JACKSON, Wyo. — The summer outlook for Jackson Lake water is a story of high flows and low fall storage, according to agency officials who perform forecasting in the Upper Snake River Basin.
On Thursday, the Bureau of Reclamation hosted its annual spring Jackson Lake Operations Meeting to give the public a status update on current levels of snowpack, streamflow and water storage. Water in the reservoir system has already been appropriated for agriculture, irrigation and power, so recipients in the basin generally know how much they’re going to get. Scientists from the Bureau of Reclamation, the Natural Resources Conservation Service (NRCS), the National Weather Service (NWS) and the Army Corps of Engineers are working together to formulate a plan and preserve water using current data, AI-guided learning and historic records.
Warm snow drought
“This weather year, the driving anomaly has been temperature,” said NWS Pocatello Hydrologist Mark Dallon. Though it felt like a dry winter, precipitation levels remained near normal. With average temperatures hovering above freezing, most of that precipitation fell as rain below 7,000 feet in elevation. This is known as warm snow drought — when warm temperatures drive a lack of snow accumulation.
For context, Erin Whorton with the NRCS said that the extremely high temperatures experienced in mid-March caused the snowpack to peak about three weeks early at just 73% of normal. She said that even snowpack at high elevations near 10,000 feet saw rapid melting. Because of that dynamic, peak streamflow is expected to be lower and earlier than normal, and the entire reservoir system could have already seen peak storage for the summer. With natural flows decreasing, those who need water for agriculture and irrigation will be forced to rely on groundwater and stored water.
“They will be tapping water in the bank earlier and need to make it stretch longer in a hotter summer,” Whorton said.
On the other side of the same coin, Dallon said that evapotranspiration is triggered by warm snow drought. The combined process of evaporation and transpiration moves water from the Earth’s surface into the atmosphere, and has a detrimental effect on the moisture content in live foliage. This can lead to plants that are unable to resist the spread of wildfire.
Water storage and outflow
At the meeting, Colin Ocker with the Army Corps of Engineers said that Jackson Lake’s full capacity is about 847,000 acre feet, and there is currently about 70,000 acre feet of space available. In a more normal weather year, the lake is required to maintain 200,000 acre feet of availability until May 1 to avoid flooding from snow runoff. The officials expect peak levels to remain in Jackson Lake until about June, barring any extreme changes to their forecasts.
An acre foot of water is enough to cover a 1-acre area at a depth of one foot. Water flow out of Jackson Lake Dam is measured in cubic feet per second (cfs). A cubic foot is a little smaller than a typical household microwave. Water is currently being released from Jackson Lake at a rate of 1,200 to 1,300 cfs.
Brian Stevens, the Bureau of Reclamation Upper Snake Field Office Water Operations Manager, compared this year’s outlook to the seasons of 1992 and 2021. Both of those years saw a nearly full lake in the spring that hit very low levels in the fall. Low lake levels this fall, potentially below 15% full, will be a result of high flows during the summer. Peak output from Jackson Lake this spring is expected to be between 2,000 and 5,000 cfs. As irrigation demand increases downriver, the projected outflow for the summer will be 3,000 to 4,500 cfs.
“I think Jackson Lake is going to get up to, like, 50 to 80% full next spring,” Stevens told Buckrail. “It’s going to be a couple of years before there’s enough natural flow.”
Recreation and fisheries
For the last 10 years, the local chapter of Trout Unlimited has been working on an initiative to restore and preserve the Snake River headwaters in order to benefit native trout populations. Especially for this project, the organization has worked closely with the National Park Service, the Wyoming Game and Fish Department, and the U.S. Forest Service.
Trout Unlimited’s Wyoming Water Policy Advisor Kathy Lynch told Buckrail that high flows in the summer will fill tributaries and side channels, giving native fish more habitat options. However, the concern lies with the fall rampdown, when the Bureau of Reclamation lowers the flow of water out of Jackson Lake to start collecting for next year.
“My hope is that, given this great collaborative structure that we all have now, that people can give input into the timing of that rampdown in an effort to allow the fish and the macroinvertebrates sufficient time to move out of those side channels that they’ve benefited from all summer, and find a new location with lower flows,” Lynch said.
Lynch does not foresee the Snake River’s flow rate to be a cause for above-average concern regarding recreation. Data from a U.S. Geological Survey monitoring location near Swinging Bridge puts the current Snake River flow rate between 8,000 to 9,000 cfs. That rate is the result of numerous tributary inputs that are flowing high from runoff. Over the next few weeks, the meltoff will inevitably slow down, and the Bureau of Reclamation will increase the flow through the Jackson Lake Dam. The river is expected to reach peak streamflow soon.
“It is going to be a higher water year for sure, especially up in the braided sections, say, up in the park,” Lynch said. “Are there going to be potential strainers or downed trees or channels that you don’t want to go in? Probably. … But I think as a whole system, I think it’ll be okay. And I also think that the whitewater section is probably going to be really fun all summer.”
