Mass erosion occurrence and debris torrent impacts on some streams in the Willamette National Forest

Graduation date: 1983This study focused on extensive soil mass movement occurrence\ud in the Willamette National Forest of western Oregon and on intensive\ud measurements of some physical and biological changes in\ud streams following debris torrents. Debris torrents are a rapid\ud movement of water-charged debris confined to steep headwater channels.\ud The frequency (events/ha/yr) of mass failures identified\ud from aerial photos increased in the presence of clearcuts and\ud roads relative to forest conditions.\ud Approximately 71% of hillslope\ud mass failures entered the stream channel, and an estimated\ud 43% of hillslope mass failures resulted in debris torrents.\ud Standing crop of large organic debris and annual input to\ud streams affected by debris torrents were highly variable. Silvicultural\ud conditions of upslope vegetation and morphological\ud features of debris torrent tracks influenced woody loading and input.\ud Old-growth streams and depositional stream sites contained\ud higher amounts of large organic debris and received higher inputs\ud of large organic debris than streams in clearcuts and erosional\ud stream sites.\ud Depositional sites along debris torrent tracks have higher\ud pool area and depth relative to erosional stream sites. Stream\ud channel gradient and channel cross-sectional form influence the\ud character of erosional or depositional sites sluiced by a debris\ud torrent. Higher total pool ratings in depositional stream sites\ud indicate spatial complexity of channel form that is related to the\ud availability, transport, and stability of particulate bed materials.\ud Herbs produced the majority of foliar biomass (> 50% of total)\ud in one-fifth of all stream sites. Shrubs produced the majority\ud of foliar biomass in one-third of all stream sites. Stream sites\ud where estimated foliar biomass of post-torrent plant strata exceeded\ud estimated foliar biomass of residual plant strata were more\ud numerous. Successional changes in clearcut riparian zones included\ud a shift to intolerant deciduous overstory species and more\ud numerous shrub species than in old-growth riparian zones. The\ud diameter growth of tolerant late successional overstory species\ud in these riparian zones was inconsistent, whereas diameter growth\ud of early successional species was rapid.\ud Case studies of two streams permitted analysis of riparian\ud recovery relative to that of an undisturbed stream site upstream\ud of the debris torrent track. Warfield Creek is a stream where a\ud high energy debris torrent remarkably altered stream conditions.\ud Simmonds Creek is a stream where a low energy debris torrent imperceptibly\ud changed stream conditions.\ud Channel slope, channel cross-sectional form, severity of\ud debris torrent, position of stream segment within the drainage,\ud and presence of channel obstructions all affect stream response\ud to debris torrents, as do riparian vegetation and litterfall inputs\ud to the stream.\ud Riparian recovery remains difficult to quantify because of\ud physical and biological structures of stream ecosystems that vary\ud in space and time