Two key characteristics of the LOM site are the absence (mostly) of hillslopes adjacent to the channel and the abundance of large, fluvially transported LWD. Among cross section sites, the LOM site is unique in being mostly unconstrained by surrounding topography (Table 9.1, Figure 9.7). The channel is adjacent to a hillslope only on the left bank of the lower 100 m of the reach. Elsewhere the channel is bordered by extensive floodplain surfaces (mostly less than 1.5 to 2 m above the low-flow channel), covered with old-growth conifer and mixed conifer-hardwood forest. On the left bank, the channel is bordered by a floodplain approximately 1 to 2.5 m (mostly <1.5 m) above the current summer low-flow water surface. This surface is vegetated with old-growth Douglas fir, hemlock, cedar, and bigleaf maple, extending from approximately 75 m upstream of the uppermost cross section down to XS 7, a distance of approximately 260 m. At the upstream end of this surface, a large side channel branches off from the main channel and follows a large arc to the south (presumably a former meander bend of Lookout Creek), returning to the main channel just upstream of the x-post for XS 7 and 8. The entrance of the side channel is partially blocked by LWD and sediment, and no surface water enters under summer low flow conditions, although some flow in the channel is maintained via subsurface flow. At approximately the southernmost extent of the arc, a small perennial tributary flows into the side channel. The outside edge of the side channel and the floodplain surface it bounds is marked by a short, abrupt 6 to 10-m high slope that is most likely the edge of a dissected alluvial fan.
Downstream of where the side channel rejoins the main Lookout Creek channel, the left bank of the channel takes on a scalloped appearance and becomes a sheer wall at the foot of a steep hillslope. Since the February 1996 flood, portions of the bank where the current has been forced against the bank by a massive accumulation of LWD and associated sediment have slumped down into the channel carrying several large conifers with them.
On the right (north) bank, bank height decreases fairly abruptly from about 2 m upstream to <15 m downstream of XS 2. A forested flooplain surface extends approximately 50 to 125 m north of the channel edge, to the edge of Road 1506 in the upstream portion of the reach or the edge of a 2 to 4-m high terrace bordering the downstream end of the reach. A network of back channels, some of which are shown in Figure 9.6, dissect the forested floodplain surface downstream of XS 5; less well-defined channels and evidence of lateral overflow from the main channel extend upstream as far as XS 3. Major back channel entrances, partially plugged by LWD and sediment, are located by cross section posts 5Z and 6Z in the central portion of the LOM reach (Figure 9.6, location D). The main back channel is lower than the main Lookout Creek channel, and its entrance by XS 6 is less than one meter above the summer low-flow water surface. This back channel has year-round flow via hyporheic flow from the main channel. As noted above, the LOM site contains abundant LWD including old-growth conifer logs. Unlike the Cold Creek and Mack Creek sites, most of the LWD within the LOM reach, including large logs, has been fluvially transported, and is concentrated in large marginal or channel-spanning accumulations. Many of the logs have cut ends. A major feature of this reach is a massive accumulation of LWD along the right (north) bank of the channel extending downstream from the vicinity of XS 5 and spanning the channel in the vicinity of XS 9. Much of this LWD predated the February 1996 flood and probably was emplaced by the 1964 flood, but a channel spanning accumulation, which partially blocks the main channel in the vicinity of XS 9, was created by the 1996 flood.
The LOM site is typical of many 4th-order reaches in the western Cascades in that it is downstream of dozens of clearcuts (but is not bordered by any). It is atypical in its geomorphic setting within a wide alluvial valley floor segment caused by deposition upstream of a massive earthflow (Fautini thesis Section 9.1; Swanson and James, 1975a). Wide valley floors are more typical of the Washington Cascades, which have been more heavily glaciated. The LOM site is upstream of most documented debris flows, and where the valley floor is wide tributary debris flows often stop before reaching Lookout Creek (Grant and Swanson, 1995), so it is probably not strongly affected by mass movement processes. Several known active earthflows occur upstream of the Mack Creek confluence, but are too far upstream to have much direct influence on the channel at the LOM site.
