The Boise River has twice its usual volume of water this spring. How high will it get?

A cold, snowy spring has been a boon for Southwest Idaho’s snowpack and reservoir levels, but as temperatures begin to warm and mountain snow melts, the Boise River is running high.

Parts of the Greenbelt are already closed for flooding, and with warm temperatures this weekend and rain in the forecast for next week, flows could increase.

The Boise River’s water level is controlled in part by the U.S. Army Corps of Engineers, which manages flows from the mountain reservoirs that feed into the river. Jon Roberts, the water management lead for the Corps’ Walla Walla District, told the Idaho Statesman in a phone interview that the Corps should be able to avoid raising water levels to flood stage, when flows can start to create “a hazard to lives, property or commerce,” according to the National Weather Service.

River flows are measured in cubic feet per second (cfs), which denotes the volume of water that is moving past a specific point in one second. A cubic foot of water is roughly 7.5 gallons.

According to U.S. Geological Survey data, the average Boise River flow for this time of year is around 2,800 cfs. The river is hovering just under 6,000 cfs, and Roberts said it could fluctuate as high as 6,500 cfs over the weekend. That’s nearly 50,000 gallons of water flowing past a point — the USGS uses the Glenwood Street Bridge as its reference point — in one second.

The Boise River has been flowing at around 6,000 cfs for most of the week. Warm weekend temperatures in the mid- to high 80s will likely cause more mountain snow to melt into the reservoirs that supply the river.

Reservoir space should keep flows below flood stage

Roberts said despite the high snowpack — which is more than 150% of normal for the Boise Basin — the Boise River is unlikely to reach flood stage, which is 7,000 cfs.

“We have a lot of space in the reservoirs compared to some similar years where we’ve had this much snow,” Roberts told the Statesman.

By Thursday, the three reservoirs in the Boise Basin system had room for snowmelt runoff and were at 56% of capacity basin-wide. Arrowrock Reservoir was only 33% full, while Lucky Peak Reservoir was 52% full and Anderson Ranch Reservoir was 75% full.

“It’s a three-part approach where step one is we’re evaluating the snowpack,” Roberts said. “Step two, we’re looking at how much space we have (in the reservoirs) and how much irrigation water we need to provide. And then we’re looking at, how do we stretch that out over time to minimize sudden spikes in the river so that we don’t run out of space in the reservoir.”

Increasing flows on the river this spring has created room in the reservoirs for the snowmelt that’s on the way, Roberts said.

But while the river may not reach flood stage, the high water is still having impacts in Boise. The city’s Parks and Recreation department began closing portions of the Greenbelt more than a week ago, starting with a portion of the Bethine Church River Trail in southeast Boise.

Since then, two other portions of the path have been closed. Parks and Rec director Doug Holloway told the Statesman people can find closures on the interactive map on the city’s website.

The city keeps tabs on which areas of the Greenbelt typically flood at different river flow levels, Holloway said, but he noted that can always change since the river’s natural path changes each year.

Between 5,000 and 6,000 cfs, the entire Bethine Church River Trail is usually under water, Holloway said. Around 6,000 cfs, paths in Marianne Williams Park start to flood. Parks and Rec has staff checking the Greenbelt multiple times a day to monitor for flooding.

“We’re really watching Marianne Williams and some tunnels downtown to see if water is creeping in there,” Holloway said.

Other closures include portions of the Greenbelt at the East and West Parkcenter bridges.

The water table is also starting to show at Ann Morrison Park where the park meets the river, Holloway said. In the past, that hasn’t happened until the river reaches flood stage.

According to Holloway, the flooding likely won’t damage the Greenbelt itself.

“The path is pretty resilient,” he said. “It’s more about erosion on the banks. We won’t know what that looks like until the water recedes.”

Holloway said extensive repairs were made on the banks following widespread flooding in 2017, when the river flows reached as high as 8,600 cfs. Those repairs should help prevent damage to the riverbanks this year, he said.

Holloway also urged people to respect the Greenbelt closures and find alternate paths in those areas. On Friday, the Boise Fire Department issued a “Dangerous River Condition” notice warning residents of the potential threats to humans and pets if they enter the frigid, fast-moving water.