Influence of logjam-formed hard points on the formation of valley-bottom landforms in an old-growth forest valley, Queets River, Washington, USA

Field surveys and radiocarbon dating of buried logjams in the floodplain of an old-growth forest river demonstrate the formation of erosion-resistant “hard points” on the floodplain of the Queets River, Washington. These hard points provide refugia for development of old-growth forest patches in frequently disturbed riparian environments dominated by immature forest. Our surveys show that local bed aggradation associated with logjams not only influences channel patterns and profiles but leads to development of a patchwork of elevated landforms that can coalesce to form portions of the valley bottom with substantial (i.e., 1 to >4 m) relief above the bankfull elevation. In addition, logjam-formed hard points promote channel avulsion, anastomosing morphology, and growth of mature patches of floodplain forest that, in turn, provide large logs needed to form more logjam-formed hard points. Hence, our findings substantiate the potential for a feedback mechanism through which hard points sustain complex channel morphology and a patchwork floodplain composed of variable-elevation surfaces. Conversely, such a feedback further implies that major changes in riparian forest characteristics associated with land use can lead to dramatic simplification in channel and floodplain morphology.     

Wood-mediated geomorphic effects of a jökulhlaup in the Wind River Mountains, Wyoming

A jökulhlaup burst from the head of Grasshopper Glacier in Wyoming's Wind River Mountains during early September 2003. Five reaches with distinct sedimentation patterns were delineated along the Dinwoody Creek drainage. This paper focuses on a portion of the jökulhlaup route where erosion of the forested banks created 16 large logjams spaced at longitudinal intervals of tens to hundreds of meters. Aggradation within the main channel upstream from each logjam created local sediment wedges, and the jams facilitated overbank deposition during the jökulhlaup. Field surveys during 2004 and 2006 documented logjam characteristics and associated erosional and depositional features, as well as initial modification of the logjams and flood deposits within the normal seasonal high-flow channel. Overbank deposits have not been altered by flows occurring since 2003. Field measurements supported three hypotheses that (i) logjams present along the forested portions of the jökulhlaup route are larger and more closely spaced than those along adjacent, otherwise comparable stream channels that have not recently experienced a jökulhlaup; (ii) logjams are not randomly located along the jökulhlaup route, but instead reflect specific conditions of channel and valley geometry and flood hydraulics; and (iii) the presence of logjams facilitated significant erosional and depositional effects. This paper documents a sequence of events in which outburst floodwaters enhance bank erosion and recruitment of wood into the channel, and thus the formation of large logjams. These logjams sufficiently deflect flow to create substantial overbank deposition in areas of the valley bottom not commonly accessed by normal snowmelt peak discharges, and through this process promote valley-bottom aggradation and sediment storage. Changes in the occurrence of glacier outburst floods thus have the potential to alter the rate and magnitude of valley-bottom dynamics in these environments, which is particularly relevant given predictions of worldwide global warming and glacial retreat. Processes observed at this field site likely occur in other forested catchments with headwater glaciers.