River channel response to invasive plant treatment across the American Southwest

Abstract

Invasive riparian plants were introduced to the American Southwest in the early 19th century and contributed to regional trends of decreasing river channel width and migration rate in the 20th century. More recently, efforts to remove invasive riparian vegetation (IRV) have been widespread, especially since 1990. To what extent has IRV treatment reversed the earlier trend of channel narrowing and reduced dynamism? In this study, paired treated and untreated reaches at 15 sites along 13 rivers were compared before and after IRV treatment using repeat aerial imagery to assess long‐term (~10‐year) channel change due to treatment on a regional scale across the Southwest USA. We found that IRV treatment significantly increased channel width and floodplain destruction. Treated reaches had higher floodplain destruction than untreated reaches at 14 of 15 sites, and IRV treatment increased the rate of floodplain destruction by a median factor of 1.9. The effect of treatment increased with the stream power of the largest flow over the study period. Resolving observations of channel change into separate measures of floodplain destruction and formation provided more information on underlying processes than simple measurements of channel width and centerline migration rate. Restoration practitioners who perform IRV treatment projects often focus on wildlife or vegetation response; however, geomorphic processes should be considered in restoration planning because they drive aquatic habitat and vegetation dynamics, and because of the potential for damage to downstream infrastructure. Depending on the restoration goal, management practices can be used to enhance or minimize the increase in channel dynamism caused by IRV treatment.

Experimental Observations of Floodplain Vegetation, Bedforms, and Sediment Transport Interactions in a Meandering Channel

“…For example, Wieting et al. (2023) found that channel width increased significantly with the removal of floodplain vegetation.…”

“…Bar-pool morphology likely would show a different response had the banks been highly erodible, and the channel width may have been influenced by the presence and density of floodplain vegetation. For example, Wieting et al (2023) found that channel width increased significantly with the removal of floodplain vegetation.…”

“…Conversely, Wieting et al. (2023) found that channel width increased as invasive floodplain vegetation was removed in sand‐bedded rivers. Bywater‐Reyes et al.…”

“…For instance, Török and Parker (2022) found that in channels with fine particles (D 50 < 2 mm), increases in floodplain vegetation were positively correlated with channel width, but they found the reverse for coarse-bedded channels. Conversely, Wieting et al (2023) found that channel width increased as invasive floodplain vegetation was removed in sand-bedded rivers. Bywater-Reyes et al (2017) measured the geomorphic effect of in-channel and floodplain vegetation and concluded that riparian vegetation and bedform topography may co-evolve at a range of scales.…”

Recognizing the ephemeral stream floodplain: Identification and importance of flood zones in drylands

“…Invasive species can thrive at higher densities than native species (e.g., Figure 9), thus increasing floodplain roughness, which can decrease flood power (Birkeland, 2002) on ephemeral stream floodplains. Floodplain stabilization associated with invasive species encroachment can cause the narrowing and incision of ephemeral stream channels (Cadol et al, 2011; Graf, 1978; Wieting et al, 2023), thus altering the sediment regime and ultimately disconnecting channels and floodplains in ephemeral settings. Invasive species can additionally alter canopy dynamics and organic matter inputs, which can impact invertebrates and cascade through food webs (e.g., Going & Dudley, 2008).…”