Pikes/Pines | Where does Capitol Hill’s rainwater go?

Where the water of Interlaken flows to. Portage bay with trouble and triumph on display: 520 porting directly into it and a beaver lodge in the foreground.

Where the water of Interlaken flows to. Portage bay with trouble and triumph on display: 520 porting directly into it and a beaver lodge in the foreground.

A drop of water falls on the intersection of 11th and Pike, slowly working its way through the grime of cars into the storm drain before slipping through a series of pipes ending in Puget Sound. Simultaneously, a drop falls on 20th Ave E, rolling down the street before crossing into Interlaken Park bound for Portage Bay. To the west, a drop slides into a drain along Belmont, coursing a storm drain that routes it, along with waste water into a treatment facility. What I’m describing is the urban watershed of Capitol Hill, in which water falling on us will flow downhill into a larger body of water, which eventually outlets into Puget Sound and eventually into the Pacific Ocean.

This is a simple concept but one worth remembering during storm season: water flows downhill. In our case downhill in all four cardinal directions . Of course, the watersheds of urban Seattle are altered; obviously, we don’t have creeks running through the middle of the city.

Only two partially daylighted creeks can be found on the Hill, out of what were probably a great deal more before we started changing things to suit our needs. Interlaken Creek (actually three separate creeks), runs through the park before joining Portage Bay, and Washington Park Creek runs through the Arboretum into Union Bay (part of Lake Washington). Deep “V” marks make them plain on a topographical map, as are others throughout the city. A rather significant one can be seen just south of St. Mark’s, but doesn’t bear running water regularly to my experience. It is likely countless forgotten creeks have been piped and co-opted into the greater water system that keeps things to our liking.


A look at the only two flowing creeks still known and visible flowing off the Hill. The three green indentations are Interlaken creeks flowing into portage bay right next to 520. Washington Park Creek is not shown but it runs along Lake Washington Blvd until it meets the waters near Foster Island.

Cement is largely impervious. That fact can create a lot of issues for us and the environment alike. Pollutants do an excellent job of coalescing on pavement before being washed away down the watershed as “stormwater.” Ideally stormwater is collected in basin to be treated before it joins one of our lakes or the Sound but there’s not always a capacity (for example SR 520 washes right into Portage Bay). You’ve no doubt seen the “Puget Sound Starts Here” images stenciled by a storm drain (of which there are over 80,000 in Seattle alone), a gentle reminder that washing your car on the street or unloading unwanted chemicals into the drain is a no-no. We don’t need any more stuff in our water than we already have.


Washington Park Creek where it outlets toward Union Bay (Lake Washington) flowing out of a culvert that moves the water downstream but doesn’t allow for anything alive to move upstream.

Storm drains are a part of our artificial watershed, helping direct water that can’t move through our cement streets toward an adequate outlet, like say Lake Union. Depending on where you live on the Hill, all water, storm or otherwise may end up flowing into a waste water treatment facility (also allowing overflow to mingle with waste water during heavy rain and flow directly into the sound…yuck). In other areas, we have partially combined systems, where storm drain separation projects in the 1960s separated street runoff (but roofs still drain into the combined system). Nowhere in Capitol Hill, except near Washington Park Arboretum are there completely separated storm drains. When storms flood the systems of Capitol Hill, it can mean gross water downstream.

Drain System

A look at our stormwater system: Yellow areas are combined sewers. Green is partially separated. Purple is fully separated.

At this point it is fair to ask why a natural history column is telling you about drains. Firstly, as urban naturalists, I think it is our responsibility to consider such things because of the complexities of the ecosystems we are linked to. At one point we had massive trees and functioning wetlands to suck up and filter massive amounts of water (now we recognize them are tools to be reintroduced in the system), alleviating flooding we now mitigate with artificial drainages. Water is a huge part of any ecosystem, a fundamental of life, so we’d do well to consider it. Even if that water isn’t directly a part of our drinking supply, dirty will round back on us in other ways.

Exploring the drainages of Capitol Hill was an educational jaunt. I found unhealthy sludge and trash along the sterile ditches and culverts that replace meandering streams in Washington Park Arboretum. I also found trees shading creek beds and beaver sign in the combined watersheds of Interlaken. At one point amphibians lived in streams going down our slopes and one might presume that salmon could have spawned in a few of them. That hope of biodiversity should be enough to consider where our water goes and what’s in it, but if it isn’t, there’s always our health to consider.


A storm drain along Interlaken Way with one of the three main drainages behind it.

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8 thoughts on “Pikes/Pines | Where does Capitol Hill’s rainwater go?

  1. Thanks for researching and writing this article on storm water and urban watersheds. In the future people will orient themselves in regard to the local watersheds rather than the nearest interstate.

  2. The part about 520 draining into portage bay is worrying. I assume some of it must have been attached to the sewer system, but now all the downspouts simply empty on to the street or run off.

    If I owned a large freeway seattle would make me build a storm water system…

  3. Thanks for posting this. Stormwater is fast becoming a top management priority for the city, county, and state, and in a lot of ways Washington state is at the forefront of handling the associated pollution carried by urban runoff.

    There are a ton of great resources at local agency websites, but maybe a good first step to poking around this issue is seeing where your local stormwater goes, how it gets there, and with what, if anything, it is comingling.

    This link to the City of Seattle side sewer card page can help with just that: