Erosion sources and sediment pathways to streams associated with forest harvesting activities in New Zealand
Forest harvesting and associated soil disturbances (e.g., earthworks) can increase catchment sediment yield, with potentially negative consequences for water quality and aquatic habitat. A major challenge for water quality protection is to anticipate where concentrated surface runoff and sediment will reach the stream (a.k.a., a ‘breakthrough’). Improved understanding of sediment sources and pathways can guide forestry practices to control erosion and disrupt concentrated surface runoff. This study involved walking along intermittent and perennial stream channels associated with 23 recent plantation forest harvests throughout New Zealand to quantify the spatial frequency of breakthroughs and characterise their sources. For road-stream crossings that contributed sediment to the stream, the Universal Soil Loss Equation modified for forest land was used to estimate annual sediment delivery rates. Time since harvest completion was typically 2 to 12 months, while harvest area ranged from 4 to 67 hectares. Breakthrough frequency for timber harvests using ground-based skidding was 1.9 times that of cable-yarding extraction (6.2 versus 3.3 breakthroughs per stream kilometre). Overall, 73% of breakthroughs were associated with concentrated runoff from roads, skid trails, stream crossing approaches, and ruts from machine tracks on hillslopes directed toward streams. Estimates of sediment delivery at roadstream crossings, which accounted for 23% of all breakthroughs, were highest for skid trails (median = 70.8 tonnes/ha/year), followed by truck road ditches and running surfaces (3.9 and 1.6 tonnes/ha/year, respectively). These results emphasize the importance of implementing surface cover and adequately spacing water control structures on highly trafficked areas during harvesting activities and site closure. While surface runoff connections between roads and streams cannot be eliminated, the severity of accelerated erosion and their impacts can be minimised.