Nearly 8 million gallons of untreated water poured into the Spokane River on a single day in December

Millions of gallons of water overwhelmed Spokane's sewer system on Dec. 27, 2022, as temperatures reached 44 degrees, with large amounts of rain melting snow that was already on the ground and causing untreated overflow to go into the river.

While the city has invested millions to build massive underground storage tanks that are intended to help when there are large amounts of sewer and stormwater, some events overwhelm that system, resulting in untreated water dumping into the Spokane River. The untreated water can be a mix of mostly stormwater (think rain, melted snow, and all that water that runs into the gutters and storm drains) and waste water from residences in areas where combined sewer and stormwater pipes move the water to the treatment plant.

Dec. 27 was one of those days, as 7,889,099 gallons of water overflowed throughout the city's "combined sewer overflow" tank system. The spots hit hardest in December included tanks at Sprague and Adams, Riverside and Crestline Street, the west end of Liberty Park, and Northwest Boulevard at Kiernan Avenue.

It's important to note the unusual circumstances that led to that overflow, says Kirstin Davis, spokesperson for Spokane's Public Works department. The days just beforehand were especially frigid, and the frozen ground was not able to absorb the water when it started melting, she says.

Also, there was an unusually large amount of snow and rain that month. While Spokane typically sees about 2.34 inches of precipitation in December, we had 3.59 inches that month, and 1.25 inches of that rain came just on Dec. 26 and Dec. 27, according to the National Oceanic and Atmospheric Administration (NOAA).

"Had those high temperatures not been combined with rain in such a short amount of time, overflows most likely would not have happened at all," Davis says.

The last significant overflow also happened on a rainy day, in June 2022, when about 2.7 million gallons were released, according to city data that's compiled monthly.

Despite those two major events last year, the city's system is working as intended, Davis says. In the years before all of the tanks came online, the city used to see far more overflows.

In 2016, for example, about 80 million gallons of untreated water overflowed into the river, with the tanks that were in operation capturing more than 20 million gallons to be treated. In comparison, in 2022 about 15 million gallons of untreated water overflowed, while the tanks captured more than 100 million gallons to go through treatment. 

For context, the city's wastewater treatment plant treats 34 million gallons of water each day, Davis says.

Cities can face fines from the Washington state Department of Ecology when untreated water is released, but the city proactively reports its overflows and makes adjustments throughout the day to ensure the system is working the best it can, Davis says.

"It is important to us that we are in compliance at all times and want to know why overflows occur. The compliance measure is a way for all of us to work together for solutions if there is a consistent problem," Davis says. "We are all passionate about protecting our river."