Irrigation demands have taken over and flow out of Mackay Reservoir are close to 500 cfs making wading below the dam dangerous. Water in the reservoir is limited, so flow will decrease eventually, and we will watch this situation and relate when flows drop to safer levels.
Fishing season is open in the park. As usual, the Firehole River offers the best stream fishing with BWO and caddis activity interesting fish. Soft hackled patterns make great emerger imitations for both insects. Spring spawning rainbows in the Madison River are done and are returning to Hebgen Lake providing some streamer fishing. Most other streams are high with run-off.
Here is the park fishing license situation. Lack of supplies to print and then send hard copy versions to retailers is behind, but should be solved soon. When they arrive here, we will post such on this report. For now,obtain park fishing licenses by going on line to recreation.gov.
Those big bugs have been hunkering down for days along the river. With the improving weather, they will be out any where along the lower river and at Coffee Pot with those in Box Canyon not far behind.
Dr. Rob Van Kirk’s Henry’s Fork drainage snow-water situation as of today follows.
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Even by Memorial Day Weekend standards, the weekend’s storm was wetter than expected. Four-day precipitation totals ranged from 0.4 in Rexburg to 3.4 inches at Black Bear and White Elephant, both located on the Continental Divide at the northern edge of the watershed. Island Park received 2.6 inches, 1.5 of which occurred on Saturday alone. Saturday was the wettest day in the watershed since September 20, 2019. Subwatershed totals were 2.8 inches in upper Henry’s Fork, 2.15 inches in Fall River, 1.6 inches in Teton River, and 0.69 inch in the valleys. Water-year total precipitation increased from 82% of average on Friday morning to 87% of average this morning.
Mean temperature over the four-day period was 5 degrees F below average, despite warm temperatures on Friday. Accordingly, most precipitation fell as rain through Saturday evening but had turned to snow at most locations by Sunday night. For the four-day period as a whole, all but the highest SnoTel stations lost 1-2 inches of snow water equivalent (SWE). Grand Targhee and Black Bear gained enough new snow yesterday to make up for melt earlier in the weekend. At the watershed scale, current SWE is 51% of this year’s peak, compared with 42% on average. Despite a long, slow snowmelt season so far and above-average precipitation in April and May, current SWE is still only 87% of average for the date.
As expected, the combination of high snowmelt last Wednesday-Friday and very heavy rain on Saturday produced a dramatic increase in streamflow; natural flow doubled between Thursday and yesterday. As mentioned above, mid-elevation SnoTel stations lost 1-2 inches of SWE, and those same locations received 2-3 inches of rain. The net effect was to put an additional 3-5 inches of water onto the watershed over a four-day period, resulting in large increases in streamflow. I had expected streamflow to peak on Saturday, but rain was heavy enough to delay the peak in headwater areas until late on Sunday. All streams reached or even exceeded average for the date but are receding this morning. Even with the dramatic increase, watershed-total natural flow was only 96% of average yesterday and remains at 69% of average for the water year to date.
As expected, rain was heavy enough in valley areas Saturday and Sunday that diversion dropped a little yesterday to 87% of average. Cumulative moisture availability in the valley areas increased by 1 inch over the past few days but is still 5 inches below average. The difference between supply and demand increased substantially and is a little above average for the date for the first time this spring. Basin-wide stream flow exceeded diversion over the past few days, allowing upper Snake River Reservoir system storage to increase from 56% full to 57% full. We will need to wait another 5-6 days to see whether and how the physical reservoir gain translates into storage-account accrual. More on that below.
Physical reservoir storage in the Henry’s Fork watershed benefited greatly from the weekend’s precipitation. Henry’s Lake gained over 2,000 ac-ft since Thursday, is 99.2% full, and will likely fill, given forecasts for continued wet, cool weather into next week. Grassy Lake gained 700 ac-ft over the past three days to reach 91% full. Grassy Lake could fill in the next 2-3 weeks if draft is not needed to meet irrigation demand before then. My April-1 simulation model gave only around a 5% probability that Grassy Lake would exceed 92% full before draft was needed to meet irrigation demand. The biggest factor in fill of Grassy Lake will be how long precipitation and physical natural flow in Fall River will meet irrigation demand there. Island Park Reservoir was within 0.5% of full pool on Friday and within that margin over the weekend, as outflow increases just kept pace with inflow. You can see that in the inflow/outflow graphs today. Outflow will be reduced this morning from 1,200 cfs to 980 cfs to keep the reservoir at full pool. Additional decreases will be made over the next few days as needed.
