INTRODUCTION TO SPECIAL ISSUE ON HYDROLOGIC AND GEOMORPHIC EFFECTS OF FOREST ROADS

Roads have been a part of human landscapes for more than 40 centuries. During the 20th century, technological advances have increased our ability to construct new roads at unprecedented rates and into steeper terrain. In the last half of that century, an extensive network of roads has been constructed in forests and other wildlands to facilitate use and management of natural resources. They are the transportation system that allows transport of timber and minerals from forests and access for recreationists, land managers, fire fighters, and residents of villages or vacation homes. Unfortunately, forest road construction may result in adverse changes to the environment. Roads fragment terrestrial and aquatic ecosystems by acting as a barrier to movement of some animals and plants. Roads can act as transportation corridors for plants, animals and fungi, some desirable, some not. Roads also affect the movement of water and sediment through landscapes. The combination of effects can be detrimental to native terrestrial and aquatic organisms, and negative correlations between road density and fish stocks have been noted (Lee et al., 1997; Thompson and Lee, 2000). The linear nature of roads and their tendency to run across topographic gradients yield an influence on watershed scale hydrologic processes that is much greater than one might expect from the small fraction of the land area they occupy. The concentration of runoff from nearly impervious road surfaces and intercepted subsurface flow into ditches effectively increases the drainage density, shifting the distribution of water on hillslopes and potentially increasing peak flows of streams. When there are few drainage features along roads there can be substantial inter-basin transfers of water between first-order streams. The effect is exacerbated when stream crossing culverts are plugged by debris, diverting a stream to other places in the landscape such as other streams or previously unchannelled hillslopes. These hydrologic effects are partially responsible for changes to geomorphic processes and sediment budgets in roaded basins. Runoff from the road tread and intercepted subsurface flow contribute to surface erosion from soil bared by road construction, use and maintenance. Lateral concentration of water collected along the length of the road and discharged at ditch relief or stream crossing culverts increases the risk of landslides, gullies and destabilization of existing stream channels. Diversion of water from stream channels onto hillslopes or into other stream channels occurs more rarely but with greater consequence. The papers in this special issue address many of the hydrologic and geomorphic issues identified above and contain important advances in the fields of hydrology and geomorphology on the subjects of hydrologic modelling, surface erosion and landscape evolution. They examine how the flowpaths of water are altered by the presence of roads and how these changes in flowpaths change geomorphic processes. Many of the efforts were spurred by recent work using digital elevation models (DEMs) to improve modelling of basin hydrology and landscape evolution. In a sense, roads are an experiment on the landscape; by perturbing the topography, we can view changes in the spatial patterns in hydrologic and geomorphic processes caused by roads, allowing us to test the principles derived in the earlier studies. Two papers primarily address hydrologic issues. They examine changes in flood frequency following road construction using distributed hydrologic models. Because stream flows are affected by both roads and harvest areas, which are created within a short time of one another, it is difficult to derive statistically the independent effect of roads and harvest from stream gauge data alone (Jones and Grant, 1996; Thomas and