Effects of beaver dams and ponds on water quality in urban streams of the Tualatin River Basin, northwestern Oregon

Significant Findings

American beavers (Castor canadensis) are native to the Pacific Northwest, and their populations have increased in many locations after being nearly removed by historical trapping. Beaver dams have well-documented effects on water quality in forested streams, but their effects on water quality in urban streams have not been well characterized. The study documented the water-quality effects of beaver dams and beaver activity in selected urban streams of the Tualatin River Basin in northwestern Oregon. Variations in water quality upstream, downstream, and within ponded areas behind beaver dams were quantified with continuous measurements of water temperature, specific conductance, dissolved oxygen, and pH from May 2016 to November 2017 in two intensively monitored reaches of urban streams (Fanno and Bronson Creeks). Five other urban stream reaches were monitored upstream and downstream from beaver ponds using water-temperature sensors to document water-temperature changes in additional beaver-affected reaches. Spatial water-quality variations within a beaver pond along Fanno Creek were characterized in more detail on four hot summer afternoons with numerous measurements of temperature and dissolved oxygen. Results from the study were used to document and derive insights from measured patterns in the water-quality data, such as the following:

• Shallow and unshaded ponds created by beaver dams (such as at Fanno Creek) capture more solar radiation than channelized and shaded stream reaches, resulting in substantially warmer water temperatures during summer.
• A large beaver pond along Fanno Creek had variable water depths and riparian shade, resulting in a wide range of water-temperature and dissolved-oxygen conditions. Some conditions measured during summer were stressful for sensitive aquatic species, with temperatures higher than 18 degrees Celsius (°C) and (or) dissolved-oxygen concentrations less than 2 milligrams per liter (mg/L).
• Although water flowing out of beaver-affected reaches was warmer than water entering the reaches, water tended to cool downstream when streams returned to a more-shaded, confined channel (such as at Fanno Creek) or received subsurface inputs and exchange (such as at Bronson Creek).
• The magnitude of water-temperature increases in beaver ponds depended on site characteristics, such as the surface area and depth of the ponds, the extent of riparian shade, and the potential for subsurface exchange. Beaver dams caused Fanno Creek to overflow its banks, resulting in a wide and shallow floodplain pond that had little riparian shade in most places. In contrast, Bronson Creek remained in its stream channel behind the beaver dams, was often deeper and more shaded, and had more subsurface water exchange than the Fanno Creek reach. Despite the longer reach length, these differences caused the Bronson Creek reach to warm less than the Fanno Creek reach.
• Beaver dams trapped sediment and organic matter, and ponding increased the time available for organic matter to decompose in the trapped sediments, thus consuming dissolved oxygen. The construction of a beaver dam in the monitored reach of Bronson Creek during the study caused an increase in oxygen demands and a rapid decrease in dissolved-oxygen concentrations.
• Dissolved-oxygen concentrations in monitored study reaches were affected by primary production (algal photosynthesis) and respiration, and hypoxic (low to zero dissolved oxygen) conditions were measured in the two intensively monitored reaches for multiple weeks during summer. Two single-station stream metabolism models were used to calculate net ecosystem production in the two intensively monitored urban stream reaches. Results indicated that the reaches were heterotrophic, with respiration demands consuming more oxygen than what was produced through photosynthesis.
• Beaver ponds at all sites had a measurable effect on water quality, causing wider ranges in temperature and dissolved-oxygen conditions than would have occurred without ponding. That wider range (both spatially and temporally) might support a variety of aquatic organisms, but also is likely to increase the frequency of water-quality standard violations in those beaver-affected reaches. Any water-quality effects or standard violations associated with beaver dams and ponds, however, may be localized or transitory along the stream network. Restored stream reaches can attract beavers. Considering the potential for beaver colonization when designing habitat restoration plans will require evaluation of the potential water-quality changes associated with beaver dams and ponds alongside other desired physical changes.