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Portland's Watersheds

Information
Aerial image of creek meandering through a green space full of trees and plants.
A watershed is an area of land where all the rain and snowmelt drains to a common body of water such as a river, stream, lake, or slough. People, fish, and wildlife need healthy watersheds to survive.
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What Is a Watershed? 

We all live in a watershed. A watershed is an area of land, large or small, where all the water—rain (or stormwater), groundwater, and snowmelt—drains into one body of water. This body of water could be a river, stream, lake, or slough. A watershed can also be called a drainage basin. Watersheds are named after the body of water into which they drain.

What Are Portland’s Watersheds?

If you live within Portland's city limits, you are most likely in one of the following major watersheds:

Map showing rivers and streams with overlay of Portland City limits in yellow. The Columbia River runs east to west along the top and Willamette River flows south to north through the middle of the map. Five watersheds are labeled: Columbia Slough, Willamette RIver, Johnson Creek, Fanno Creek, and Tryon Creek.
There are five major watersheds in the city limits of Portland.
  • Columbia Slough 
  • Fanno Creek 
  • Johnson Creek 
  • Tryon Creek 
  • Willamette River  

Another key watershed that Portland relies on, which is not within the city limits, is the Bull Run Watershed, where we get our drinking water from. Read more about the Bull Run Watershed.

Watersheds can contain many smaller watersheds. For example, in the Willamette River watershed, Stephens Creek flows into the Willamette River. Stephens Creek can be called a sub-watershed. Portland’s five main watersheds are also a part of larger, regional watersheds, like the Columbia River Basin. 

How Do Portland’s Watersheds Fit into the Larger Region? 

Map shows Columbia River Basin and the Pacific Ocean in relation to Portland. The basin stretches across seven Northwest states in the USA and part of Canada.
The Columbia River Basin stretches across seven states and British Columbia in Canada.

Portland’s watersheds are part of two larger drainage basins: the Willamette River Basin and the Columbia River Basin. The Willamette River stretches almost 200 miles to the Columbia River. Its headwaters are at Waldo Lake near Eugene. The Willamette River Basin is the largest watershed in the state. Portland is at the lower end of the Willamette River drainage basin and is the most urbanized part of the watershed. 

The Columbia River is one of the largest rivers in the United States. It flows more than 1,200 miles from the base of the Canadian Rockies in British Columbia to its mouth at the Pacific Ocean. The Columbia River carries a lot of water—it delivers more flow to the Pacific Ocean than any river in the world. The Columbia River watershed includes parts of seven states and is roughly the size of France. The City of Portland covers only a tiny fraction of the entire Columbia River Basin. Still, the Columbia River is a defining part of the region’s geography, culture, and history.   

The Willamette and the Columbia Rivers come together at the northernmost part of Portland. Salmon pass through this confluence as they migrate from Canada and the Willamette Valley to and from the ocean. Many species of birds also travel through this area as they migrate between South America and the Arctic, and from the Pacific Ocean through the Columbia Gorge. The confluence is also important for Portland’s economy. The Columbia and Willamette Rivers have significant port facilities that help link Portland to the global economy. 

Why Are Healthy Watersheds Important?  

Even in a big city, it's possible to have a healthy environment for people, fish, and wildlife. Healthy watersheds are important because they:  

  • Have good water quality. People, fish, and wildlife must have clean, naturally flowing water.
  • Provide places for people to access nature. Healthy rivers and natural areas provide places for people to play outside, exercise, and connect with nature. Studies show that access to nature in the city improves physical and mental health.  
  • Protect our homes and businesses from climate change. As temperatures increase, so will threats from floods and fires. Healthy urban forests, floodplains, and wetlands are important for climate resilience. 
  • Support a variety of wildlife. Herons, beaver, hummingbirds, pollinating insects, and so many more creatures call watersheds "home." 
  • Have thriving salmon populations. Salmon migrate, rest, spawn, and mature in many of our city's waterways. Portland is a key location for salmon. Healthy habitat will ensure salmon are here for future generations.
  • Help recharge underground water. When rainwater soaks into the soil, it replenishes groundwater. Groundwater is the underground water supply that keeps streams flowing when it’s hot and dry, and it's a secondary drinking water source for Portland. Learn more about groundwater and its role in Portland's drinking water supply.
  • Protect the City’s investments. Managing watershed health makes good financial sense. It costs less to prevent erosion and landslides than it does to replace pipes, roads, bridges. 

