Island Park Reservoir Continues to Fill Rapidly

Early October snow at Island Park Dam

Conditions as of 12:00 a.m. Oct. 11, 2016

  • Reservoir contents: 33,384 acre-feet (24.7% of capacity)
  • Outflow: 127 cfs by USBR gates; 76 cfs by USGS gage (see below)
  • Inflow: 60 cfs from Henrys Lake + 425 cfs reach gain from Henrys Lake to IP
  • Mean storage rate since last flow adjustment, made October 6: 695 ac-ft per day
  • Total volume of water stored since storage began on September 13: 12,904 ac-ft


Continued cool, wet weather has increased water supply throughout the Henry’s Fork watershed over the past few weeks. Over the past ten days, cumulative precipitation amounts have exceeded one inch throughout the watershed, with much higher amounts in the mountains (see Table 1 below). Natural flow in the watershed upstream of Island Park Reservoir has reflected this precipitation (see Figure 1 below). While still below average, natural flow at Island Park has generally been much higher that it was at this time last year and is now closer to average than to last year’s record low levels. This additional streamflow has greatly increased the rate of fill of Island Park Reservoir in the last week. In the lower watershed, diversions have remained low and fairly steady, although some canals have slightly increased diversion rates in recent weeks as more natural flow has become available. Periodic reductions in outflow at Island Park have very closely matched increases in streamflow and reductions in irrigation demand; in between rainstorms, streamflow at the St. Anthony gage has remained very close to the 850 cfs target established by the Drought Management Planning Committee (DMPC) in August (see Figure 2).

Table 1. Cumulative precipitation since October 1 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 Dam

Figure 1. Watershed yield (natural streamflow) at Island Park, September 1 – October 10, 2016, compared with that in 2015 and the 1979-2016 average.

Graph of streamflow at St. Anthony

Figure 2. Streamflow in the Henrys Fork at St. Anthony, September 1 – October 10, 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 13,000 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. Given that flow at St. Anthony is currently right at the 850 cfs target, this reduction will probably not occur until the next precipitation event increases streamflow in the lower watershed. A period of relatively heavy precipitation is forecast to begin this Friday and extend well into next week, providing the opportunity to reduce Island Park outflow to around 70 cfs while still maintaining streamflow at St. Anthony above 850 cfs. Buffalo River flow has remained steady at around 190 cfs, increasing somewhat in response to precipitation events. Thus, the lowest streamflow we anticipate through Box Canyon and Last Chance is 260 cfs.

Island Park outflow will remain at its minimum until very cold weather arrives later in the fall, at which time outflow will be increased. On September 18, I modeled a potential winter scenario in which I assumed that outflow will be increased around November 20. That model predicted that reservoir content would be 33,053 acre-feet today, compared with today’s actual value of 33,384 acre-feet. Thus, the model has performed extremely well so far this fall, only slightly under-predicting reservoir inflow. This model predicts that the reservoir will fill to last year’s April 1 value if outflow is increased to 120 cfs on November 20. This outflow from the dam will produce a little over 300 cfs in the river from the Buffalo River confluence through Harriman State Park. However, if wet weather continues, reservoir inflow will continue to exceed model projections, and mid-winter outflow greater than 120 cfs becomes increasingly likely.

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 (see Figure 3). 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 127 cfs according to the gate setting, whereas the USGS gage is reading 76 cfs. Reliance on the gates to set outflows has resulted in a much higher degree of precision in management than last year and one reason the DMPC objectives have been met so far this fall.

My calculations suggest that the USGS gage at Ashton is also reading lower right now than actual streamflow by about 60 cfs. Based on streamflow in Fall River and at St. Anthony and on diversion rates between Ashton and St. Anthony, streamflow at Ashton is probably closer to 700 cfs than to 643 cfs, which is the average of Ashton gage readings over the past three days. By the time USGS conducts the next round of field measurements and rating-curve adjustments in the Henry’s Fork watershed, aquatic plant decay should be nearly complete, and the resulting stream gage readings should remain accurate through the early part of the winter.

Graph of streamflow at Island Park, as measured by USGS gage and by gate opening.

Figure 3. 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.