream—this is the water that maintains fisheries, fills reservoirs, and is diverted for irrigation. The discrepancy between precipitation and streamflow is extremely concerning—why does precipitation at 80-90% of average yield streamflow of only about 70% of average? We theorize that the fraction of precipitation that actually ends up as streamflow is declining due to a variety of factors that could include increase in growing-season length, earlier snowmelt, and increase in forest cover on upland areas of the watershed over the past few decades. We will be carefully analyzing these kinds of data this winter to develop better predictive models of streamflow in the Henrys Fork and neighboring watersheds and increase our understanding of how climate change is affecting water supply.

Table 2. Total precipitation for water-year 2016 at snow survey sites in and adjacent to the upper Henrys Fork watershed.



2016 precipitation (inches)

1981-2010 average (inches)

Percent of average

Black Bear




Crab Creek




Grassy Lake




Island Park




Lewis Lake Divide




White Elephant