Reductions in road sediment production and road-stream connectivity from two decommissioning treatments

Abstract

Unpaved forest roads can be an important source of sediment to streams. Road decommissioning is an increasingly common technique to eliminate these impacts, but few pre- and post-treatment studies have rigorously assessed its effectiveness. The objectives of this study in the northern Colorado Front Range were to: (1) quantify the effects of key variables on road sediment production before decommissioning; (2) quantify the changes over time in segment-scale sediment production from two decommissioning treatments (ripping only, and ripping plus mulching) versus untreated controls; and (3) quantify the factors affecting road-stream connectivity and the changes in connectivity due to decommissioning 12.3km of roads. Median sediment production rate in the first year prior to decommissioning was 0.3kgm−2, but values varied from 0.0kgm−2 to 3.0kgm−2. Traffic, precipitation intensity, and road segment area had the greatest effects on road sediment production. In the first two years after decommissioning the median road sediment production was zero kgm−2, as the furrows created by ripping trapped nearly all of the eroded sediment. Decommissioning also reduced road-stream connectivity from 12% of the total length to only 2%, with most of the connected segments being immediately adjacent to a stream. While both decommissioning treatments were effective, the ripping plus mulching treatment had visibly less surface erosion and no segment generated any measurable sediment. These results can help guide the design and quantify the benefits of future road decommissioning projects.