Peakflow responses to forest practices in the western cascades of Oregon, USA

The effects of clearcut silviculture (road building, clearfelling, cable logging, and site preparation) were evaluated using long-term peakflow records for three small watersheds (60–101 ha) and six large basins (62–640 km2) in the western Cascades of Oregon, USA. After a calibration period, two of the small watersheds were treated while the third remained untreated (control). Analysis indicated that peakflow increases following treatments were dependent upon peakflow magnitude. Peakflow increases averaged approximately 13–16% after treatment for 1-yr recurrence interval events, and 6–9% for 5-yr recurrence interval events. For the six large basins, multiple linear regression analyses of peakflows relative to: (1) peakflow magnitude; and (2) difference in percent area harvested provided mixed results. While significant (p<0.05) relationships were found in half of the analyses, the explained variance (Δr2) due to harvesting was generally small (1–7%).