Widespread wood placement and regrading drive lateral connectivity and reworking of the channel and floodplain in a valley bottom reset to Stage 0

Abstract

Valley bottom process reset, or the excavation of high surfaces and fill of incised channels combined with large wood addition, is a new method for creating multi-channel river-wetland corridors (also referred to as Stage 0 valley bottoms). Valley bottom process reset seeks to increase lateral flow and sediment connectivity, retain water and sediment, and kickstart geomorphic processes that may sustain aquatic and riparian habitat. This study uses this anthropogenic intervention to examine relationships among wood-induced hydraulic roughness, valley bottom topography, and geomorphic processes such as overbank flow and sediment transport, avulsion, sediment retention, and pool scour. Here, I present a 6-year case study of a two-phase valley bottom process reset along Deer Creek, Oregon. Kickstarting the floodplain reshaping processes required a substantial increase in roughness and hydrologic connectivity. The first phase of construction enhanced hydrologic but not sediment connectivity, failing to kickstart avulsion and floodplain reworking even during a likely 2- to 5-year recurrence interval flood. A second, more intensive phase of construction substantially reduced the threshold discharge necessary for overbank flow and floodplain reworking, as evidenced by the occurrence of these processes after only a <2-year recurrence interval flood. During this flood, a spatially distributed wood lattice rearranged into discrete jams that scoured pools, retained sediment, and drove geomorphically effective overbank flows. While valley bottom regrading likely contributed to these geomorphic effects, the spatial correlation between newly incised floodplain channels and areas of wood aggregation indicates wood's important role in driving floodplain reworking. I derive from these findings a conceptual model that details how wood drove floodplain reworking via two mechanisms: In-channel wood jams backwatered and constricted flow into the floodplain, and small wood jams formed in the floodplain forest further constricted flow through nascent channels to facilitate channel incision.