Warm weather has caused the mountains above the Wood River Valley to slough off their winter snowpack, and that means heightened flows on the Big Wood River and potential flooding.
Last week, crews with the U.S. Geological Survey were out at Lions Park in Hailey taking measurements of streamflow with an acoustic Doppler current profiler.
The measurements and data collected help the USGS provide accurate information to the National Weather Service and emergency planners with Blaine County so they can relay that information to the public in warnings and alerts about potential flooding along the river, said Timothy Merrick, a USGS spokesman.
The National Weather Service office in Pocatello takes the USGS data and compiles it into its hydrograph, which includes recent, current and forecast conditions at various points along the Big Wood River. One of the most closely watched is the hydrograph for the river at Hailey, because it relates to the flooding that can occur in neighborhoods closest to the river, including Della View.
The hydrograph showed that the river’s flow was above action stage—3.7 feet—on Monday, Tuesday and Wednesday. It was forecast to above minor flood stage, which is 5 feet, on Thursday and to top out at 5.28 feet Friday before dropping again over the weekend. Moderate flood stage is at 6 feet, according to the hydrograph.
The Weather Service offers flood-impact information at the various stages. Minor flood stage means street flooding along Cedar, War Eagle and Della Vista drives, according to its website. Draper Preserve would be inaccessible.
Moderate flood stage would entail flooding on Cedar, War Eagle and Della Vista drives and possibly make them inaccessible. Draper Preserve would also be inaccessible.
But the forecasts need to have the most accurate data possible—which is why the USGS had its hydrographers in the field last week. Their stop in Hailey was part of a regional effort, because rivers throughout south-central and eastern Idaho are flush with spring snowmelt, Merrick said.
The federal agency maintains more than 250 stream gauges in Idaho and has staff based from field offices in Idaho Falls, Boise and Post Falls.
The stream gauge in Hailey was established in July 1889 and has an uninterrupted record of measurements from June 1915 to the present, Merrick said.
Using the acoustic Doppler current profiler is a common practice for determining streamflow. The devices use sound waves to measure the velocity of a cross-section of a stream or river.
Merrick said it’s important to take those measurements when the rivers are flowing high and fast. The observative flow in early June was above the median for that time, but still well below the highest observed flow from 2017. Major flooding hit that area of Hailey in May and June of 2017, following a record-setting snowpack that winter.
“The more data points we have, the more confident we can be that the stream gauge is reporting accurately,” Merrick said. “Thankfully, we’re not where we were at in 2017. [The USGS] is strictly a science agency. Our role is to provide the reliable, unbiased information. We tend to be a little incognito.”