The Mosquito 3100, 4500 and 6000 road systems were constructed 20 to 40 years ago using bulldozer construction, as was common during that era. There had been road prism failures in several places along the road systems. Prior to this project there was high potential for additional failures in the road prism and downslope. This project involved deactivating the road systems. It was done to improve the road prism and downslope stability, and to return subsurface and surface runoff to pre-road conditions. The Mosquito Creek road system covers a lengthy east-west bearing ridge that is located along the south San Juan River valley. Except for MOS 4500, the deactivated roads are situated on the north aspect of the ridge. The MOS 4500 road wraps around the ridge nose, with the hillslope changing to a westerly aspect and drains into Falls Creek. Area sediment is comprised of a glacially pulverized slate that consists of fine textured silt and small, platy rock detritus. Due to the sediment?s high silt content disturbed soils are prone to erosion. The surficial mantle depth is generally shallow on the MOS6000 system, with the exception of some deep in-fill wedges that occur within larger gully corridors. Deeper till sediments were observed along the lower elevation of the MOS 3100 Road and spur roads. The colluvial surficial mantle depth was observed to be generally shallow along the MOS 4500 Road. Outcrops were found throughout the deactivation sites. The road systems are estimated to be 20 to 40 years old. Because of the cut and fill dozer construction, used at the time, failures are common along road sections crossing the steeper terrain or incised drainage corridors. The objective of this deactivation work was to reduce the slide potential within sections of over-steepened sidecast and to return subsurface and surface runoff to pre-road conditions.
Pollmer, A.. 2003. Deactivation of Mosquito 3100, 4500 and 6000 road systems San Juan Watershed, South Island Forest District: project completion abstract. Forest Investment Account (FIA) - Forest Science Program. Forest Investment Account Report. FIA2003MR103