Stormwater is the polluted runoff that flows from paved and impervious surfaces—roads, roofs, parking lots, and sidewalks—after rain or snowmelt. In developed areas, water that once soaked into the ground now runs off quickly, picking up oil, fertilizers, trash, pesticides, and other contaminants. This runoff is routed through underground pipes to the nearest waterway—usually without any treatment.

The City of Spokane’s drainage system is a large, complex network designed to move stormwater away from streets and property to prevent flooding. But this same system also serves as a major pathway for pollution to enter the Spokane River and its tributaries.

Stormwater Impacts on the River

Stormwater is a major source of pollution to the Spokane River. Each rainfall event can wash large amounts of pollutants from roads, construction sites, lawns, and parking lots directly into creeks and the Spokane River.

Common stormwater pollutants include:

  • Metals (like zinc, copper, and lead) from brake pads and tires

  • Nutrients from fertilizers that fuel algae growth

  • Sediment that clouds the water and smothers fish habitat

  • Oil, grease, and chemicals from roads and vehicles that can contain harmful toxics like PCBs

Unlike sewage, stormwater generally receives no treatment before entering waterways. Because of this, nonpoint source pollution from stormwater is one of the largest ongoing contributors to water quality problems in the Spokane River watershed.

The City of Spokane and Spokane County are both expanding stormwater management programs to reduce this impact, including infiltration swales, drywells, and other green infrastructure projects that capture runoff where it falls and allow it to soak naturally into the ground. But it is up to us as a community to ensure that these projects are built to proper capacity and maintained to continue protecting our waterways well into the future.

Combined Sewers and Overflows

In downtown Spokane and on the South Hill, stormwater and sewage still share the same pipes. These combined sewer systems were part of Spokane’s original infrastructure and were designed before modern treatment standards existed. When heavy rains or snowmelt overwhelm the system, the mixture of sewage and stormwater can overflow directly into the Spokane River—a Combined Sewer Overflow (CSO) event. This old system continues to be used because the basalt layer under these portions of the City make constructing separate systems costly.

CSOs can carry harmful bacteria, viruses, oils, and other pollutants into the river. Even diluted, these events pose potential health and environmental risks, particularly for people engaged in boating, swimming, or fishing.

Reducing CSOs and Improving Water Quality

Spokane has made major progress over the last several decades to separate combined sewers and control overflows. Since the 1980s, roughly 85% of CSO volume has been eliminated by building separate stormwater systems across most of the city. A key strategy has been constructing large underground CSO storage tanks and basins that temporarily hold excess combined wastewater during storms. Once the system’s flow returns to normal, these tanks slowly release the stored water to the treatment plant for full processing. Learn more about the CSO system within the City of Spokane here.

Even with the recent upgrades, we still have overflow events dunig heavy rainstorms.

How do I find CSO outfalls?

The City of Spokane has posted signs, like the one illustrated here, at the combined sewer overflow locations along the Spokane River. The City also has maps showing CSO locations on their website. The signs provide necessary information for the City's Wastewater Management Department to respond quickly if an overflow is detected during dry weather conditions.

What should I do during a CSO event?

City of Spokane residents should avoid activities in the water (swimming, boating, fishing) during rainfall and snowmelt conditions that may result in an overflow event. You can check if there has been an overflow event in the last 48 hours or an ongoing event on the City’s website.

If you see discharge from a combined sewer overflow pipe, you can help by calling 509-625-7900. Make sure to note the CSO number on the sign so City staff will know exactly where the overflow occurred. The City will then confirm whether the discharge is a combined flow or stormwater only.