Conditions at Island Park Reservoir as of 12/17/2016

  • Reservoir contents: 72,446 acre-feet (53.6% of capacity)
  • Mean outflow since 12/10/2016: 164 cfs by USGS gage; 161 cfs by USBR gate setting
  • Mean streamflow through Box Canyon since 12/10/2016: 354 cfs
  • Inflow: 0 cfs from Henrys Lake + 360 cfs reach gain from Henrys Lake to IP
  • Mean storage rate since 12/10/2016: 386 ac-ft per day
  • Total storage since 9/13: 51,965 ac-ft (starting value was 20,481 ac-ft)

Water Supply Overview

Following the Halloween rain-on-snow event, streamflows have receded to expected winter-baseflow levels (Figure 1). The increase over the past few days represents both increased streamflow during warm-sector precipitation on Thursday 12/15 prior to Friday’s cold front and the large amount of snow that fell directly on the reservoir surface during the most recent storm. Because of above-average precipitation since October 1 (Table 1), watershed inflow between Henry’s Lake and Island Park Dam has exceeded my September prediction by 869 ac-ft.

Graph of inflow to Island Park Reservoir

Figure 1. Reach gains between Henry’s Lake and Island Park, September 1 – December 17, 2016, compared with the prediction I made in September, observed values in 2015, and the 1979-2016 average. Note that I have changed the color scheme from that I have used in previous posts to match that used in U.S. Bureau of Reclamation Hydromet graphs.

Despite above-average precipitation since the beginning of the water year, current snow-water-equivalent (SWE) remains slightly- to well-below median at the four snow survey sites most relevant to streamflows in the upper Henry’s Fork watershed (Table 1). As of this morning (December 18), only four snow survey sites in the entire upper Snake River basin have below-median SWE on the ground; three of these are in the upper Henry’s Fork watershed (Black Bear, Island Park, and White Elephant sites), and the fourth is in the Teton watershed near Teton Pass (Phillips Bench site). Note that the Crab Creek site, while in the Centennial Range, is technically in the Sinks drainage and not directly in the upper Snake River drainage basin. Snow that had accumulated at these sites back in October melted during the Halloween rain event. November was much warmer than average and slightly dry, so snowpack at these sites has only recently begun to accumulate again. If current short- and long-term forecasts for below-average temperatures and above-average precipitation hold, SWE at these four sites should exceed the 30-year median by early January. Effect of this winter’s snowpack on streamflow and reservoir management will depend heavily on how early the snowpack melts, regardless of how much accumulates during the winter. Because of this, we will be monitoring temperatures and snow conditions carefully throughout the middle and late winter.

Table 1. Summary of water year-to-date snow-water-equivalent (SWE) and precipitation at the four snow survey sites that reflect water supply in the upper Henry’s Fork watershed. Median SWE and mean precipitation are calculated over water years 1981-2010. Data are from the Natural Resources Conservation Service (



Elev. (ft)

SWE (in)

Median SWE (in)

% of Median SWE

Precip. (in)

Mean precip. (in)

% of mean precip.

Crab Creek

Centennial Range








White Elephant


Henry’s Lake








Black Bear

Yellowstone Plateau








Island Park

HF Caldera








 Island Park Reservoir Management and Streamflow

As decided at the November 21 meeting of the Island Park Drought Management Planning Committee (DMPC), outflow from Island Park Reservoir was increased during the week of November 28. The weather forecast at the time indicated that the first single-digit lows of the season would occur around December 2. Accordingly, outflow was increased on November 30 (Figure 2) from 100 cfs to 150 cfs (Figure 2). A slight upward adjustment was made on December 9 to accommodate an adjustment of the USGS stream stage recorder made on December 7 (see next section). Since December 10, outflow from the dam has averaged slightly above the DMPC’s winter objective of 160 cfs, and total streamflow downstream of the Buffalo River has averaged 354 cfs, compared with last year’s 299 cfs and the long-term average of 531 cfs (Figure 2). The reservoir continues to fill at a steady rate of around 380 ac-ft per day, which will put reservoir content right at average on April 1 (Figure 3), a target set by the DMPC.

Graph of streamflow in Henry's Fork through Box Canyon.

Figure 2. Total streamflow in the Henry’s Fork through Box Canyon (IP Dam outflow + Buffalo River flow) since October 1 2016, compared with observed values in 2015, and the 1979-2016 average. The color scheme matches that in figures 1 and 3.

Graph of Island Park Reservoir content.

Figure 3. Island Park Reservoir contents, compared with last year and the long-term average. Graph from the USBR Hydromet,  

Streamflow measurement

The November 8 field measurement and rating adjustment produced a 0-shift rating, which remains in place. However, the November 8 field measurement of river stage (depth) deviated slightly from that measured by the continuous pressure recorder. The December 7 field measurement showed no deviation, and the subsequent adjustment to the stage record resulted in a slight adjustment to streamflow (Figure 4). Prior to this adjustment, apparent streamflow was around 160 cfs, compared with an actual measured value of 145 cfs. The slight increase to outflow made on December 9 brought actual outflow in line with the DMPC’s desired winter outflow of 160 cfs and with the outflow indicated by the setting on the USBR outflow gates.  

Graph of streamflow downstream of Island Park Dam, as measured by three methods.

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 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.