Revisiting Sediment Disconnectivity

Abstract:

In recent decades, the geomorphology community has become captivated by the concept of connectivity but lacks consensus around a general definition. Disconnectivity is often an afterthought even though it likely dominates at most spatiotemporal scales. In response, I suggest defining disconnectivity as the dominant but inefficient state of a 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. Disconnectivity is then placed within an open systems framework. The definitions and framework are illustrated, taking examples from the Tahoma Creek watershed in Washington and discussing the prevalence of disconnectivity over the last 100 years despite it being considered a dynamic landscape. This study presents a geomorphic map created in the field, sources and patterns of disconnectivity, and connectivity within a sediment budget framework. Bio-geomorphic characteristics and landforms that cause disconnectivity are abundant and widespread, resulting in a highly fragmented network. Buffers significantly reduce connectivity between the colluvial and fluvial process domains. The upper 6 kilometers of channel are coupled to the hillslopes, while the lower 8 kilometers are predominantly decoupled. As a result, only approximately 15% of the watershed contributes sediment to the outlet. Debris flows provide an important link between glaciofluvial sediment in the upper watershed and the fluvial network, while most of the other upland sediment reservoirs and processes are disconnected. Fluvial processes moderate the total coarse sediment export from the system. Barriers and blankets are common within the fluvial process domain and result in the temporary storage of over half of the total mobilized sediment. Sources of disconnectivity are relatively easy to map in the field and explain the spatial patterns of sediment dynamics. I suggest greater emphasis be placed on disconnectivity within the literature to help explain the spatial variability of sediment transfer efficiencies.