PARENT SESSION
Contributed Oral Session 37: Floodplain Restoration
Tuesday, August 9, 8:00 AM - 11:30 AM, Meeting Room 513 A, Level 5, Palais des congrès de Montréal
Response of riparian vegetation and water temperature to debris torrents in mountainous streams of western Oregon.
D'Souza, Lana*,1, Reiter, Maryanne1, Six, Laura1, 1 Weyerhaeuser Company, Federal Way, Washington, USA
ABSTRACT- Disturbances such as fire, floods and debris torrents profoundly affect the physical and biological character of streams. The magnitude of the impact and subsequent recovery of stream form and process is dependent upon the landscape context of the disturbance since the unique biophysical components of a landscape determines what tools a stream has available to rebuild itself. This study examines differences among watersheds in the natural recovery of riparian vegetation and stream temperature after extreme flood events in 1996 triggered numerous debris torrents throughout the Pacific Northwest. We studied twelve impacted streams in two different geo-climatic regions of Oregon, the Western Cascades and Coast Range. We found site factors such as substrate and landform to be similar across regions and minimally impacted riparian recovery. Red alder (Alnus rubra) and willow (Salix spp.) were the dominant species found in both regions. While willow was similar between regions, red alder density, basal area and height was significantly greater at Coastal sites (7112.7 tph, 17.5 m2/ha and 5.4 m) than Cascade sites (3861.4 tph, 8.9 m2/ha and 4.2 m). Stream channels at the Coastal sites had greater average shading (95.1%) and a lower average maximum daily summer stream temperature (16.9°C) than streams at the Cascade sites (71.6% and 19.0°C). Stream temperatures in both regions have decreased as riparian vegetation has re-established. Coastal streams appear to have recovered more quickly and now reflect pre-disturbance temperatures while the Cascade streams do not. Since shade recovery in western Oregon is predominantly influenced by hardwood (specifically red alder) recovery, those areas where red alder growth is favored will have more rapid shade recovery and subsequently more rapid temperature recovery.
Key words: disturbance, natural recovery, Alnus rubra, riparian vegetation