Effects of input and redistribution processes on in-stream wood abundance and arrangement in Lookout Creek, western Cascades Range, Oregon Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/pr76f537p

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  • This study explored how selected input and redistribution processes affect the amount and arrangement of in-stream wood within the 64 km2 Lookout Creek watershed in the Andrews Forest, western Cascade Range, Oregon. A longitudinal inventory of in-stream wood was conducted over approximately 20 km of stream length in 2nd to 5th-order channels along with reconnaissance activities in stream sections that had originally been mapped in 1970's. The study had two major parts. The first focused on patterns of in-stream wood amount and arrangement within the study watershed. Flood transport capability of inventoried stream segments was assigned based on channel width. Historical observations of debris flows and earthflow activity over the 50-year period prior to our study were used to delineate influence of those processes on inventoried channel segments. Wood amounts decreased with increases in channel transport capability. As channel transport capability increased the number of wood accumulations also decreased and there was a higher proportion of accumulations created by floods. These observations signal an increase in influence of fluvial transport of wood in the downstream direction. There were lower than expected amounts of wood at locations where debris flows entered mainstem channels and where earthflows were in contact with mainstem channels. This is attributed to the relatively high mobilization capability of channels in these locations. Congested wood break flood pulses is thought to be responsible for observed differences in wood conditions among segments affected by these processes and segments affected by floods only during the last fifty years. Some evidence exists to support the idea that debris flow entry to mainstem channels and dam-break flood pulses may promote a several decade period of increased wood input as a result of damage and disruption to trees in and adjacent to the channel in disturbed area. However, analysis of the effects of debris flows and earthflow activity on wood amount and arrangement was limited by small sample sizes. The second part of the study examined relative changes in wood amount and arrangement as well as changes to individual wood accumulations over a 25-year period within 5 stream reaches that had been mapped in detail in the early 1 970s. The effect of channel size, input and redistribution processes, piece size, as well as arrangement and location of pieces with respect to the active channel on changes in wood conditions were analyzed. Overall, changes in in-stream wood amount and arrangement were greater in larger channels than in smaller channels. In-stream wood changes resulting from fluvial redistribution also increased with channel size while decomposition and toppling were the processes responsible for most changes in smaller basins. In-stream wood changes were greater among small pieces than large pieces, although substantial changes in large pieces did occur in larger channels as a result of fluvial transport and in smaller channels as a result of toppling. In-stream wood changes were greater among single pieces than pieces in accumulations. Flood transport probably reduced the number of single pieces by collecting them in accumulations. Significant changes in in-stream wood occurred among pieces located both in and out of the channel suggesting that recent flooding, particularly in larger basins, was able to affect wood located along the margins of the channel. Changes in accumulations were generally minor in all sites except the 5th-order channel site.
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