What Challenges Do Portland’s Watersheds Face? 

Portland's watersheds face many challenges. 

Pollution, erosion, and other challenges affect the health of our rivers and streams. Some challenges, such as declining salmon populations, are regional or global.  Many challenges are local:  

  • Old or inadequate infrastructure. 
  • Poor development choices made as the city grew. 
  • Individual Portlanders’ actions.  

Find more detail below about our watershed problems and solutions. 

Stormwater Runoff  

In cities, rain runs off streets, roofs, and parking lots instead of soaking into the ground. That stormwater runoff washes pollutants from hard surfaces into streams and rivers.  

Solutions include green infrastructure. Trees, rain gardens, and ecoroofs are examples of green infrastructure. They soak up rain, slow down runoff, and filter pollutants. Learn more about stormwater and what the City is doing to manage stormwater runoff.

Flooding  

Before urbanization, streams and rivers flooded often. Floodwaters replenished the land and soaked into the ground. City development disconnected streams and rivers from their natural floodplains. Water has no place to go during high flows. This can mean flood damage to homes, businesses, and infrastructure. Learn more about flooding in Portland.

One solution to flooding is to restore floodplains and return stream banks to more natural conditions where possible. Floodplain restoration not only protects people and property, but also provides several other benefits for watershed health like improving habitat for fish and wildlife.

Pollution

Many things we use at home or on our cars like chemicals, fertilizers, and detergents pollute rivers and streams. They are also toxic to fish and wildlife. 

Do your part to improve watershed health by reducing use of home and lawn chemicals, and learn how to dispose of them properly.

Erosion 

High volumes of stormwater can cause erosion, which is when rushing water washes away soil. Erosion can wash away stream banks, cut down hillsides, and damage roads and buildings. The eroded soil washes into streams and rivers, harming habitat and water quality. 

One solution to erosion is low-impact development, which reduces hard surfaces. We can limit parking lot size, build narrower structures, and line properties with trees. Keeping stormwater on site is a solution, too. Learn more about how you can manage the rain on your property.

Culverts  

Photo shows a steel culvert or pipe running under a road shown in the top of the photo. Water is pooling at the base of the culvert that prevents fish from passing.
The old culvert under SW Boones Ferry Road blocked fish from moving upstream.

Many species of fish and native salmon in Portland are threatened or endangered. As Portland developed, in some places free flowing streams were piped into culverts under streets. These culverts often block fish from swimming to habitat in the upper reaches of streams. Other culverts are too small for water to flow naturally, causing water to back up and cause flooding.

To solve this challenge, we can replace old culverts with new ones designed to allow fish to pass and water to flow more naturally. Read about the SW Boones Ferry Bridge and Restoration Project that replaced a culvert on Tryon Creek to restore connections for fish, wildlife, and people.

Hardened Riverbanks 

Rock and concrete walls along rivers and streams may cause problems. Hardened riverbanks:  

  • Leave no place for fish to hide. 
  • Prevent animals and people from safely accessing the water. 
  • Sometimes make flooding worse. 

Where possible, we can remove concrete and restore stream banks to more natural conditions. We can bolster this solution by protecting existing habitat from new damage.  

Habitat Loss 

Fish and wildlife need access to habitats rich in food and shelter. Urban development can disturb habitats and keep fish and wildlife from the resources they need to survive.  

One solution is to protect and restore valuable habitat areas in the city. Another solution is to add trees and green spaces to our neighborhoods. Find tree planting opportunities.

Heat Island Effect 

Hard surfaces such as roads, parking lots, and buildings absorb and retain heat. Natural landscapes such as forests and water bodies reflect heat. Urban areas that trap heat cause what's called a "heat island effect." They raise the temperature of air and water, which can impact wildlife and plant species. 

One solution is to plant more urban street and yard trees.  Another is to build more green infrastructure such as rain gardens and ecoroofs

Invasive Plants and Animals

Invasive species are plants and animals that are brought to this area by humans either intentionally as garden plants or accidentally like through transported firewood. Because they reproduce quickly and spread rapidly, they can become problems in our parks, forests, and natural areas. If left unchecked, they can do costly damage.

One solution is to curb new invasive species before they become established.

How Can You Help Portland’s Watersheds?

Everyone can help improve Portland’s watersheds through actions big and small. From building a rain garden to picking up neighborhood litter, we can all help improve watershed health. Take action today: