Hydrology is how water flows across Portland’s land, our rivers and streams, and in natural and constructed drainage systems. Stream hydrology affects fish, wildlife, infrastructure and property.

Upstream dams have a major influence on the flow of water in the Willamette and Columbia rivers, but local conditions also affect hydrology.

Before Portland was here, rain soaked into the ground. Trees and plants kept the water clean and provided habitat for salmon and other wildlife. Urban development reshaped Portland’s landscape and its hydrology. Today, there are fewer trees and hard surfaces like streets, parking lots and roofs cover much of the ground.

Stormwater runs off hard surfaces instead of soaking into the ground. When it rains, stormwater flows into streams too quickly. Rain that doesn’t soak into the ground can’t replenish groundwater, which is an important source of cold, clean water for streams during dry summer months.

Many Portland streams that once flowed freely are now buried under development. Some streams run through pipes under neighborhoods or through culverts under roads. Streams squeezed into pipes and culverts flow faster, wash fish and wildlife downstream, and increase flooding risks upstream. Pipes remove streams from their floodplains and the natural processes that filter and store the water.

Portland works to improve hydrology to comply with state and federal water quality regulations and improve stream flows for salmon and other native fish. Two ways to do that are:

• Reducing impervious area – An impervious surface is a hard surface, like asphalt, that prevents rain from soaking through it. Portland uses green street planters and rain gardens to collect stormwater runoff, slow the flow and let water soak into the ground.
• Replacing culverts and daylighting streams – Replacing culverts that are too narrow or high and bringing sections of piped streams back to the surface let streams flow more freely and reconnect to their floodplains.

Hydrology is typically measured by the amount, duration and timing of water flows. Environmental Services collects this information to plan and design specific stormwater management projects. For the Watershed Report Cards, the hydrology score is based on information about impervious area and piped streams. This information is more readily available for the whole city and helps illustrate areas where we can make a difference.