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Exploring the importance of sediment disconnectivity and connectivity in glacierized catchments, Tahoma Creek, WA Turley, Michael

Abstract

In recent decades, connectivity has emerged as a prominent topic of discussion within the geomorphology community but lacks consensus around a general definition. On the other hand, disconnectivity is often an afterthought even though it is prevalent at most spatiotemporal scales. In response, we suggest defining disconnectivity as the dominant but inefficient state of the system in transferring matter and energy within and between system components at all spatial and temporal scales. Connectivity is then a special case within disconnectivity in which the efficient transfer of matter and energy occurs within the spatiotemporal scale of interest. In this study, we explore whether disconnectivity controls the spatial patterns of sediment dynamics, and how well current methods of quantifying connectivity captures these spatial patterns. We conducted a case study within the Tahoma Creek Watershed of Mount Rainier National Park, WA, in which we present fieldwork and historical data in the form of a geomorphic map and conceptual sediment budget and map all sources of disconnectivity. These analyses are compared to methods of measuring the influence of landscape history and hillslope-channel coupling, followed by several semi-quantitative connectivity indices. Slope-Area plots clearly show the topographic signature of Pleistocene glaciations within the confines of relict cirques, while the mainstem channel appears fully adjusted to contemporary fluvial processes. Hillslope-channel coupling estimates based on the method proposed by Whiting and Bradley (1993) generally match fieldwork evidence, where the uppermost 6 km of the channel are coupled to the hillslopes, and the lowermost 7 km are decoupled. We found that the spatial distribution of sources of disconnectivity and their upslope affected areas explains the spatial patterns of sediment transfers and assumed transfer efficiencies within the watershed. Even locations with intense morphodynamics, such as Mount Rainier, are predominantly disconnected over human-timescales. The methods of quantifying sediment connectivity all performed rather well within their stated limitations and inherent resolution, although discrepancies exist. The primary sources of error result from inaccurately modelling runoff pathways and overlooking the effects of vegetation. We suggest explicitly integrating sources of disconnectivity within disconnectivity indices for improved performance and physical grounding. Supplementary materials available at: http://hdl.handle.net/2429/77301

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