I’ve had more than a few people ask why the reservoir teacup graphs don’t look full after such a heavy winter snowfall and why the drought monitor maps are still showing drought in Idaho. The answer to this question is that the snow is still in the mountains. Snowmelt runoff typically peaks in May or June, and reservoir operators are typically trying to get the reservoirs full by mid-June to early July.
David Hoekema, hydrologist with the Idaho Department of Water Resources, gave a recent update of Idaho’s reservoir fills, snowpack conditions and outlook for the 2023 growing season, and explained the situation.
Southern Idaho
Going into the winter of 2022-23 with an especially dry October on the books, hydrologists were expecting drought to continue in southern Idaho. An extremely dry October means that runoff is likely to fall below snowpack values. For example, snowpack peaked on the Snake River at 119% of median, but runoff is expected to be about average. The lack of October rains means that the initial melt will go into the soils to replenish soil moisture rather than into the rivers and streams. However, because the snowpack came in much stronger than expected, the dry soils will help mitigate some of the flood risk that comes with extreme snowpack.
“So one of the striking things about this winter is if you can compare the snow water equivalent [or snowpack], we missed the October precipitation across most of the state, and that's really key to getting soil moisture into the soil kind of locked in place through the winter and produces more efficient runoff,” Hoekema said.
By April 10, 2023, the drought was disappearing out of southern Idaho, but it didn’t correct all of the drought that happened throughout the state.
Upper Snake River Basin
“The Upper Snake River Basin is the part of the state we were most concerned about coming into this water year. We were about a million acre-feet short of normal carryover. Right now, we are optimistic that the Upper Snake water users will get a full water supply because of the cool spring that has delayed irrigation by about three/four weeks. Typically, irrigation starts on April 1. But this year, irrigation really didn’t pick up until the warm weather this past weekend. We have made up some of that deficit over the winter, but now that irrigation has kicked in, the reservoirs will stop filling and may need to be drafted briefly to meet early season irrigation demand. Once the snowmelt begins in the Upper Snake River Basin [which should start within a week or two], reservoir operators will make their best effort to store as much water as possible by mid-June to early July. We have no flow as of last week past Milner Dam [late April], which indicates all water being released from dams is being diverted by water users.
At the beginning of the year, the hope was for 120 percent of normal snowpack and runoff, to get a full irrigation supply on the Snake River. The area received 119 percent, which is good, but with the delayed irrigation season and cooler temperatures, that has really increased the probability of having an adequate water supply in the Snake system."
North Idaho
In northern Idaho, there is some moderate drought, and there is risk of severe drought developing. Hoekma said, “We don't consider it likely at this point, but we're definitely keeping our eye on it.”
Middle Snake River Basin
The Middle Snake River Basin includes everything in eastern Oregon and western Idaho that drains into the Snake River. Snowpack typically peaks during the first week of April. This year, the snowpack peaked during the last week of April in many basins. Unusually warm weather has accelerated snowpack melt. Some cool weather this weekend will hopefully slow down some of the melt in the higher elevations. The snowpack has declined by about one-fifth according to the NRCS, and it is likely that we'll see runoff peak in many basins over the next month. “And we're going to get another system coming in [the first week of May], which will bring snow up into the higher mountains, but it'll bring significant precip into the lower elevations,” Hoekema said. “So there will be a continuation of the surge we're seeing in runoff leading to potential flooding risk in various parts of the state, especially the southern part of the state.”
Overall, in eastern Oregon, reservoirs are likely to fill (with the exception of the Owyhee Reservoir that is likely to receive a runoff exceeding a year’s irrigation supply), which would be the first time in quite a while.
The Owyhee Reservoir has recovered quite a bit since 2022 and is expected to have adequate water supply for this year, especially with the delay in the irrigation season. Typically, they start irrigating from the Owyhee area in early April, but cool spring temperatures delayed irrigation until the end of April, and combined with cooler temperatures and precipitation in the forecast going forward, should help drought recovery there.
Boise Basin
“If we look at the Boise system, they’re in really good shape,” Hoekema said. “Anderson Ranch is pretty full. The USBR (U.S. Bureau of Reclamation) has been evacuating storage in Lucky Peak and Arrowrock so that they can capture the surge in runoff that is expected over the next month or two. There is, of course, risk that if that snow comes off too fast, we could be in another 2017 situation. The Boise River is running pretty high, and if they're forced to push it up too much higher, we could start getting some damage to diversion structures downstream. So this cool weather is going to help kind of slow down the snowmelt in the higher elevations. But we do have quite a bit of low snowpack and that's a little harder to track, so we'll be watching runoff closely as this storm system moves in.”
Bear River Basin
“The Bear River Basin – they’ve had just crazy, crazy amounts of snow up there,” Hoekema said. They’ve had record-setting snow in a lot of areas. “So they're kind of at their top water supply and that should really help them recover. They've had four or five years of declining lake levels at Bear Lake; this should push the lake probably not full – but should get us several years of water supply. They may even pass some flow just out of precaution.” In 2019, the reservoir was full; in 2020, it went down; and in 2021, they really used a lot of storage – almost a full allocation, which is really rare in that basin. By the end of 2022, Bear Lake had lost over half the reservoir content. “And so [this year] we're going to see that spike back up, and that'll give them a few more years of pretty solid predictable water supply in that basin,” Hoekema said.
Southeast Idaho
“Portneuf, Blackfoot and Willow Creek – I think they've lost about a third of the snowpack over the last week,” Hoekema said, “which is an incredible loss rate and that's why they're predicting flooding on the Portneuf River at this point.”
Idaho has had warm temperatures to kick of the month of May. However, weather services are calling for a good storm system coming in with cooling temperatures during the first seven days.
“It'll be really key how warm that snow is when the rain hits and how quickly temperatures cool down and rain converts to snow,” Hoekema said. “The higher basins are not expecting a surge just because of the cooler temperatures – they're expecting the runoff to decline. But if you look at Willow Creek, Salmon Falls Creek, probably Goose Creek, all of those are likely to see a surge down in southern Idaho with this precip coming in.”
It's not all about the weather models
“We love to hear from agricultural producers,” Hoekema said. “If you have any kind of drought issues – especially if you don't think the map is lining up with what you're experiencing in the field – if we can get field reports, it makes a big difference. In 2020-21, a sheep producer in southern Idaho told us a lot of streams that typically hold water late in the year had already dried up. And so it gave us an opportunity to dig into the data, and we could see from satellite imagery that what he was telling us was verifiable. We were able to get that to the drought monitors, and they were able to increase the drought categorization.
“I sit in an office in Boise, and I really don't have a view other than the data of what's going on. So if we can hear from users, that really helps either verify what we're seeing or if we're missing something; we can get help [getting] that onto the drought map.”
Aquifer impact
Last year, hydrologists were predicting a shortfall in the Swan Falls minimum stream flow down in the Snake system, which is a key indicator of the aquifer conditions in eastern Idaho, especially with the East Snake Plain Aquifer. “But last year,” Hoekema said, “because of that unusually cool condition in the spring, we actually saw a little bit of aquifer recovery. And so having another cool spring and late start to the irrigation season, we'll have quite a bit of natural recharge to the aquifer system; we might see even further recovery of that aquifer, which would be spectacular. We're not going to get a lot of surplus recharge this year to the aquifer because of the lack of carryover in the Snake system … but the natural recharge hopefully will be substantial this year.”
David Hoekema presented on University of Idaho’s Ag Talk Tuesday series on May 2, 2023. Click here to register for the 2023 Ag Talk Tuesday video-call presentations.
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