Seventy years of watershed response to floods and changing forestry practices in western Oregon, USA

This study examined the 70-year history of clearcutting of old-growth forest and associated road construction, floods, landslides, large wood in rivers, and channel change in the 64?km2 Lookout Creek watershed in western Oregon, where forestry practices began in 1950 and largely ceased by the 1980s. Responses differed among three zones with distinctive geomorphic processes within the watershed: a glacially sculpted zone, an earthflow-dominated zone, and a debris slide and debris flow-dominated zone. Watershed response to floods was more related to the timing of road construction and clearcuts, past geomorphic events, and forest dynamics than to flood magnitude. Even small (1?3?year) floods generated geomorphic responses in the period of initial road construction and logging (1950?1964) and during ongoing logging in the early part of a 30-year period between large flood events (1966?1995). The floods of 1964/65, 15?years after the onset of logging, produced much larger geomorphic responses than the flood of record (1996), more than a decade after logging ceased. Geomorphic response was negligible for the third largest event on record (2011) during the last period (1997?2020), when former clearcuts were 20 to 70-year-old forest plantations. Watershed response in each of five distinct time periods depended on conditions created during prior periods in the three zones. Understanding of watershed response to forestry requires integrated observation of forestry practices, floods, landslide susceptibility, wood delivery and movement, and channel change on time scales that capture responses to past and ongoing management practices and geophysical and biological factors and events.
Keywords: channel adjustment, disturbance cascade, harvesting, large wood, process domains, sediment supply, stochastic forcing