Although stream inflow responded to the weekend’s rain, over half of the additional inflow to the watershed’s three reservoirs came from direct precipitation on the respective reservoir surfaces. Net gain in reservoir storage since Friday was 3,441 ac-ft, of which 2,091 ac-ft came from direct precipitation. This gain will likely make the difference between filling or not at Henry’s Lake. If Grassy Lake fills, it will be because the rain increased natural flow and decreased irrigation demand enough to provide a longer fill season. In any case, the primary short-term effect of the weekend’s rain was to put the watershed’s three reservoirs within striking distance of full, something that looked very unlikely a month ago.
What does this mean for the drought? At the risk of seeming alarmist and overly pessimistic, I will answer this question at three time scales, based on the data at hand. In the long term (years), two months of above-average precipitation during the spring will not make any difference in the ongoing drought. Had we received this above-average precipitation during February and March, the effect on long-term conditions would have been a little better. Long-term drought conditions improve in our region with heavy snowpacks, not rain. The three-year average precipitation is the same today (11% below average) as it was in early April.
In the medium term (6-12 months), cool wet weather during April and May has slightly improved soil moisture and offset some of the effects of dry conditions during February and March. For example, the one-year accumulated moisture deficit in agricultural areas of the watershed improved from 6 inches below average on March 1 to around 5 inches below average today. This is still a substantial deficit but is less bad than it would have been without recent precipitation.
In the short term (seasonal), recent cool weather has had the effect of doling out this year’s poor snowpack a little at a time. At both the watershed and basin scales, natural streamflow has been close to irrigation demand—a little ahead in the case of the Henry’s Fork and a little behind in the case of the upper Snake River basin. As a result, the upper Snake River reservoir system as a whole has been dropping slowly. This means that junior storage rights have not filled much this spring. The upper Snake River reservoir system achieved its peak of 59% full in mid-April, dropped to 56% full last week and has regained only around 51,000 ac-ft (1.2% of system capacity) over the past few days. This will probably result in a little accrual in storage accounts, primarily in the American Falls 1921 account.
On that note, the U.S. Bureau of Reclamation is today initiating delivery of upper Snake River reservoir storage for salmon flow augmentation in the lower Snake River. In most years, around 200,000 ac-ft of rented storage from the upper Snake River system is delivered for this purpose. This year, no rental water was available, so Reclamation’s powerhead storage in Palisades will be used for this purpose. That is around 53,000 ac-ft. This delivery of storage out of the upper Snake River system is equal to the physical gain the reservoir system made over the weekend.
Current weather forecasts call for continued relatively wet and cool conditions. If those forecasts materialize, we can expect to see continued slow snowmelt and relatively little additional physical fill in the basin-wide reservoir system. If this weather continues well into the summer—remember 1993, if you were here—natural flow will stay slightly higher than it otherwise would, and irrigation demand will stay slightly lower. The net effect will be to decrease reservoir draft, especially because junior storage users will gain little if any additional paper storage. This situation will benefit senior natural flow users, but the overall cost of a wet, cool summer would likely be decreased crop yields across the board. The best hydrologic outcome of a cool, wet summer (this is a big “IF”) would be better-than-expected reservoir carryover (bad instead of really bad), which would put the basin in a better position to recover from the drought next spring if snowpack is well above average next winter.
In the meantime, enjoy average streamflow for a few days.
Rob Van Kirk, Ph.D.
Senior Scientist
P.O. Box 550
Ashton, ID 83420
208-881-3407 CELL
Thanks to our recent rains, flow out of Palisades Dam was reduced a few days ago about 1000 cfs to 9180 cfs (now 10100 cfs at Heise, 5080 cfs at Lorenzo). This change should not impact streamer fishing or offering such as rubber leg patterns. Some BWO activity may be happening at the very heads of riffles because of the weather, so concentrate BWO life cycle presentations there.
The lake is beginning to warm and fish are moving deeper to cooler water. So they are now dispersed around the lake meaning a slow down in fishing success.
