Characteristics of channel steps and reach morphology in headwater streams, southeast Alaska

The effect of timber harvesting and mass movement on channel steps and reach morphology was examined in 16 headwater streams of SE Alaska. Channel steps formed by woody debris and boulders are significant channel units in headwater streams. Numbers, intervals, and heights of steps did not differ among management and disturbance regimes. A negative exponential relationship between channel gradient and mean length of step intervals was observed in the fluvial reaches (<0.25 unit gradient) of recent landslide and old-growth channels. No such relationship was found in upper reaches (≥0.25 gradient) where colluvial processes dominated. Woody debris and sediment recruitment from regenerating riparian stands may have obscured any strong relationship between step geometry and channel gradient in young alder, young conifer, and recent clear-cut channels. Channel reaches are described as pool–riffles, step–pools, step–steps, cascades, rapids, and bedrock. Geometry of channel steps principally characterized channel reach types. We infer that fluvial processes dominated in pool–riffle and step–pool reaches, while colluvial processes dominated in bedrock reaches. Step–step, rapids, and cascade reaches occurred in channels dominated by both fluvial processes and colluvial processes. Step–step reaches were transitional from cascades (upstream) to step–pool reaches (downstream). Woody debris recruited from riparian corridors and logging activities formed steps and then sequentially might modify channel reach types from step–pools to step–steps. Scour, runout, and deposition of sediment and woody debris from landslides and debris flows modified the distribution of reach types (bedrock, cascade, and step–pool) and the structure of steps within reaches.