• Rob Van Kirk

Summer 2017 Streamflow Predictions: Much Better than 2016

To provide streamflow information for all river stakeholders, we have constructed a computer simulation model of the Henry’s Fork watershed stream, reservoir, and irrigation system. Using early-April water-supply conditions and long-term temperature trends as inputs, we expect streamflow conditions to be generally better than average and much better than last year across the watershed. Focusing specifically on Island Park Reservoir and the river immediately downstream, we predict:

  • Streamflow during the second half of June at Island Park Dam will be roughly equal to the river’s natural flow. With 90% probability, this natural streamflow will range between 400 and 750 cfs.

  • Irrigation delivery from Island Park Reservoir will begin around July 1 and peak in mid-July. With 95% probability, releases from the reservoir during July will be lower than 1,400 cfs, and with over 95% probability will be much lower than 2016 releases between the middle of June and the first week of August.

  • With 95% probability, Island Park Reservoir contents at the end of the September will remain above 58,000 ac-ft (43% full), very close to the long-term average.

  • Because of lower outflows and higher reservoir contents, turbidity (how “dirty” the water appears) in the river immediately downstream of Island Park Dam is expected to remain lower than the high values observed in 2016 during the mid-July cyanobacteria bloom and late-summer reservoir drawdown.

What, when, where and why?

We have constructed a dynamic, randomized (the technical term is stochastic) computer simulation model of the Henry’s Fork water system, including natural streamflows, reservoir storage and delivery, river reach gains and losses, and irrigation diversion. In addition, because water clarity (measured by “turbidity”) downstream of Island Park Dam is critically important to the early- and mid-summer fishing experience, we have used statistical analysis of turbidity data collected in 2016 by our continuous-recording instruments to develop a simulation model of turbidity. A more detailed description of model inputs, methods, and assumptions is given in the “How the model works” section at the bottom of the blog.


The outputs are a series of hydrographs of the various system components over the period April 1 through September 30, 2017. Our predictions for 2017, including the range of values that will occur with 90% probability, are compared to conditions in 2016 and to long-term averages. A guide to interpreting the graphs is given in a separate section following the graphs. These predictions are designed to provide timely and relevant information to help anglers, fishing outfitters and guides, other river recreationists, water users, and water managers anticipate river, reservoir, and irrigation-delivery conditions during the upcoming summer. Model outputs, in the order we report them in this blog, are:

  1. Island Park Reservoir contents, and streamflow and turbidity in the Henry’s Fork immediately downstream of Island Park Dam.

  2. Henry’s Lake contents and outflow.

  3. Streamflow in the Henry’s Fork at Ashton, Fall River at Chester, Henry’s Fork at St. Anthony, and Teton River immediately upstream of the Crosscut Canal inflow point.

  4. Grassy Lake contents and outflow.

  5. Diversion of water into the Crosscut Canal at Chester Dam to be delivered to the Teton River to meet irrigation demand.

Island Park Reservoir




The reservoir fills around May 20, natural inflow is passed through the full reservoir until around July 1, modest irrigation delivery is required through July and August, and the reservoir reaches its minimum volume by the middle of September. With 95% confidence, ending reservoir content is very near or above its long-term mean of around 58,000 ac-ft (43% full). Expected ending content is around 110,000 ac-ft (81% full). Outflow is expected to remain below 1,000 cfs for the entire spring and summer, although flows up to 1,400 cfs during runoff in late May and again during irrigation season are possible within the 90% prediction interval.


One important thing to note is that between date of reservoir fill and the need for irrigation delivery, outflow from Island Park will be equal to the river’s natural flow (as if neither Henry’s Lake nor Island Park Reservoir existed). Because natural streamflow in the upper Henry’s Fork watershed is predicted to be only 85-90% of average, and because runoff is expected to occur slightly earlier than average, these natural streamflows during June may be as low as around 400 cfs and are unlikely to be much higher than 750 cfs—below average in any case. These values are completely determined by the natural water supply of the river, since the Island Park Reservoir management objective for all stakeholders is to keep as much water in the reservoir as possible. Delivering water during late June to keep river flows higher for recreational purposes when it is not needed for irrigation delivery will result in decreased water quality later in the summer and in decreased outflow during the subsequent winter.


Turbidity is expected to be a little higher than last year during spring and the early part of the summer, due to higher inflow during snowmelt. However, the high spikes in turbidity observed last year during mid- to late-summer are very unlikely to occur this year because the majority of the outflow will pass through the power plant rather than the west-side gates and because the reservoir will remain much higher than last year.


Henrys Lake



Henry’s Lake is expected to fill around its normal time in mid-June and end up pretty close to its long-term average by the end of the summer. Outflow is expected to increase to almost 200 cfs during runoff but could get much higher if runoff is higher than expected and occurs in a short time window in early June.


Streamflow in the lower watershed





With 90% probability, streamflow in the Henry’s Fork at Ashton will be lower than the long-term average from the middle of May until the middle of August. This reflects the prediction of below-average streamflow and slightly earlier runoff timing for the upper Henry’s Fork watershed. However, streamflow during April and May will nearly certainly exceed last year’s streamflow. Late-summer flow at Ashton is expected to be near the long-term average and much higher than last year.