Passive Restoration of a Small Mountain Stream in Eastern Oregon

Abstract In this study, I documented the results of 11 years of passive restoration of a small mountain stream in northeastern Oregon. Removal of the main stressors on the system, grazing and agricultural practices, resulted in a strong recovery of the riparian vegetation over the middle two-thirds of the reach. Cover of trees and shrubs tripled, tree density increased sevenfold, and stream shading fourfold over the period 2007 through 2018. Average maximum daily stream temperature declined by 2.1 to 2.8 °C, and average maximum diurnal temperature range narrowed by 3.7 to 4.6 °C. Recolonizing American beavers (Castor canadensis) played an essential role in improving the condition of the streambed over 18% of the reach. Their dams and ponds initiated the process of streambed aggradation and transformed the single-thread, incised channel into a multi-thread configuration within beaver complexes. Vegetation expansion was much stronger in impounded than in un-impounded parts of the stream. Passive restoration was not effective in two sections, together comprising one-third of the reach. In the upper part of the stream, recovery stalled because of continued (unauthorized) trespass grazing during late summer and fall. The channelized lower section of the stream was too severely modified for measurable recovery to occur. I concluded that 11 years of passive restoration improved the functionality of the stream from 24% to 32%, and within beaver complexes, up to 57% of its pre-disturbance condition.