Stream channel response to peak flows in a fifth-order mountain watershed

This investigation explored how the magnitude, style, and frequency of channel adjustments vary spatially and over time within a 5th-order mountain watershed. Historical data sets, including repeated cross section surveys spanning up to 20 years at five sites on 2nd to 5th-order channels and streamflow records spanning up to 50 years, were supplemented by mapping and field reconnaissance activities.
The study had two major parts. The first focused on two adjacent, contrasting stream reaches to examine the influence of large woody debris (LWD) on channel morphology and channel response to peak flows in a 3rd-order stream. The upper reach flows through old-growth forest with abundant LWD, while the lower reach was clearcut in 1964-65 and contains little LWD. A 25-year flood in 1996 caused deposition upstream of LWD steps in the old-growth reach alternating with scour between steps, resulting in no net gain or loss of sediment within the reach, while extensive scour and coarsening of the bed occurred in the clearcut reach. These observations suggest that reach-scale channel response was strongly influenced by LWD abundance, but that response at finer scales depends critically on the details of the location and arrangement of LWD.
The second part of the study examined the dynamics of channel response to peak flows over two decades, and to two particular large floods during that period, in different portions of the channel network. The cross section data show that the streambed at the study sites is very stable, particularly in lower-order channels. Peak flows that produced detectable change at 90% of cross sections-flows able to cause significant channel adjustments-recur approximately three times as frequently (every 6-7 years) in 4th to 5th-order Lookout Creek as in 3rd-order Mack Creek (20-25 years). Flows that produced detectable change at 25% of cross sections are estimated to occur on average every 1.7 to 3.0 years at the study sites. It is estimated that if peak flows of all sizes were increased by only 10% due to anthropogenic impacts (e.g., logging) or climate change, the frequency of peak flows of a magnitude observed to produce significant channel adjustments would increase by approximately 30% in Lookout Creek and 60% in Mack Creek.