Location Name: Cold Creek (cross-section reach) - HJACOC

Parent: Cold Creek - COLD

Geology:

The Cold Creek (COC) site is a high elevation (1000 m) headwaters channel with a drainage area of approximately 71 ha (0.71 km2). While Cold Creek likely experienced some degree of past glaciation, the study reach is narrow (5.3-m average channel width), highly constrained by valley sidewalls, and quite steep (average gradient of 17%). The channel contains abundant woody debris (Figure 9.2), but most of the larger pieces and the majority of the volume of LWD is suspended well above the channel bed. The longitudinal profile of the channel has a decidedly stepped appearance, with most of the fall occurring as boulder or log steps. The channel would be classified as a cascade channel type in the Montgomery and Buffington (1997) classification scheme.

Hydrology:

The channel contains many large boulders and logs (Figures 9.2 and Appendix C, Figures C.1 through C.6) which appear to be immobile under the current hydrologic regime, at least with respect to purely fluvial transport processes. Even small boulders and larger cobbles are typically moss-covered, indicating lack of recent transport. However, pools and other low-gradient channel units between longitudinal steps contain a substantial amount of gravel- to cobble-sized sediment (as well as minor sand) which appears to be actively transported.

While the canyon drained by Cold Creek is narrow, its floor and lower sidewalls exhibit a distinctly trough-shaped cross-section, which may be the result of repeated scour by debris flows. Within this U-shaped trough are small (several m2 to several tens of m2), low (0.25-0.5 m above the low flow water surface) floodplain-like surfaces adjacent to the channel, which in at least some instances appear to have been formed by slumping of the valley sidewalls. Vegetation on these surfaces showed no sign of disturbance following the major flood of February 1996. The absence of large trees adjacent to the channel within the U-shaped trough (despite the lack of evidence of any recent disturbance) suggests that the channel may have experienced debris flows within recent decades. However, debris flow surveys conducted in the Andrews Forest in the 1960s, 1970s and 1990s show no evidence of debris slides or debris flows in this part of the watershed and none originating above an elevation of approximately 3500 ft (1070 m); most have occurred between elevations of 2000 and 2600 ft (600 to 800 m) (Dyrness, 1967; Swanson and James, 1975b; Snyder, in preparation).