Heavy Rain Produces High Streamflow: How high is high?

Photo of Henry's Fork in Box Canyon
  • May 22-24 precipitation totals were over 1 inch at most locations; water-year precipitation jumped from 105% of average to 109%.
  • Snowmelt continues at average rates, and SWE remains at 102% of average.
  • Watershed-total natural flow has increased to its highest level so far this year and higher than last year’s peak.
  • Inflow to Island Park Reservoir is around 1,600 cfs, and outflow is currently just a hair over inflow, allowing the reservoir to drop very slowly. Current reservoir content is a little higher than full pool.

NOTE: This blog post is essentially the same as my daily water report for Friday, May 25. If you would like to receive these daily reports via email, send me a message at rob@henrysfork.org.

Heavy Precipitation Fell May 22-24

The big story is a widespread, heavy rain event on Wednesday. Although rain had been in the forecast, amounts exceeded even short-term predictions. Three-day precipitation totals ranged from 0.7 inch at Crab Creek to 2.4 inches at Grand Targhee. All other stations received between 1.1 and 1.7 inches of rain. Highest amounts fell from Ashton eastward to the Fall River headwaters and northern end of the Teton Range. The valleys received just as much as the high-elevation areas, ranging from 1.29 inches at Rexburg to 1.68 inches at Ashton. Moisture availability in the agricultural areas is now 2 inches above average for the date.

Graph of water-year precipitation

Water-year precipitation as a percent of average, by subwatershed.

Graph of moisture deficit

Net moisture surplus/deficit in agricultural areas of Henry's Fork watershed.

Snowmelt Rate Remained Near Average

Meanwhile, afternoon high temperatures have generally been a little below average, but nighttime lows have been a little above average. The net effect was watershed-mean temperature around 2-4 degrees F above average over the past week.

Graph of temperature relative to average

Seven-day mean temperature, departure from average.

Despite the rain, snowmelt has continued to track average very closely, primarily because of cool temperatures during the peak of the precipitation. Much of Wednesday’s precipitation fell as wet snow at the higher elevations, and the low-elevation sites didn’t have any snow left to melt. Thus, Wednesday’s precipitation was truly a rain event and not a rain-on-snow event. Snow-water-equivalent (SWE) is currently 48% of this year’s peak, compared with an average of 55% remaining on May 25. Current SWE over the watershed is 102% of average, where it has been for the past week.

Graph of SWE in Henry's Fork watershed

Watershed-total snow-water-equivalent.

Streamflow Responded Quickly to Rain

Streamflow throughout the watershed responded rapidly to Wednesday’s rain. Natural streamflow yesterday was 11,869 cfs, higher than anything recorded so far this spring and higher than last year’s snowmelt-driven peak of 10,365 cfs, which occurred on May 13. Yesterday’s natural flow was 152% of average: 124% of average in the upper Henry’s Fork, 151% of average in Fall River, and 189% in Teton River. Natural flow in the upper Henry’s Fork was 3,465 cfs yesterday, up 728 cfs from Sunday’s value but still short of this year’s peak flow of 3,528 cfs on May 8. Yesterday’s natural flow was the peak so far this season on Fall River and Teton River. Natural flow is receding this morning in all subwatersheds. However, the streamflow peak is just reaching Rexburg this morning, and streamflow in the Henry’s Fork at the Rexburg gage is still increasing. The river is currently 2.5 inches above flood stage there.

Graph of natural streamflow in HF watershed

Graph of total natural streamflow in the Henry's Fork watershed.

Watershed-total diversion has tracked average very closely over the past week and is a little over 2,600 cfs, average for the date.

Graph of diversion in Henry's Fork watershed

Graph of watershed-total diversion.

Henry's Lake and Island Park Reservoirs Full

Both Henry’s Lake and Island Park Reservoir gained a lot of volume over the past week, due both to increased streamflow and direct precipitation. Henry’s Lake is 99.6% full, just a few hundred acre-feet short of full pool.

