Conditions at Island Park Reservoir as of 11/10/2016

  • Reservoir contents: 54,480 acre-feet (40.3% of capacity)
  • Outflow: 100 cfs, according to recently adjusting USGS rating
  • Estimated streamflow downstream of the Buffalo River: 290 cfs
  • Inflow: 0 cfs from Henrys Lake + 375 cfs reach gain from Henrys Lake to IP
  • Mean storage rate since Henry’s Lake Dam outflow was reduced: 518 ac-ft per day
  • Total storage since 9/13: 33,999 ac-ft (starting value was 20,481 ac-ft)


Record rainfall October 28-29 on top of mid-elevation snow from the previous week produced some record runoff in unregulated streams throughout the upper Snake River basin. Fall River (Figure 1), Teton River, and Greys River all set daily flow records for one or more days around Halloween. This increased streamflow was also evident in inflow to Island Park Reservoir (Figure 2). However, streamflow from that rain-on-snow event has been steadily receding for the past 10 days due to an extended period of warm, dry weather. In the lower watershed, diversions have decreased to values typical of the off-season, around 150 cfs from the Teton River, 40 cfs from Fall River, 100 cf from the Henry’s Fork upstream of St. Anthony and 170 cfs from the Henry’s Fork downstream of St. Anthony. As a result of increased reach gains and decreased diversion, flow at St. Anthony has exceeded the 850 cfs target (Figure 3) established by the Drought Management Planning Committee (DMPC) back in August. Now that irrigation season has ended, I expect streamflow at St. Anthony to remain above 1000 cfs.

Graph of streamflow in Fall River.

Figure 1. Streamflow in Fall River upstream of all diversions, 9/1/2016-11/10/2016.

Graph of inflow to Island Park Reservoir, showing average, 2015 and 2016.

Figure 2. Reach gains (natural streamflow) between Henry’s Lake and Island Park, September 1 – November 10, 2016, compared with that in 2015 and the 1979-2016 average.

Graph of streamflow at St. Anthony

Figure 3. Streamflow in the Henrys Fork at St. Anthony, September 1 – November 10, 2016.

Streamflow measurement

The biggest event in management of Island Park reservoir that has occurred over the past two weeks was field measurement of flow at the Island Park gage and associated adjustment of the rating curve. This occurred on Tuesday, November 8, although as I write this, the field measurement and rating shift have not yet been entered in the online database. In my last report (October 27), I estimated that had the rating curve been adjusted on that date, the USGS gage would show a streamflow of 96 cfs, even though the gage was showing an apparent 76 cfs at the time. After the rating shift the other day, October 27 streamflow turned out to be 91 cfs. Figure 4 compares streamflow estimates at Island Park Dam, as determined from the settings on the USBR gates, USGS-measured flow using the previous rating shift (real-time, “unadjusted”), and USGS-measured flow using the current rating shift (“adjusted”).

Graph of streamflow at Island Park, showing three different flow estimates.

Figure 5. Outflow from Island Park Dam as measured by the USGS gage and USBR gates. The “USGS unadjusted” values are those that appear as current real-time 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.


For now the following observations are worth making.

  1. During this late-summer to early-fall period of rapid decay in aquatic plants from the stream channel, rating-curve shifts provide accurate real-time streamflow estimation for about 10-15 days following the shift.
  2. Immediately following the rating adjustments on August 27 and September 22, flow as estimated from the USBR gate openings was equal to that estimated from the USGS streamflow gage.
  3. Since the November 8 measurement and rating adjustment, the USBR gate opening indicates a flow of 112 cfs, whereas the USGS gage indicates a flow of around 100 cfs.
  4. Since the outflow reduction on October 6, the USBR gate settings have overestimated streamflow by 10-30 cfs, relative to the adjusted USGS gage readings.
  5. The three observations above suggest that at outflows greater than about 200 cfs, the USBR gate settings provide a reliable estimate of streamflow but at lower outflows, they may not. Discrepancies may vary depending on whether both gates are open or only one gate is open.


I will present a more careful analysis of flow-measurement issues later.

Looking ahead

As anticipated, outflow from Henry’s Lake Dam was reduced to 0 for the winter on November 5. That adjustment reduced inflow to Island Park Reservoir by 50 cfs. However, given reach gains around 375 cfs, fill rate is remaining good at over 500 acre-feet per day, even at an outflow of 100 cfs, which is about 20 cfs higher than we had originally thought would be necessary to keep reservoir fill rate high. Buffalo River flow has averaged around 190 cfs between rainfall events, leaving a total flow of about 290 cfs in the river through Box Canyon and Last Chance right now. Long-range weather forecasts call for a return to more average precipitation starting next week, with temperatures cooling enough to accumulate snow above 7,000 feet. However, no very cold weather is forecast, which at this point is good for fish survival at current streamflow. If the forecast warm temperatures hold through the end of the month, there should be no need to change outflow from Island Park until our intended target date of December 1. At a 500 acre-feet per day fill rate, reservoir content will be close to 66,000 acre-feet by that time, nearly equaling last year’s December 1 content of 66,878 acre-feet. Even prior to the record October rainfall, my baseflow model had predicted mid-winter inflow to be about 20 cfs higher this year than last year. Accounting for the recent rain has improved that prediction to as much as 40 cfs higher than last year. In addition, flow in the Buffalo River has remained about 10 cfs higher than last year. Thus, the prospects remain good for mid-winter outflow from Island Park around 20-40 cfs better than last year, producing 30-50 cfs more flow through Box Canyon than last year.

The DMPC next meets on Monday, November 21, at which time we will have a better idea of how baseflows will end up after recession from the October rain, whether the warm weather continues to hold, and how much water remains to be filled in the reservoir over the remainder of the storage season. The strategy for managing mid-winter outflow and reservoir fill will be set at that meeting.