This study assessed how logging-access roads may have contributed to
observed historical increases in peak discharges associated with small and large
logged basins in the western Cascades of Oregon. The study was conducted on
the Lookout Creek (62km��) and the upper Blue River (118km��) basins. Potential
road effects on hydrology were examined using a combination of field surveys
and spatial modeling with a geographic information system (GIS). Road
networks were similar in both basins with respect to hillslope position,
orientation, and stream crossings, but roads in Blue River were constructed one
or two decades later than roads in Lookout Creek. A total of 20% (62 km) of the
road length was sampled to assess routing of surface flow, using 31 2-km
transects stratified by decade of construction and hillslope position. Along each
transect, ditches and culvert outlets were examined and this information used to
predict the probable routing of water to (1) existing stream channels, (2) newly
eroded gullies downslope of culvert outlets, or (3) subsurface flow. Nearly 60%
of the surveyed road length appeared to route water directly to stream channels or into gullies. Over time, the length of road connected to stream crossings has
decreased, while the length of road discharging runoff that reinfiltrates to
subsurface flow has increased, as roads have progressed up hillslopes and onto
ridges in Lookout Creek and Blue River. The relatively constant proportion of
the road network draining to gullies over time suggests that roads have the
potential to become integrated into stream networks, even when constructed on
unchannelled hillslope positions. An extended stream network, assumed to
exist under storm conditions, was simulated for the basins using a digital
elevation model. Although gullies and ditches differ from natural channels,
extrapolation of field surveys using the GIS suggested that roads might extend
the stream network by as much as 40% during storm events. It is hypothesized
that such an effect could decrease the time of concentration of stormflow and
contribute to higher peak discharges observed after clearcutting and road
construction in these basins. Differences in the magnitude of road effects on
peak flow generation may occur among road systems according to hillslope
position of roads, road age, soil saturation, geologic substrate, and climate.
These differences may explain the range of observed results from paired-basin
studies examining road effects on hydrologic response. / Graduation date: 1994
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/31877 |
| Date | 21 January 1994 |
| Creators | Wemple, Beverly C. |
| Contributors | Jones, Julia A. |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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