For the last several decades, the Natural Resources Conservation Service (NRCS) Idaho Snow Survey and US Army Corps of Engineers (USACE) have collaborated on an annual spring helicopter survey of spring snowpack in the North Fork Clearwater Basin.
For the last several decades, the Natural Resources Conservation Service (NRCS) Idaho Snow Survey and US Army Corps of Engineers (USACE) have collaborated on an annual spring helicopter survey of spring snowpack in the North Fork Clearwater Basin, largely in support of the safe management of Dworshak Reservoir, the largest reservoir in Idaho.
Since snowmelt is the primary source of inflow to Dworshak Reservoir, snowpack measurements are a key component in water management and are critical in abnormal years like this. The spring of 2022 has kept water managers on their toes as they work to balance the mitigation of pre-existing drought conditions with the exceptionally cool and wet spring. The latter part led to a later than normal (by about 2 weeks) spring snowline assessment flight.
Measurements from the NRCS Snow Telemetry (SNOTEL) network are some of the primary metrics used to estimate the amount of water that is stored in our mountain snowpacks, and SNOTEL data is crucial for western streamflow forecasting. The SNOTEL network, however, has limitations in regard to spatial representation – which is inherent to all point-based measurement networks. Idaho NRCS Snow Survey Supervisor Corey Loveland noted, “These limitations often hinder our basinwide snowpack assessment, not just spatially, but by elevation as well. That is why these flights help us assess at what level the snow line is and how deep that snow is. This information also plays a key role in assessing how rain-on-snow events, like we experienced this spring can cause rapid snowmelt.”
To capture the bigger picture and see how much snow remains in the Clearwater River Mountains, scientists and engineers fly the area and map the remaining snow-covered area – which helps to verify what we already know from the SNOTEL network and fills spatial gaps in our understanding of the extent and magnitude of late season snowpack. This information is critical to USACE for managing the “final fill” of the reservoir. Loveland also said, “What this means is water managers can better predict how much water (snowmelt) is forecast to enter the reservoirs. This information helps determine if there will be too much water which needs to be carefully passed in flood control operations or if not enough water to fill, it helps water users know about how much they will have for irrigation, power generation, recreation, environmental and other uses.”
Although snowpack is well above normal for this time of year, observations from the 2022 snowline flights on June 15 confirmed that the SNOTEL Network was representative of all elevations across the basin. This helped the US Army Corps of Engineers make the decision to temporarily reduce releases from Dworshak which will allow for ample water supply and help mitigate downstream flood risks.
Jonathan Roberts, a civil engineer with the USACE said: “Based on the snowpack we saw and our hydrologic analysis, we were able to determine that there was no rain event with enough volume that would cause an issue at Dworshak, prior to the snowpack continuing to melt. This helps us generate the inflow possibilities, which in turn help set the limits for our outflow. This was how we were able to safely determine that we could reduce outflow and work to continue to slowly fill the pool.”
While this has been a unique year, collaboration between NRCS Snow Survey and water management groups are ongoing and are just one piece of the larger balancing act that is water management in the Western United States.