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 km2Lookout 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.
Long‐term watershed experiments provide the opportunity to understand forest hydrology responses to past logging, road construction, forest regrowth, and their interactions with climate and geomorphic processes such as road‐related landslides. We examined a 50‐year record from paired‐watershed experiments in the H. J. Andrews Experimental Forest, Oregon, USA in which 125 to 450‐year‐old conifer forests were harvested in the 1960s and 1970s and converted to planted conifer forests. We evaluated how quickflow and delayed flow for 1222 events in treated and reference watersheds changed by season after clearcutting and road construction, including 50 years of growth of planted forest, major floods, and multi‐decade reductions in snowpack. Quickflow runoff early in the water year (fall) increased by up to +99% in the first decade, declining to below pre‐harvest levels (−1% to −15%) by the third to fifth decade after clearcutting. Fall delayed flow responded more dramatically than quickflow and fell below pre‐treatment levels in all watersheds by the fifth decade, consistent with increased transpiration in the planted forests. Quickflow increased less (+12% to 70%) during the winter and spring but remained higher than pre‐treatment levels throughout the fourth or fifth decade, potentially impacted by post‐harvest burning, roads, and landslides. Quickflow remained high throughout the 50‐year period of study, and much higher than delayed flow in the last two decades in a watershed in which road‐related changes in flow routing and debris flows after the flood of record increased network connectivity. A long‐term decline in regional snowpack was not clearly associated with responses of treated vs. reference watersheds. Hydrologic processes altered by harvest of old‐growth conifer forest more than 50 years ago (transpiration, interception, snowmelt, and flow routing) continued to modify streamflow, with no clear evidence of hydrologic recovery. These findings underscore the importance of continued long‐term watershed experiments.
more » « less- NSF-PAR ID:
- 10441273
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Hydrological Processes
- Volume:
- 35
- Issue:
- 5
- ISSN:
- 0885-6087
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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