FORGING THE ECOLOGICAL TEMPLATE: GEOMORPHIC AND HYDROLOGIC CONTROLS ON THE STRUCTURE AND ORGANIZATION OF CHANNEL BEDS IN HEADWATER STREAMS

Headwater streams in mountainous regions sit at the boundary between hillslopes and channels, and thus have properties common to both. Although these low-order channels are small, ephemeral, and often obscured by vegetation and woody debris, they represent the greatest number and total length of streams in dissected landscapes. Despite their ubiquity, the processes controlling the spatial pattern and organization of channel features are poorly understood. These processes involve both mass wasting (i.e., landslides, rock falls, and debris flows) and fluvial (i.e., bedload and wood transport) mechanisms that interact with a highly variable, flashy, and episodic flow regime.

Drawing on examples from the Oregon Cascades and Coast Ranges, we examine the degree to which headwater streams are “organized” in the sense of semi-regularly spaced bedforms, bedrock and channel features, and explore the temporal and spatial dimensions of geomorphic and hydrologic processes that structure channel beds. We distinguish three types of headwater channels common to mountainous regions, based on the degree to which hillslope, debris-flow, or fluvial processes dominate morphogenetic events, and examine the magnitude and frequency relations of processes and resulting morphologies in each channel type. In particular we examine mechanisms generating step-pool features, distinguishing between those forced by introduced roughness elements (e.g., large boulders, wood) from those involving adjustments between sediment transport and flow hydraulics. This approach yields a semi-quantitative framework for predicting the degree of geomorphic organization of headwater channels, based on size distributions of sediment and wood in relation to channel dimensions, flow regime, and time since last resetting event. Such a framework may provide a useful means of characterizing the ecological template and disturbance regime of headwater channels.