Channel Classification, Prediction of Channel Response, and Assessment of Channel Condition

A process-based landscape and channel classification is proposed as a framework for assessing watershed response to natural and anthropogenic environmental change. Our proposed classification is based on a hierarchy of process-regimes at several spatial scales: i) geomorphic province, ii) watershed, iii) valley segment, iv) channel reach, and v) channel unit. The geomorphic province level identifies watersheds developed in similar materials, topography, and climates, reflecting comparable hydrologic, erosional, and tectonic processes. The watershed level distinguishes hillslopes from valleys, defining fundamental differences in transport processes within a contiguous drainage basin. Valley segment morphologies further distinguish transport processes and general relations between transport capacity and sediment supply of both channeled and unchanneled valleys. At the reach level, distinct morphologies may be identified based on sediment transport characteristics, channel roughness configuration. Channel reaches, in turn, are composed of finer-scale channel units. Within this framework, our discussion focuses mainly on the valley segment and channel reach levels. Valley morphology and sediment nansport characteristics define colluvial, alluvial, and bedrock valley segments. Unchanneled valleys (hollows) are characterized by a lack of fluvial processes, resulting in a transport-limited accumulation of colluvium that is periodically excavated by mass wasting processes. Channeled colluvial valleys am those in which fluvial sediment transport maintains a channel, but in which the transport capacity is insufficient to mobilize all of the colluvium delivered from the surrounding hillslopes. In mountain drainage basins, colluvial valleys am dominantly carved by mass wasting processes. Alluvial valleys contain predominantly alluvial fills and are characterized by fluvial transport of sediment over a variety of alluvial bed morphologies. Alluvial valley segments may be either confined or unconfined, reflecting general relations between transport capacity and sediment supply. Bedrock valley segments lack a continuous alluvial cover due to high nansport capacities. Valley morphology generally reflects the relation between sediment supply and transport capacity. At the channel reach-level of the classification, bed morphology is coupled with both the potential for debris flow impacts and the role of large woody debris loading to characterize channel processes and provide a framework within which to examine potential channel response. Colluvial and bedrock channels occupy cormsponding valley segments, but we recognize six alluvial channel types: regime, braided, pool-riffle, plane bed, steppool, and cascade. We hypothesize that observed systematic and local downstream changes in alluvial channel morphology and channel roughness correlate witb changes in channel slope, sediment supply (size and amount of material available for transport) and transport capacity (a function of the available shear stress). These differences provide the basis for interpreting the potential response of different areas of the channel network to perturbation. In general, steep alluvial channels (step-pool and cascade) tend to maintain their morphology while transmitting increased sediment loads. In contrast, low-gradient channels (regime and pool-riffle) typically respond to increased sediient loads through morphologic adjusmrent. In essence, steep channels effectively act as sediment delivery conduits connecting zones of sediment production on hillslopes to downslope low-gradient channels. Such distinctions allow recognition of source, transport, and response reaches. Channel morphology thus reflects the local and watershed-integrated processes influencing sediment supply and transport capacity. Evaluation of channel response potential within the context of morphologically-characteristic processes allows distinction of different response potential for different portions of a channel network. While the proposed channel classification provides insight into potential channel response that can guide impact assessment, changes in sediment supply and transport capacity may result in either similar or opposing effects. This highlights the reality that changes in discharge and sediment supply cannot be examined in isolation; both need to be considered when assessing either watershed conditions or the potential for furture impacts. In particular, it is necessary to focus on aspects of channel morphology and dynamics that am sensitive indicators of perturbation and to consider the specific channel type and position in the channel network. A number of quantitative and qualitative approaches provide insight into evaluating watershed impacts and predicting potential responses to continuing or anticipated watershed disturbance.