Graph of Henry's Lake inflow

Graph of inflow to Henry's Lake.

Graph of Henry's Lake volume

Graph of Henry's Lake volume.

Island Park is currently 880 ac-ft in excess of full, and the reservoir level is about 1.5 inches above full-pool elevation, resulting in some water spilling over the top of the spillway. Current inflow to Island Park is around 1,650 cfs. Outflow is 1,700 cfs, allowing the reservoir to drop slowly back down to its authorized full-pool elevation.

Graph of inflow to Island Park Reservoir

Graph of net inflow to Island Park Reservoir.

Graph of Island Park Reservoir volume

Graph of Island Park Reservoir volume.

Natural flow at Island Park yesterday was 1,850 cfs. This is the flow that would be present in the river at Island Park Dam if neither Island Park nor Henry’s Lake dams were in place to store water. Outflow from Island Park is still a little less than natural flow, so outflow from Island Park is still a little lower than streamflow would have been under natural conditions.

Graph of natural and regulated flow at Island Park

Graph of regulated and natural streamflow in the Henry's Fork at Island Park Dam. Natural flow is what would be in the river in absence of the dams at Henry's Lake and Island Park.

Management of Island Park Reservoir

Water managers at U.S. Bureau of Reclamation, in communication with Fremont-Madison Irrigation District and HFF, filled the reservoir earlier in the week, as planned. Also according to plan, inflow to the reservoir was matched by increases in outflow. Wednesday’s rain turned out to be heavier than forecast, and there was no choice but to increase outflow abruptly on Wednesday, given that the reservoir was already full. Despite intent to keep outflow constant over the Memorial Day weekend, changes in outflow—in either direction—are likely as conditions change. Inflow is currently receding, but the reservoir is still over capacity, and additional flow from Henry’s Lake will contribute to Island Park inflow once Henry’s Lake fills—probably in the next two days. Outflow will certainly remain high throughout the weekend, but small changes may be needed to keep both reservoirs full until irrigation delivery is needed. More rain is in the forecast, with amounts still uncertain. Some forecasts call for just as much rain in the northeastern corner of the watershed on Sunday as just fell on Wednesday.

The bottom line here is that reservoir storage is having almost no effect on river flows right now. Outflow is roughly matching inflow, and streamflow is essentially the same as it would have been under natural conditions, with no reservoirs in place.

This Week's Runoff Event in Perspective

Wednesday’s rain was rare but by no means unprecedented. Runoff events—and streamflows much higher than those seen right now—were common in the 1980s and 1990s. Natural flow in the upper Henry's Fork subwatershed (upstream of Ashton) was higher than yesterday's peak during May of 1982-1984, 1993, and 1995-1999--nine years out of 18 over the period 1982-1999. For example, natural flow at Ashton on May 24, 1984 was 6,442 cfs, almost twice yesterday’s value, after reaching a peak of 7,974 cfs a week earlier. However, runoff events like the one are currently experiencing have been rare since then, occurring only three times in the 18 years since 1999—in 2006, 2008, and 2011.

Outflow from Island Park Reservoir in selected water years

Outflow from Island Park Reservoir so far this year, compared with that in 1984, 1997, and 2011.

More Precipitation and Uncertain Streamflows Ahead

The weather forecast continues to call for showers each afternoon, and potentially heavy rain again Saturday night through Sunday night. Temperatures warm today and tomorrow but fall back to average, if not below average, after that. Over the past few days, medium- and long-term forecasts have trended toward near-average temperatures with above-average precipitation. One-week quantitative precipitation forecast calls for widespread precipitation on the order of 0.75 to 1.5 inch at all elevations. Given the uncertainty associated with rain events, just about anything is possible with streamflows over the next week. However, there is very little chance of rain-on-snow, given relatively cool temperatures and lack of snow at this point.