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Forest harvesting practices can expose mineral soils, decrease infiltration capacities of soils, disturb the stream bank and channel, and increase erosion and fine sediment supply to stream channels. To reduce nonpoint source sediment pollution associated with forest management activities and to maintain the high water quality typically provided from forests, best management practices (BMPs) were developed and implemented. While BMPs have evolved over time, the effectiveness of contemporary BMPs, particularly for harvesting practices, have not been thoroughly investigated, especially in comparison to historical practices. The objectives of this study were to (1) determine the effects of contemporary harvesting practices on suspended sediment concentrations and yields and (2) examine the legacy effects from historical harvesting on suspended sediment concentrations. The Alsea Watershed Study was an important early research site that lead to the development of contemporary forest management practices to protect water quality and fish habitat in Oregon and elsewhere. By returning to the same watersheds that were harvested in 1966, this is one of the few times that a watershed-scale study is able to directly compare and contrast the effects of historical practices with contemporary practices. The Alsea Watershed Study Revisited includes the same three watersheds as the original study. Flynn Creek (FCG, 219 ha) is an old-growth dominated reference watershed. Deer Creek (DCG, 315 ha) is an extensively managed watershed that was patch-cut during the original study. Needle Branch (NBLG, 94 ha) was clearcut harvested in the original study and again in the recent study, but with contemporary BMPs, including riparian buffers. The upper portion of Needle Branch was harvested in 2009 (Phase I), while the lower portion of the watershed was harvested in 2015 (Phase II). We monitored suspended sediments and discharge from WY 2006–2016, and analyzed this data using multiple linear regression procedures and ANCOVA. Average suspended sediment yields ranged from 55–313 Mg km−2 yr−1 in FCG, 31–102 Mg km−2 yr−1 in NBLG, and 69–127 Mg km−2 yr−1 in DCG. We found no evidence that contemporary harvesting techniques affected suspended sediment concentrations or yields. Overall, suspended sediment concentrations and yields after contemporary harvesting were similar to historical pre-treatment levels.

Research dating back to the 1950 s has documented negative effects from harvesting of primeval forests on stream ecosystems of the Pacific Northwest. By the early 1990 s, state and federal forest practice rules governing timber harvest were modified throughout North America to better protect aquatic habitats and biotic resources, principally salmonids. These rules inspired a generation of studies using a before-after-control-impact (BACI) design to document the capacity of contemporary timber harvest rules to protect salmonids in headwater streams of second-growth forests. One important unanswered question concerns the potential effects of successive clearcuts in second growth forests. Consequently, we used a paired watershed approach to evaluate the effects of two successive clearcut harvests in the Alsea Watershed, site of the seminal Alsea Watershed Study that was conducted from 1958 to 1973, on relative biomass, movement, survival, and distribution of coastal cutthroat trout (Oncorhynchus clarkii clarkii) and three physical habitat characteristics (pool area and depth, and water temperature). Although the total clearcut harvest encompassed 87% of the treatment catchment in six years, no negative effects of logging were detected for either age-1 + coastal cutthroat trout or habitat variables. Comparisons between the harvested and reference catchments suggested the survival of coastal cutthroat trout (>94 mm fork length) and total catchment relative biomass of age-1+ (i.e., > 80 mm) exhibited similar patterns, increasing from the pre-logging period (2006–2009) through the Phase I post-logging period (2009–2014), and decreasing to levels observed in the pre-logging period during the Phase II post-logging period (2014–2017). Additionally, there was no evidence for differences in movement of coastal cutthroat trout related to the harvesting treatment. In terms of habitat variables, there was a relative increase in annual total pool area in the harvested catchment during the Phase II post-logging period, but there was no evidence the 7-day moving mean maximum stream temperature changed after the Phase I and Phase II harvests. Moreover, stream water temperatures never exceeded the criterion designed to protect core coldwater habitat for salmonids (16 °C). As such, it is unlikely that cutthroat trout experienced thermal stress following either harvest. More generally, results from this and other recent studies suggest that forest practice rules developed in conjunction with current best management practices for logging in headwater catchments have substantially improved outcomes for stream biota relative to unregulated forest harvest, at least for short periods of time after logging (i.e., ≤ 8 years).

