Debris downstream of the Glenwood Bridge this week backed up water at the streamgauge the U.S. Geological Survey maintains there.
That caused the gauge to overreport the flow of water passing under the bridge, the USGS explained Thursday.
That gauge is a key one: It’s used to track flooding and river levels in Boise. It measures two things: the height of the water in feet (though that’s a relative measurement, not the river’s actual depth), and the flow in cubic feet per second.
The gauge height was correct, says the USGS. But the flow, a measure of the river’s area multiplied by its velocity, was thrown off by the debris slowing the water and increasing the area it covered at the gauge.
What does that mean? River managers use the flow figure to determine how and when to release more water from upstream reservoirs, causing mild flooding now in a monthslong attempt to avoid severe flooding later this spring.
Authorities intended to raise the water to at least 9,500 cfs by Tuesday, nearing or surpassing a previous peak flow in 1983. (That year was the highest the river has been since Lucky Peak Dam was built in 1955, dramatically changing flood management in the Treasure Valley.)
Turns out as of midday Thursday, the Boise River at Glenwood is at more like 8,880 cfs.
And there it will stay for now, says the U.S. Army Corps of Engineers.
“The way we’ve been managing the system to avoid major flooding effects has been working,” the Corps’ Walla Walla office wrote in a comment on Facebook. “At this time, no changes in discharges are planned as a result of the corrected streamgage rating.”
As always, with June and the start of summer nearing, that may change.
Rocky Barker contributed.