Those big bugs taht are out are hunkering down. Give them a warm day or two, and their numbers will increase, they will fly, then drop to the water, and the fish will respond maybe way up the river.
Memorial Day weekend is a big time for anglers and families trying to reach back country waters. In this year where spring is coming late, we contacted the USFS Caribou-Targhee National Forest Ashton office today. They relate that currently nearly all back country roads have yet to open and/or are in very wet, blocked with snow, or in muddy condition. That includes the Cave Falls, Fish and Snow Creek, Horseshoe Lake, and Warm River Spring Roads. All campgrounds, except the Cave Falls Campground, will be open by May 27th.
On the Palisades District side, the Bear Creek, Jackknife Creek, and McCoy Creek Roads are open. The Jensen Creek Road is currently closed.
The Forest Service offices remind us that there are penalties for damaging roads that are in muddy or very wet conditions and therefore not officially open .
Now that many of our irrigation reservoirs have experienced the usual spring turn-over, fishing orn each will improve. This seems to be the case for Daniels, Hawkins, Mackay, and Springfield Reservoirs where fishing success has picked up recently. We have little information on fishing success on Chesterfield and Twenty-Four Mile Reservoirs.
It looks like a late year for the giant stone fly event beginning in good numbers. Let’s see what the warming weather of the next few days brings. Looking at what Dr. Rob Van Kirk observes in the Henry’s Fork drainage with respect to effect of cold weather on the drainage, it is no wonder that insect activity is behind for the date. See below
His snow-water analysis for Henry’s Fork drainage of yesterday follows.
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Mean temperature yesterday was 6 degrees F below average, continuing what has been a very long run of below-average temperatures. Precipitation dropped to 83% of average for the water year so far. Snow melt was a little below average for the date, and 65% of this year’s peak snow water equivalent (SWE) remains yet to be melted, compared with 53% on average. Current SWE at the mid-elevation stations is about average for the date. Cold weather continues to keep aquatic insect hatches delayed by about 5-6 days in the lower watershed. Crop development also continues to lag average due to cooler temperatures.
A couple of warmer days are in store tomorrow and Thursday before cooler weather and precipitation return on Friday and persist through the Memorial Day weekend. Temperatures will be around 5 degrees below average. Precipitation forecasts have not been very consistent, due to a pretty strong gradient between a relatively dry forecast to our southwest and a very wet forecast to our northeast. The latest 7-day quantitative forecast calls for around 0.1 inch along the western edge of the watershed to around 1 inch along the Montana border. Areas just to our northeast, in Yellowstone National Park and southern Montana, could receive up to 2 inches. So, slight changes in moisture feed could mean large differences in actual precipitation amounts. At this point, rain in the valleys would be beneficial, but current forecasts suggest a continuation of the recent pattern of showers in the mountains but little if any in the valleys.
Natural stream flow continued to drop yesterday in response to cooler weather. Natural flow is currently 45% of average for the date and 70% of average for the water year so far. These numbers are truly astounding. In the 1978-2022 record, natural flow for the water year to date in the next driest year to this one (2002) is 79% of average, illustrating just how far this year is away from anything close to “normal.” Diversion stayed constant at 92% of average, dropping the difference between supply and demand down to only 600 cfs above the indicator of need for reservoir draft to meet irrigation demand. With warmer weather on the way—as well as some precipitation—natural flow will meet demand by a small amount at least into next week, although stream flow will be far less than half of average for this time of year throughout the watershed.
At an average outflow of 389 cfs yesterday, Island Park Reservoir gained a very small amount of water and is 99.6% full. Current outflow is just at the right level to keep the reservoir within a few hundred ac-ft of full for the next several days. Outflow may need to be increased a hair at the end of the week to accommodate increased inflow in the form of rain, should forecast amounts creep upward. On a full reservoir, 1 inch of rain adds 625 ac-ft, which is less than the difference between full pool and current reservoir content. Right now, forecasts call for around 0.5 inch in Island Park later in the week.
Rob Van Kirk, Ph.D.
Senior Scientist
P.O. Box 550
Ashton, ID 83420
208-881-3407 CELL
208-652-3568 FAX
A few big stone flies reported out, and plenty of boats are on the lower river. Any day now the big bugs will be out in force, and so will those who are now waiting for them whether they be trout or fly fishers.