The Influence of Knickpoint Development and Channel Incision on Riparian Vegetation in Semi-Arid River Corridors

Hydrogeomorphic processes strongly influence riparian vegetation, but few studies have determined the influence of knickpoint development on these processes. We investigate the influence of knickpoint development and channel incision on flood inundation, channel morphology, and riparian vegetation on the Fremont River in Utah. We conducted topographic surveys (n = 30 transects) and plot-based vegetation surveys (n = 336) in three 1 km-long reaches with varying proximities to a human-created knickpoint. We also developed 1D hydraulic models to assess flood inundation patterns. Our data indicate that knickpoint development affects riparian vegetation composition and abundance through its influence on geomorphic and hydrologic processes. The study reach located just upstream of the knickpoint (R2) had a deeper channel, flooded less frequently, and required higher flows to inundate the floodplain compared to the study reach located 5 km upstream of the knickpoint (R1). Overall mean vegetation abundance (percent cover) was higher in R1 (55.7%) than in R2 (30.1%), and R1 had more occurrences of wet-adapted (facultative wetland and obligate) species than R2 (198 and 79, respectively). Vegetation in the human-created abandoned meander reach near the knickpoint (R4) was predominantly dry-adapted (upland) species. Cross-reach comparisons and more than 3 m of incision in the knickzone since knickpoint creation indicate that knickpoint development has resulted in channel incision in R2, shifting its vegetation toward more dry-adapted species reminiscent of the surrounding uplands. This work informs natural resource management practices for habitat in riparian ecosystems and can be applied to rivers in drier regions.