City Beavers: Tualatin River Basin Beaver Study

Why Beavers?

An Oregon native, North American beaver is well-known for its dam building activities that help slow down and retain water and sediment. Beaver is often referred to as a “keystone species” because its dam building and habitat alterations create new habitats that can support other species, including wetland vegetation, amphibians, fishes, and waterfowl.

Historically, the streams of the Tualatin River Basin supported a large beaver population. That population was greatly decreased by trapping for their pelts in the 1800s. Today, as beavers recolonize the basin, managers face the challenges of identifying where beavers may recolonize and determining how beavers fit into their management and restoration of the modern landscape.

What Are We Investigating?

We are quantifying the benefits and effects of beaver dams and ponds on local urban streams. This research is important because past studies have focused on rural and mountainous streams – streams with substantially different hydrology, water quality, ecology, and land use pressures than urban streams. We are assessing many urban stream components that may be influenced by beaver dams, including water temperature, dissolved oxygen, flow regime changes, sediment storage, and surface water and groundwater exchange.

We are also assessing the current and potential beaver dam distributions in the basin to identify areas of potential beaver recolonization. This involves compiling known dam locations from basin partners and analyzing the inventory to identify: 1) reaches likely to support dam building; and 2) reaches where habitat restoration actions may improve dam building potential in the Tualatin basin.

Where Are We Looking?

The study focuses on the urbanized southeastern part of the Tualatin River Basin, where approximately 500,000 people live. This area is in the Portland Urban Growth Boundary where roads, municipal wastewater treatment, and other public services support dense urban development. Here streams tend to be low gradient, incised, and flowing over sand, silt, and clay. They also often receive large volumes of stormwater runoff during rain events.

Two study sites (Fanno and Bronson Creeks) are being monitored. A variety of data is being collected at these sits, including water temperature, dissolved oxygen, surface and groundwater levels, and sediment loads. These data will be used to quantify the effects of multiple beaver dams on the water quality, hydrology, and geomorphology of two urban stream reaches with different dam and floodplain characteristics.

Five additional sites (Springville, Stoller, Summer, and Derry Dell Creeks and McKay Creek tributary), have been examined using a rapid approach. At these sites we are collecting strategic water temperature and geomorphic data. These sites will help illustrate the effects of beaver dams on a wider range of urban stream types.