US Forest Service Research and Development Tree Mortality from Beetles and the Role of Large Wood in Stream Channels - Rocky Mountain Research Station - RMRS - US Forest Service
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Tree Mortality from Beetles and the Role of Large Wood in Stream Channels

On-going research on channel processes at the Fraser Experimental Forest draws on number of long-term records of flow, sediment yield, and channel form in assessing geomorphic changes associated with large-scale tree mortality from epidemic mountain pine beetle infestation. A primary change expected in response to widespread mortality is an increase in the loading of large wood in channels throughout the Experimental Forest. Large wood is a key component influencing flow hydraulics, bank stability, sediment storage, and channel structure in the form of log steps in small streams. Baseline studies initiated in 2006 indicate that about a third of the large wood in (very) small channels originates from adjacent valley walls, suggesting that tree fall associated with beetle kill will produce substantial increases in wood loading in these systems. Here, large wood is often longer than the channel is wide and large pieces are moved only rarely. Instead, trunks of trees become incorparated into the channel, forming step structures which store sediment in wedges behind the obstruction. On-going work tests the hypothesis that the channel shifts its position so that flow runs perpendicular to the introduced wood, there by maximizing channel stability and sediment storage. Over time, the wood decays and the steps break apart, releasing the sediment stored in wedges. Large pieces of wood may also re-direct flow, causing localized erosion over portions of stable channels. Hence, the presence and movement of large wood in small streams can both enhance and destabilize channel structure. Therefore, the net effect of a substantial increase in large wood is uncertain and will be documented over time by repeating baseline channel surveys on a regular basis. Changes in sediment dynamics will be assessed by comparing flow-weighted sediment yields from the "baseline" period against similar data from the period following beetle infestation. Changes in sedimentation to streams draining National Forest Lands can potentially alter the quality of water obtained from these pristine environments, thereby increasing the costs of purification for human consumption.

References

Baseline sediment and channel survey data published in:

Troendle, C.A., Nankervis, J.M. and Ryan, S.E. 1996. Sediment transport from small, steep-gradient watersheds in Colorado and Wyoming. In: Sedimentation Technologies for Management of Natural Resources in the 21st Century. Sixth Federal Interagency Sedimentation Conference, March 10-14, 1996, Las Vegas, NV. pp. IX-39 to IX-45.

Ryan, S.E. 1997. Morphologic response of subalpine streams to transbasin flow diversion. Journal of the American Water Resources Association 33(4):839-854.

Ryan, S.E., Porth, L.S., and Troendle, C.A. 2002. Defining phases of bedload transport using piecewise regression. Earth Surface Processes and Landforms 27:971-990.

Ryan, S.E., Porth, L.S., and Troendle, C.A. 2005. Coarse sediment transport in mountain streams in Colorado and Wyoming, USA. Earth Surface Processes and Landforms 30:269-288.

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