Hydrological processes and pathways affected by forest roads: what do we still need to learn?

*Correspondence to: Charles H. Luce, Rocky Mountain Research Station, Boise Aquatic Sciences Laboratory, 316 E. Myrtle, Boise, ID 83702, USA. E-mail: [email protected] Forest roads are an important environmental issue. While many scientists interested in hydrology recognize climate-altering processes as an important global issue, there are problems that are similar in scope and magnitude because human industriousness has brought them to so many parts of the world. Almost everywhere people live and work they build and use unimproved roads, and wherever the roads go, a range of environmental issues follows. Among the environmental effects of unimproved roads, those on water quality and aquatic ecology are some of the most critical. Increased chronic sedimentation, in particular, can dramatically change the food web in affected streams, lakes, and oceans and reduces the effectiveness of drinking water treatment, which compounds problems of access to safe drinking water in developing regions. Low-standard roads accessing agricultural and forest lands comprise much of the extent of roads and probably affect the greatest area. Effects of roads on sediment generation are closely tied to runoff generation and redistribution processes. The nearly impervious nature of road surfaces (or treads) makes them unique within forested environments and causes runoff generation even in mild rainfall events, leading to chronic fine sediment contributions of minor magnitude to water bodies. In some circumstances, much greater volumes of runoff can be generated by cutslopes, which may play an important role in more severe erosion processes. Lateral redistribution of runoff generated by roads (lateral to the natural hillslope) can greatly affect slope stability and peak discharges of small streams (Montgomery, 1994; Jones and Grant, 1996; Wemple et al., 1996; Thomas and Megahan, 1998; Croke and Mockler, 2001). If we look at the issue of what we need to learn or the research priorities for forest road hydrology, I would argue that the areas of cutslope hydrology and effectiveness of restoration efforts are perhaps most critical. I base this on the idea that research priorities hinge on the uncertainty exposed in existing research and the influence of that uncertainty on future decisions and designs. A great deal of literature on forest roads focuses on the road tread because of the obvious contrast with surrounding forest lands. They are described as almost ideal surfaces for infiltration excess (Horton) overland flow, with a narrow range of hydraulic conductivities (Reid and Dunne, 1984; Flerchinger and Watts, 1987; Luce and Cundy, 1994; Luce, 1997; Ziegler and Giambelluca, 1997; Croke et al.,