Precipitation Continues to Headline HF Water Situation

Henrys Fork near Marysville, with snow-covered Teton Range in background

Conditions at Island Park Reservoir as of midnight, Monday 10/17/2016

  • Reservoir contents: 37,588 acre-feet (27.8% of capacity)
  • Outflow: 129 cfs by USBR gates; 71 cfs by USGS gage (see below)
  • Inflow: 60 cfs from Henrys Lake + 560 cfs reach gain from Henrys Lake to IP
  • Mean storage rate since last flow adjustment, made October 6: 699 ac-ft per day
  • Total volume of water stored since storage began on September 13: 17,108 ac-ft


Precipitation continues to headline hydrologic conditions in the Henry’s Fork watershed. The most recent wet-weather period, which has yet to end, is the first of the season to come directly from the Pacific Ocean. As a result, the orographic effect has been larger than in precipitation events earlier this fall. Precipitation amounts since last Friday have been 2-3 times seasonal average at lower elevations but 5-10 times average at higher elevations (Table 1). This precipitation is reflected in natural flow in the upper Henry’s Fork (Figure 1) and in Fall River (Figure 2). The most encouraging trend, apparent in both of these graphs, is an increase in baseflow since September 1, during a time period when, on average, streamflow declines. This increase in baseflow indicates steady cumulative increase in soil moisture and shallow groundwater storage, which maintain higher streamflows between runoff from precipitation events. This additional streamflow continues to increase the rate of fill of Island Park Reservoir. In the lower watershed, diversions have remained fairly steady, as some canals have reduced diversion while others have slightly increased diversion rates to take advantage of recent increases in natural flow availability. Over the past week, diversion has averaged around 85 cfs from Fall River, 175 cfs from the Teton River, and 460 cfs from the Henry’s Fork. Additional outflow reduction from Island Park was not possible last week because until late on Saturday Oct. 15, streamflow at the St. Anthony gage did not exceed the 850 cfs target (Figure 3) established by the Drought Management Planning Committee (DMPC).

Table 1. Precipitation during the three-day period October 14-16 at selected sites in and adjacent to the Henry’s Fork watershed. Sites are listed in increasing order of elevation.


Elevation (ft)

Precipitation (inches)

Average (inches)













Island Park




Pine Creek Pass




Crab Creek




Grassy Lake




White Elephant




Black Bear




Graph of natural flow at Island Park, showing 2015, 2016 and average.

Figure 1. Watershed yield (natural streamflow) at Island Park, September 1 – October 16, 2016, compared with that in 2015 and the 1979-2016 average.

Graph of streamflow in Fall River, showing increase due to recent precipitation.

Figure 2. Streamflow in Fall River upstream of all diversions, September 1 through October 16, 2016.

Graph of streamflow in Henrys Fork at St. Anthony.

Figure 3. Streamflow in the Henrys Fork at St. Anthony, September 1 – October 16, 2016.

Island Park operations

Island Park operations have continued to be implemented as planned. Since minimum reservoir volume was reached on 9/13, aggressive and precise reductions in outflow have allowed storage of over 17,000 acre-feet in the reservoir, while generally maintaining flow at St. Anthony at or above 850 cfs. To meet the DMPC objectives set at meetings in August and September, one more reduction in Island Park outflow will be required. As predicted last week, the current precipitation event has increased streamflow sufficiently that a decrease in Island Park outflow to about 70 cfs will be possible early this week. Given recent rain, flow in the Buffalo River will stay above 190 cfs for the next week or two, yielding a total streamflow of no less than 260 cfs through Box Canyon and Last Chance for the next few weeks. Current weather forecasts call for a week of dry weather from the middle of this week until early next week. As occurred during the last period of dry weather—October 7 through October 14—streamflow will recede from the current precipitation event, and flow at St. Anthony will decline back toward the 850 cfs target. However, as noted above, stream baseflows continue to improve with each storm, so this week’s decrease in outflow from Island Park will be offset by increased streamflow in tributaries between Island Park Dam and St. Anthony, even as streamflows recede from the current rain. In addition, watershed-wide diversion will decrease over the next two weeks from its current value of around 700 cfs to a typical winter-time value of around 300 cfs. This latter value is the long-term average of November-March diversion from the Henry’s Fork and its tributaries for stock water and other non-irrigation uses. Thus, streamflow at St. Anthony should remain above 850 cfs for the remainder of the fall and winter, even with low outflow from Island Park Dam between now and the arrival of cold weather.

As mentioned last week, storage of water in Island Park Reservoir continues to slightly exceed my projections from mid-September, and above-average precipitation continues to increase soil moisture and shallow groundwater storage. Taken together, these two watershed-wide improvements in water supply continue to increase the probability that mid-winter outflow from Island Park Dam will exceed my earlier prediction of 120 cfs (300 cfs through Box Canyon and Last Chance).  

Streamflow measurement

Since the last rating adjustment at the Island Park stream gage, the apparent flow recorded by the gage continues to drift away from actual outflow, as measured by the dam gates on the west side of the river (Figure 4). Remember that the hydroelectric plant is not operating right now due to outflows lower than the minimum operating requirement of the plant, so all outflow is being delivered through the gates. Continued decay of aquatic vegetation in the stream channel has resulted in gage readings lower than actual outflow. Right now, outflow is 129 cfs according to the gate setting, whereas the USGS gage is reading 71 cfs. Current gage readings at Henry’s Lake Outlet and Ashton are also somewhat lower than actual streamflow at those locations for the same reason.

Graph of apparent streamflow at Island Park, as measured by USBR gates and USGS gage.

Figure 4. 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.

Photo of HFF truck in snow, October 17, 2016.

First snow of the season in Ashton, Monday morning, October 17, 2016.