Biogeomorphic strategies in stream restoration

Abstract:

Starting around 2000, fluvial geomorphology turned from the view of physical processes dominant with vegetation responding, to increasing recognition of vegetation as a driver of channel form and physical processes (Hickin 1984; Corenblit et al 2007). Subsequent research produced more sophisticated models of how vegetative and physical processes combine to create habitat conditions. This research can support strategies and methods for using vegetation as a tool in restoration of riverine aquatic habitat. In this paper I review key biogeomorphic concepts relevant to stream restoration, provide examples of potential applications, and identify knowledge gaps and important questions.

Riparian zone vegetation, active channel vegetation, and aquatic macrophytes all can influence flow hydraulics, erosion and deposition. Several authors (O’Hare et al 2016; Solari et al 2016; Diehl et al 2017) have classified functional effects (roughness, sediment stabilization, flow deflection, mechanical bank strength, etc.) of plant traits (size, stem density, flexibility, root form, resistance to uprooting, etc.) in fluvial systems. Vegetation effects will vary depending on stream type (suspended-, mixed- or bedload-dominated), stream power, and hydrologic regime, so restoration approaches need to be classified by stream characteristics.

Vegetation effects on channel form have been recognized in a wide variety of fluvial system types. For example, in low-energy suspended-load streams that often incise in response to disturbance, sedge and graminoid species may trap sediment (Gurnell et al 2012), build benches and potentially reverse incision. Mixed-load and sand rivers are susceptible to channel instability and widening, and woody shrubs control subsequent narrowing (Schumm Lichty 1963). Island formation in braided rivers is driven by large wood deposits and tree regeneration (Gurnell Petts 2006). Wetland herbaceous vegetation and beaver dams were probably important in maintaining multithread channels in pre-disturbance streams in the eastern U.S. (Walter Merritts 2008). Even in montane gravel-cobble streams, tussock sedges can influence hydraulics and channel form (Goslin 2019). Vegetation may be an effective, cost-efficient tool in process-based restoration, in combination with other approaches.