Gauge tells 100-year-old tale

Reaching the ripe age of 100 is still a big deal in this part of the world, worthy of celebration for institutions such as Lewis and Clark High School, St. Aloysius Catholic Church and Hudson’s Hamburgers.

There was no cake or champagne toasts when Station 12419000 hit the century mark last fall. Apart from a few guys with the U.S. Geological Survey, barely anyone noticed the occasion for the humble Spokane River stream gauge just below Post Falls.

But for those who watch the ebb and flow of the river and seek to understand the pulse of hydrology in this watershed, it’s a big deal to have such a long record detailing how much water drains out of Lake Coeur d’Alene and courses toward Spokane.

“Our overall knowledge through these gauges has just increased profoundly and is irreplaceable,” said Dave Evetts, USGS data chief in Boise.

The Spokane River swells this time of year, carrying melted snow and spring rains down to Long Lake and the Columbia River.

The peak flow at the Post Falls station so far this season came a week ago, when about 20,000 cubic feet per second rushed past the gauge a mile down from Avista’s power plant. That’s 150,000 gallons a second – enough to fill 818 Olympic-size swimming pools in one hour.

Five years ago, the crest was twice that amount, prompting restrictions on recreation downstream and turning Spokane Falls into a thunderous spectacle.

“It comes and goes fast on the Spokane River. It’s a pretty flashy system,” said Steve Esch, senior operations engineer for Avista, which operates the Post Falls dam and other hydroelectric projects downstream.

River geography can change with floods, but the Post Falls station is at a fairly stable point, said Pete Elliott, a hydrologic technician with the USGS field office in Post Falls.

“This is armored with rock, and it’s pretty solid,” Elliott said on a visit to the stream gauge last week. “Boulders and rock all set in place through years of pounding water, they’ve shifted and settled in, and they’re pretty much there to stay.”

The technology measuring the flow at the Post Falls station has changed over the decades. The modern method releases a tiny nitrogen bubble through a hose fixed to a point underwater to read the pressure and equate it to a water level, known as the river stage. The stage is converted to the approximate cubic feet per second of water flow.

A reading is taken every 15 minutes and the data is sent via satellite telemetry, with the numbers updated online once an hour.

Avista taps into the gauge for even quicker feedback – useful for when it adjusts the outflow from the dam. The numbers also show if the utility is meeting federal requirements for maintaining a minimum streamflow, in part to keep some whitewater for people to enjoy in downtown Spokane.

“Now that gauge is the measurement of how well we’re doing, so our compliance is pretty much based on that gauge,” Esch said.

Kayakers, rafters and river guides check the gauge readings before heading to the water, biologists use the data to assess fish habitat and weather forecasters tap the numbers to estimate how bad floods may get.

“On that gauge we have certain stages that we know through history cause flooding,” said Matt Fugazzi, a forecaster with the National Weather Service office in Spokane.

The agency monitors 26 gauges daily in the region, and a centennial gauge like Post Falls is valuable when formulating flood watches and warnings, Fugazzi said.

“It makes the forecast somewhat more accurate because we have that long period of data that we can cross-reference and find analogous situations that have occurred in the past,” he said.

Even though the Spokane River is regulated by dams, heavy rainfall on top of rapidly melting snow in the upper basin can thwart those controls.

“Mother Nature is going to have her own way, and if she throws curve balls at us, then occasionally they have to release enough water from these dams to cause flooding downstream,” Fugazzi said.

Data from the Post Falls gauge is used to analyze long-term trends as well. Those include the impact of climate change and urban growth, and estimating how much water seeps into the ground and joins the Spokane-Rathdrum Prairie Aquifer, Evetts said.

The station also is used to monitor water quality – how much lead, zinc and other trace metals wash down from historic mining waste in the Silver Valley, he said.

“So it’s a really useful and needed tool to help monitor that valuable water resource,” Evetts said. “And again, when you have 100 years of record, you can really see how things are changing over time.”