The geomorphic significance of log steps in forest streams of the Oregon Coast Range Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/1z40kx49b

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  • A log step develops when large woody debris extends across the active channel, creating a change in the water surface elevation as the stream spills over the log step. By forming a series of vertical falls, log steps reduce the amount of potential energy available for conversion to kinetic energy used for water and sediment routing. By trapping sediment supplied from upstream and adjacent hillslopes, log steps introduce an additional storage component into the sediment budget for forest streams. Simply reporting the percent of potential energy dissipated by log steps fails to reveal the significance of this function to equilibrium conditions. Forest management related addition and removal of log steps and stream adjacent trees have occurred without appreciation for the consequences to equilibrium conditions and associated trends in sediment production. Collection and analysis of data is undertaken for 13 study basins in the central Oregon Coast Range, including field surveys of 102 miles (164 km) of streams. The basins are divided into four landtypes using lithology and cluster analysis of seven morphometric variables which reflect the hydrologic and erosional regimes. Analogies between thermodynamic systems and stream systems enable derivation of equilibrium criteria which account for potential energy dissipation by falls and log steps. Calculations using the chi-square (x²) test statistic produce a quantitative measurement of the proximity to equilibrium of the stream network. A morphometric erosion factor is used to estimate mean annual sediment yield for each basin. The estimated mean annual sediment yield is then expressed as a percent of the sediment volume stored by log steps in all third, fourth, and fifth order streams. The dissipation of potential energy by log steps amounts to 6,09 percent, approximately equal to that of falls. Landtypes do not account for the spatial variation of log step development among the study basins. However, differences in the percent of potential energy dissipated by log steps between second, third, fourth, and fifth order streams are found to be statistically significant. Silvicultural activities account for only 20 percent of all log steps, in direct proportion to the percent area clearcut (25 percent) in the 13 study basins. The mean height of log steps created by silvicultural activities is 4.2 feet (1.3 m), one and one-half times as great as the mean height of log steps created by natural processes. The sum of mean annual sediment yield estimates for the 13 study basins amounts to 82 percent of the total volume estimated in storage by log steps in third, fourth, and fifth order streams. Statistical analyses indicate that falls and/or log steps do not cause a significant differences in the proximity to equilibrium of the stream network. Because of the long duration time of instream large woody debris, the accumulation of log steps over time with future silvicultural activities may lead to significant impacts on equilibrium conditions which now depend more on structural controls (e.g. uplift, faulting, and lithology) imposed over geologic time. In any case, the equilibrium criteria and sedimentation impacts described provide only two standards by which to manage instream large woody debris. The geomorphic functions of instream large woody debris not incorporated as log steps and the biological functions of instream large woody debris must also be addressed in forest management decisions.
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