Recent heavy rains will impact the dry fly fishing for a few more days,. The river is running higher than normal throughout and fish have enjoyed life forms washed into it from the impact of high water on banks and stream bottoms. The Harriman State Park reach opens tomorrow, but expect a slowdown in fly fishing there for a while.
Dr. Rob Van Kirk water status report compiled yesterday for the Henry’s Fork drainage follows. It contains information on how recent rains will impact fishing throughout the drainage for a while.
Headlines:
- Record-setting rain and flooding just to our north and east in Montana and Yellowstone National Park made national headlines yesterday, and these conditions extended down into Island Park as well.
- Natural streamflow appears to have peaked at most locations in the watershed late yesterday and was 120% of average for the date.
- Outflow from Island Park Reservoir will be increased to 1100 cfs this morning to keep pace with increasing inflow.
- Flooding during a drought? Short- and medium-term drought indicators continue to improve, while long-term low water supply conditions continue.
Details:
By now, most of you have seen regional and national news coverage of record-setting rain and a rain-on-snow event causing extreme flooding and damage in Yellowstone National Park and adjacent areas. Some streams set new all-time record highs. Island Park sat on the southern edge of the heaviest precipitation and has also experienced flooding, especially from streams that drain the Centennial and Henry’s Lake mountains.
Two-day precipitation totals for our watershed ranged from 0.67 inch at Alta to 3.3 inches at White Elephant. For those of you who are not familiar with the SnoTel stations, White Elephant sits on the eastern side of Mt. Sawtelle in Island Park and is a good indicator of conditions along the Continental Divide. The watershed average was 1.57 inches, which brought the water-year total up to 91% of average. Total for the first 13 days of June is 2.5 inches, which is the average total for the whole month of June. More precipitation is in the forecast, and this June’s monthly precipitation is likely to end up in the top 25th percentile of all years since 1989.
Mean temperature over the past two days was 5 degrees F below average, which kept melt of remaining snow below average. Some locations even received a little new snow this morning. Remaining snow water equivalent (SWE) is 19% of this year’s peak and 67% of average for the date. Warmer temperatures later this week will increase high-elevation snowmelt and maintain relatively high streamflow in Teton River well into next week.
Natural streamflow peaked in headwater areas late yesterday, and that peak is now making its way down through the lower watershed. Streamflow at most locations yesterday was above average or even well above average for the date. Total natural flow yesterday was 120% of average: 89% in upper Henry’s Fork, 131% in Fall River, and 136% in Teton River. The reason for the low figure in the upper Henry’s Fork is because spring-fed base flow is still very low and reflective of long-term drought. Discharge from the large springs at Big Springs, Buffalo River, Warm River and other locations responds to changes in precipitation and snowmelt on the scale of years to decades. We will need to receive above-average precipitation—and particularly snow—for several years to see those base flows increase from near-record lows back up to average.
However, Fall River and Teton River both respond rapidly to rain and snowmelt, and both increased substantially due to both factors over the past two days. As mentioned several times over the past week, the Memorial Day rain appears to have saturated soils to the point where more recent rains have produced strong and rapid streamflow response. I expect Fall River to drop over the next few days, while Teton River will increase again later this week due to melt of high-elevation snow. Diversion decreased by 13% yesterday, and I see no indication that reservoir draft will be needed any time soon to meet irrigation demand.
For some context on current streamflow, a reader suggested I look at some all-time high flows in the watershed. Current watershed-wide natural flow is around 7,000 cfs, which ranks in the bottom quartile (25th percentile) of the annual peak in water years since 1978. The highest natural flow in that record was 17,423 cfs in 2011. That was a year in which heavy rains during the spring and summer fell on top of an above-average snowpack. As mentioned above, this year’s heavy spring rains fell on a very poor snowpack and low base flows.
Current natural flow at Island Park is 1,330 cfs, which ranks 20th lowest out of the 51 years since 1972. The maximum natural flow in that record was 3,155 cfs, in 1984. In the lower watershed, current flow in the Henry’s Fork at Rexburg is around 5,500 cfs. That ranks 15th lowest among maximum annual flows in that same set of 51 water years. Streamflow at Rexburg will continue to increase for another day or two as water makes its way to the lower watershed. The highest daily flow recorded at Rexburg was 79,000 cfs during the Teton Dam flood. Otherwise, the highest annual maximum was 15,100 cfs, in 2011. So, current flows, while near or above average for the date, are generally in the bottom one-third of annual peak flows
Island Park Reservoir fill rate slowed yesterday after an increase in outflow to 950 cfs, but outflow is still lower than inflow. Outflow from Henry’s Lake was increased yesterday to around 110 cfs to keep pace with very high inflow. That will increase inflow to Island Park Reservoir later today. Outflow from Island Park is being increased this morning to 1,100 cfs, which should get ahead of inflow by tomorrow and put the reservoir on track to stay at full pool for the next week. Average outflow on the Harriman Ranch opener is 1,000 cfs. With any luck, outflow will not change for at least several days, but precipitation keeps beating expectations, and more is forecast for Sunday and Monday. The upper Snake River reservoir system continued to gain volume yesterday and is 61% full, its highest level of the year so far. The American Falls storage right is currently accruing paper water, which has happened on only a very few days since early April.