Historical forest harvesting practices, where the riparian canopy was removed, generally increased energy loading to the stream and produced higher stream temperatures. As such, contemporary forest management practices require maintenance of streamside vegetation as riparian management areas, with an important function of providing shade and minimizing solar radiation loading to streams to mitigate stream water temperature changes. The Alsea Watershed Study Revisited in the Oregon Coast Range provided a unique opportunity to investigate and compare the stream temperature responses to contemporary forest harvesting practices (i.e., maintenance of riparian vegetation) with the impacts from historical (1960s) harvesting practices (i.e., no riparian vegetation). Here we present an analysis of 6 years (3 years pre-harvest and 3 years post-harvest) of summer stream temperature data from a reference (Flynn Creek) and a harvested catchment (Needle Branch). There was no evidence that the (a) 7-day moving mean of daily maximum ( T 7DAYMAX ) stream temperature, (b) mean daily stream temperature, or (c) diel stream temperature changed in the study stream reaches following contemporary forest harvesting practices. The only parameter of interest that changed after forest harvesting was the T 7DAYMAX when analyses were constrained to the Oregon regulatory period of July 15 to August 15 and all sites in each catchment were grouped together—in this case stream temperature increased 0.6 ± 0.2 °C ( p = 0.002). However, over the entire post-harvest study period, the warmest maximum daily stream temperature observed in Needle Branch was 14.7 °C—in the original Alsea Watershed Study, maximum daily stream temperatures rose to 21.7 °C (1966) and 29.4 °C (1967) in the first two post-harvest years, providing evidence that current harvesting practices have improved protection for stream water temperatures.

Elevated fine sediment transport to streams can negatively affect aquatic ecosystem health, downstream infrastructure, and community water supply. Forest harvesting activities can increase the delivery of fine sediment to streams due to intensified erosion or mass wasting from hillslopes, roads, and stream channels. However, quantifying the effects of forest harvesting on sediment inputs to streams and the effectiveness of current best management practices (BMPs) at mitigating these effects remains a challenge. In this study, we used sediment source fingerprinting techniques to quantify and compare the sources of suspended sediment to a stream draining a recently harvested catchment and a nearby, unharvested reference catchment in the Oregon Coast Range of the U.S. Pacific Northwest. In each stream, we quantified the proportional contributions of suspended sediment from three potential source areas: hillslopes, roads, and streambanks. The primary source of suspended sediment in the harvested catchment was streambank sediment (90.2 ± 3.4%) with lesser amounts of hillslope (7.1 ± 3.1%) and road (3.6 ± 3.6%) sediment. Interestingly, the proportional contributions of suspended sediment in the reference catchment were similar, with the majority from streambanks (93.1 ± 1.8%) followed by hillslopes (6.9 ± 1.8%). There were no contributions from roads in the reference catchment, despite a similar road network as the harvested catchment. We also quantified monthly sediment mass eroded from 36 × 1 m2 hillslope plots. The sediment mass eroded from the general harvest area (96.5 ± 57.0 (SE) g) was ~10.6-times greater than the sediment collected in the riparian buffer (9.1 ± 1.9 g) and ~4.6-times greater than the sediment collected on the unharvested, reference hillslope (21.0 ± 3.3 g). While this study provides evidence of effectiveness of contemporary BMPs (e.g., riparian management areas, limits to cutblock size, reduced impact forest harvesting techniques, road building and maintenance) at mitigating sediment delivery to streams, additional research is needed as existing studies do not adequately reflect the broad range of climate, geology, topography, and vegetation in the Pacific Northwest, which drive highly variable hydrologic and geomorphic processes in the region.