Spatial Patterns of Disconnectivity Explain Catchment-Scale Sediment Dynamics and Transfer Efficiencies

While connectivity studies are becoming common in the Earth sciences, disconnectivity has received much less attention. Sediment storage is the direct result of sediment disconnectivity and can provide concrete evidence of the spatial patterns of disconnectivity at the catchment-scale. In this study, we explore the catchment-scale sediment dynamics of the Tahoma Creek watershed, a high-gradient glacio-volcanic landscape, within a sediment budget framework and identify and map sources of disconnectivity to determine whether they explain the spatial patterns and estimated efficiencies of sediment transfers. We found that up to 80% of the total eroded sediment is sourced from the proglacial zone. The proglacial zone is characterized by high connectivity resulting from frequent debris flows and floods, and rapidly responds to changing conditions. Down valley, however, sources of disconnectivity become increasingly more prevalent, the hillslopes become decoupled from the channel, and a majority of the eroded sediment is redeposited with as little as similar to 15% reaching the outlet. The spatial distribution of sources of disconnectivity and their upslope affected areas explains, to a large degree, catchment-scale sediment dynamics and sediment transfer efficiencies and is in close agreement with quantitative connectivity estimates. We find that steep, glaciated watersheds are predominantly disconnected over human timescales and suggest that disconnectivity is the dominant state of landscapes over most timescales of interest. Mapping sources of disconnectivity provides a straightforward and concrete approach to estimating system disconnectivity and can increase confidence when paired with quantitative indices. Plain Language Summary Mountain watersheds supply freshwater and sediment to downstream river systems affecting flooding, fish habitat, and water quality. We argue that understanding the spatial and temporal patterns of sediment movement within a landscape requires knowing where and for how long sediment is stored and the overall efficiency of sediment transfer between different parts of the landscape. We illustrate the importance and utility of mapping landforms and landscape characteristics that delay or disrupt sediment movement using the Tahoma Creek watershed as an example. This watershed drains a portion of Mount Rainier, a volcano shaped by glaciers in Washington, USA, and is incredibly dynamic. We find that landforms and landscape characteristics that limit sediment movement are common and that mapping them gives an accurate picture of sediment movement patterns. Glaciers dramatically reshape the landscape, which controls sediment supply and movement patterns, and their legacy remains for thousands of years after they retreat. We also make use of high-resolution elevation data representing the land surface in 2002, 2008, and 2012 to quantify how much erosion, transport, and deposition occurred and how efficiently sediment was transported. We find that sediment is transported more efficiently during extreme events, such as the 2006 floods.