Sediment trapping by streamside management zones of various widths after forest harvest and site preparation
Recommended widths for streamside management zones (SMZs) for sediment protection vary. The objectives of this study were to compare the effects of SMZ widths and thinning levels on sediment moving through SMZs. Four SMZ treatments were installed within 16 harvested watersheds where intermittent streams graded into small perennial streams. Sites were clearcut, prescribed burned, and planted with loblolly pine (Pinus taeda L.). Treatments were 30.4-, 15.2, and 7.6-m-wide SMZs without thinning and 15.2-m-wide SMZs with thinning. Three to seven treatments replicated within four blocks created a randomized incomplete block design. Erosion rates from watersheds and sediment trapping within SMZ treatments were monitored with modeling and sediment pins. A second study evaluated 24 subwatersheds within eight watersheds. Three subwatersheds were located within each watershed so sediment traps collected inputs into SMZs from harvest site-prepared areas, firelines, or at streams. SMZ treatments had no significant differences regarding sediment trapping. All SMZs widths were generally effective in trapping sediment. Within the 16 intermittent-perennial watersheds and 24 ephemeral subwatersheds, erosion to sediment delivery ratios from harvests ranged from 3 to 14%. For ephemeral stream subwatersheds, firelines adjacent to SMZs contributed 14% of total sediment. Sediment trap data collected within SMZs indicated that 97% of watershed erosion was trapped before reaching streams. In three subwatersheds, sediment penetrated SMZs due to channelized flow from failed or inadequate water controls on roads and firelines. Results support the common recommendation for SMZ widths of 15.2 m in which partial harvests may occur and emphasize the importance of implementation of best management practices for roads and firelines.