Ecohydraulics of a Disappearing Stream: Flow-Tree-Sediment Interactions in Karst

Sinking Creek in the Appalachian Mountains of Virginia disappears into a subsurface cave network near its downstream end. For roughly 7 km downstream of this location (the "sink"), it only flows intermittently throughout the year, exposing a dry creek bed. Sinking Creek is a gravel-bedded stream with a channel width of about 15 m and draining a watershed area of roughly 200 km². There are trees (mostly Sycamore; Platanus occidentalis) growing in the channel: either near the toe of the bank, in the thalweg, or on presumably self-formed bars. We hypothesize that there have been low-flow conditions in the past conducive to tree-seedling establishment. During high flows, the entirety of Sinking Creek is flowing and young trees could easily be ripped out. Once in-channel trees establish, it appears that they form sediment bars in their wake, which has provided suitable conditions for additional trees to establish. Currently, the mix of in-channel trees and sediment bars has created a channel morphology distinct from perennial streams of the eastern U.S. We are currently testing our hypothesis on the processes forming this channel morphology through field measurements and data analysis of streamflow, stage, precipitation, topography, tree cores, and sediment grain size distributions. We have found that the oldest trees in the channel are at the upstream end of sediment bars, providing support for our working hypothesis. This work highlights the conditions for tree establishment in streams, which is an important threshold not only for this system, but other streams under projected future drying.