TRANSPORT OF ROAD‐SURFACE SEDIMENT THROUGH EPHEMERAL STREAM CHANNELS¹

Abstract

Since the majority of road drainage points in western Washington and Oregon enter small, often ephemeral streams rather than large, fish‐bearing waters, impact of road‐surface sediment on biota in permanent streams depends, to a large extent, on transport through these small watercourses. A series of experimental additions of road‐surface sediment was made to two ephemeral streams to examine the downstream transport of this material as a function of discharge and channel characteristics. These small streams were found to store large amounts of sediment washed from road surface. In no instance did either stream transport more than 45 percent of the added material to their mouths, distances of 95 and 125 m. Larger‐sized sediment particles were delivered at a lower rate than finer material. Added sediment <0.063 mm in size was transported efficiently through the systems at all but the lowest flows tested. Material between 0.5 and 0.063 mm and from 2.0 to 0.5 mm in size were retained at progressively higher rates, with sediment in the coarser size category never exceeding a delivery of 10 percent of the added material. There were significant differences in the transport of sediment in the two larger size categories between the two streams. These differences were due to a much greater amount of woody debris in the stream with the lower delivery rates, which acted to trap and hold sediment, as well as a slightly longer and less steep channel.

Sediment transport, redistribution and storage on logged forest hillslopes in south-eastern Australia

“…Forest roads and tracks also have a speci®c sediment delivery pattern whereby runo is discharged at concentrated outlets often resulting in channelized¯ow paths or gullies that form very ecient delivery paths to streams (Duncan et al, 1987;Montgommery, 1994;Wemple et al, 1996). In the case of temporary roads or forest snig tracks, sediment is diverted on to the hillslope via a network of cross-banks (diversion banks) where channelized¯ow paths are rarely observed.…”

The influence of historical forestry practices and climate on the sediment retention function of wetlands.

“…and the Yellow Water Lily (Nuphar variegata), however, the presence of these species and their spatial distributions could have changed depending on the extent of the water level fluctuations. This second point is not of major concern as it was pointed out by Duncan et al (1987) that wetland vegetation did not have an impact on the deposition of the finest grain sizes (<63 /im) in channels, and stream discharge was a more important factor for sediment deposition. However, it is important to note that under certain conditions (i.e.…”

“…Finer material requires less stream power to remain in suspension and can therefore be transported over greater distances than coarser material (Duncan et al, 1987). However, the physical characteristics of the material near the outflow (i.e.…”

“…As well, the ephemeral nature of flooding in this channel would have pre vented the growth of wetland vegetation which play an important role in sediment trapping and stabilization of the wetland bottom. A study by Duncan et al (1987) tested the ability of two ephemeral channels to capture various sediment grain sizes derived from logging roads.…”

Grays River Watershed and Biological Assessment, 2006 Final Report.

“…The specific impacts of logging roads, some of which have already been discussed, include increased surface-water runoff (Harr et al 1975;Jones and Grant 1996;Wemple et al 1996;Bowling and Lenttenmeier 1997), road-surface erosion (Reid and Dunne 1984;Duncan et al 1987;, and landslide initiation (Megahan and Kidd 1972;Swanson and Dyrness 1975;Sessions et al 1987;Megahan 1991;. The typical forest road network interfaces with the natural streamchannel network to influence the routing of water and sediment, as well as soil movements (i.e., landslides, debris flows, and mass-wasting events) on forest hillslopes (Figure 4.16).…”