South Coldwater Creek valley was buried to a depth of 10 to 75 meters by volcaniclastic debris during the May 18, 1980, eruption of Mount St. Helens. To avert catastrophic breaching of nearby Spirit Lake blockage, a tunnel was bored from Spirit Lake basin into South Coldwater Creek valley and was opened in April 1985. Formation and development of the South Coldwater Creek channel during 1980-85 and the response of South Coldwater Creek to augmented discharge during April to September 1985 illustrate interactions among changes in channel slope, width, depth of incision, and sediment load. These interactions relate to the characteristics of unstable stream channels, effects of augmented discharge that exceeds previous channel-forming discharge, and effects of large fluctuations in water and sediment discharge on stream channels.
During water years 1981-85, nearly 2 million cubic meters of material was eroded from a 5.7-kilometer reach of South Coldwater Creek as a result of natural streamflow. Erosion rates declined exponentially during 1980-85, and the location where erosion rates were greatest shifted downstream. During water year 1981, channel formation, incision, and widening along a steep, upstream reach dominated net sediment yield. During water years 1982-84, channel avulsion and widening shifted the location of maximum erosion downstream to a more gently sloping reach. During the final winter in which South Coldwater Creek received only natural flow, total sediment production declined to within the range observed in drainage basins not affected by volcanoes, and channels along the most gently sloping downstream reaches were incised.
During April through September 1985, flow in South Coldwater Creek was augmented by release of 5.44 to 14.7 cubic meters per second from nearby Spirit Lake. The effect of these flows on the channel system through South Coldwater Creek was dependent on (1) the magnitude of flows relative to prior channel-forming flows, and (2) the sediment delivery (or the quantity of net upstream erosion) of the stream at any given point relative to discharge and slope.
When the 1985 augmented flows did not substantially exceed prior channel-forming flows, considerable erosion occurred along steep, upstream reaches as bed material and alluvium were mobilized, but the occurrence of rapid erosion lasted only about a day. Downstream, sediment-laden channels were widened as a result of increased sediment discharge and eroded 5-meter-high terraces at a rate as much as 15 meters per day.
When the augmented flows did substantially exceed prior channel-forming flows, channel enlargement and migration of relatively clear streams along steep, upstream reaches eroded 8-to 16-meter-high terraces. Sediment-laden downstream channels again widened, but most of the necessary channel width had already been excavated by prior flows.
A pattern was observed as the stream channels changed in response to both natural and augmented discharges. Stream-channel initiation, incision, and widening along steep, upstream reaches greatly increased the sediment load along more gently sloping downstream reaches. Downstream reaches commonly aggraded and widened when sediment discharge was increased. Where the channel was bounded by erodible terraces, channel widening was responsible for the largest rates of erosion observed. After a period -- years for natural flows, days or weeks for augmented flows -- channel incision along upstream reaches declined, and thus the sediment load to downstream reaches declined. Downstream channels became narrow and were incised through both alluvium and 1980 volcaniclastic deposits.
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