From the late 1800s through the 1940s, engineers designed sewers to carry sewage and stormwater to the nearest body of water. At that time, planners believed that diluted pollution would not harm the water bodies. The system took care of horse manure and garbage on the streets along with human waste. Today, cities in King County build separate pipes; one to carry sewage to a treatment plant and another to carry stormwater to the nearest water body. However, there are still combined sewers in the oldest neighborhoods in Seattle. (source: King County website)
In Montlake, there are four CSO locations that spill combined sewage and stormwater during heavy rains. Here is a link to a map of CSO locations that also shows the current status of each site. This combined sewer overflow status page is provided and maintained by King County.
Typical overflow contents include approximately 10% sewage and 90% stormwater. While discharging untreated sewage into our public waters is certainly gross, the water temperatures are generally too cold for most bacteria in sewage to survive for very long. However, due to health concerns (contact with polluted water can make you sick), King County recommends that people stay out of the water for 48 hours after a combined sewer overflow event.