A survey of the road network in the Lookout Creek and Blue River watersheds was conducted to inventory road-related sedimentation resulting from the February 1996 flood. During the spring and summer of 1996, the entire road network in both basins was inventoried for erosional and depositional features associated with roads. Each feature was field located on 7.5' topographic maps and later digitized into a geographic information system (GIS).
To distinguish among forms of erosion and deposition associated with roads, a typology of features, based on the process type and the point of origin relative to the road zone (defined as the cutslope, road surface and ditch, and fillslope), was developed. Two general process types were considered: mass wasting and fluvial. Mass wasting involves en masse detachment and transport of sediment and organic debris on hillslopes (debris slides) and in channels (debris flows). Mass wasting can also involve displacement of soil without evacuation from the site (e.g. slumps). The features produced by these processes are referred to in this paper as mass movements. Fluvial processes involve particle by particle transport of sediment by flowing water in channels (bedload transport) and on hillslopes or roads (incision and gullying), and produce what are generally refer to here as fluvial features. Four points of origin were considered: above the road, the cutslope, the road surface and ditch, and the fillslope. Three types of debris slides were distinguished for this study: those initiated on hillslopes above roads were termed hillslope slides, those originating on cutslopes were termed cutslope slides, and those from road fills were termed fillslope slides. Other processes originating above roads included debris flows and bedload transport that plugged culverts. Processes originating within the road zone included slumps, ditch incision, and gullying.
The length, width and depth of erosional scars and deposits were measured as appropriate for each feature to determine sediment volumes. Volumes are reported here to the nearest 5 m3. The accuracy of volume estimates, however, varies by the size of the feature, and reported volumes are considered to be precise only to two significant figures.
The study was conducted in the Lookout Creek and Blue River watersheds, located approximately 70 km east of Eugene in the western Oregon Cascades. Elevations in the basins range from 400 meters to over 1500 meters, with slopes ranging from 0 to 80 percent. The basins are underlaid by Tertiary and early Quaternary volcanic rocks, primarily ash flows, mudflows and pyroclastic flows at the lower elevations (below 800 meters) and andesite and basalt lava flows in the higher elevations. The geomorphic history of the basins is shaped by glacial, fluvial and mass wasting processes. Glacial deposits are evident in the southeast portion of the Blue River drainage. Lacustrine sediments and varve deposits are found along the banks of lower Blue River and Lookout Creek. Geologic mapping (Swanson and James, 1975) and landslide inventories (Dyrness, 1967; Swanson and Dyrness, 1975) document the history of deep-seated earthflows and shallow landsliding in the area.
Mean annual precipitation ranges from 2300 to over 2500 mm at the upper elevations (Greenland, 1994). Over 80% of the precipitation falls between November to April, typically as rain below 400 meters. Elevations between 400 and 1200 meters have been termed the "transient snow zone" (Harr, 1981) where precipitation falls alternately as rain and snow. A permanent snow zone develops during most winters above 1200 meters. Floods in the region often result from relatively warm rain events on an accumulated snowpack (Harr, 1981).
Summary of characteristics of study basins
Lookout Creek basin
Blue River basin (upper)
Notes:
a Estimated winter baseflow drainage density (see Wemple et al., 1996).
b Computed using an average width of road cut, surface, and fill of 16 meters from Silen and Gratkowski, 1953.
Vegetation in the basins consists of Douglas fir (Pseudotsuga menziesii), western hemlock (Tsuga heterophylla) and western red cedar (Thuja plicata). Stands of Pacific silver fir (Abies amabilis) occur above 800 m. Forest harvesting and road construction have occurred in both basins since the late 1930's (Jones and Grant, 1996). Harvesting and road construction in the Lookout Creek watershed occurred primarily in the 1950's and 1960's. Most of the harvesting and road construction in the Blue River watershed occurred between 1960 and 1985.
