Channel and vegetation recovery from dredging of a large river in the Gulf coastal plain, USA

Abstract

Anthropogenic impacts in large rivers are widely studied, but studies of recovery once a disturbance has stopped are uncommon. This study examines the biogeomorphic recovery of a 40‐km river corridor on the mid‐Apalachicola River, Florida following the cessation of dredging, disposal, and snag removal in 2002. This failed navigation project resulted in vegetation losses (~166 ha between 1941 and 2004), river widening, and increased point bar areas. We used paired sets of imagery for a 10‐year period during the recovery process at two different flow levels to assess sand bar change, land cover change, and their spatial variations. Most large sand bars decreased significantly in area due to growth of pioneer species, typically from the bankside of the bar. Mean bar area shrank 0.17 and 0.20 ha for the 30th and 1st percentile flows, respectively. For the entire study area, both water‐level comparisons showed gains in vegetation (23.36 and 15.83 ha), compensated by losses in the extent of water (16.83 and 8.55 ha) and sand bar losses (6.53 and 7.28 ha). Overall, these gains during the 10‐year passive recovery period are equivalent to ~15% of the vegetation losses that resulted from the navigational dredging. As found in other studies, most of the pioneer vegetation grew approximately 2 m relative elevation above the low‐water surface. The initial length of the tree line and the area of herbaceous growth both had a significant and positive relationship with the area of new vegetation growth over the study interval. As parts of the river are healing, reduced channel capacity from narrowing and tree growth will benefit the floodplain. As elsewhere, understanding of a river's biogeomorphology, hydrology, and disturbance history can help in selecting appropriate recovery metrics to further advance the understanding and management of disturbed floodplains. © 2020 John Wiley & Sons, Ltd.

Geomorphic response to historic and ongoing human impacts in a large lowland river

“…River bar area reflects hydrologic changes (Hazel et al, 2010;Wiele et al, 1996), a legacy of landscape disturbance, or historical/modern human activities (Jacobson & Gran, 1999;Martin & Pavlowsky, 2011;Mossa & Marks, 2011). Point (Mossa et al, 2020). Examining the entire river for the same period, we found that the sand bar area decreased 18% (Figure 13).…”

“…Therefore, we selected a pair of aerial photos taken at low to medium water level from 2005 (320 m 3 /s) and 2015 (284 m 3 /s), which was at $30th flow percentile. We adopted the sand bar data fromMossa et al (2020) in RM 40 to 64 reach, and digitized sand bars along the rest of the river at a 1:2500 mapping scale for standardized results. We labelled each point bar according to the closest river mile at the middle of the sand bar and paired bars from both years to compare the changes over time after dredging ended.4 | RESULTS4.1 | Thalweg elevation changeSubtracting the 1960 thalweg elevation from the 2010 thalweg elevation, most of the changes are less than a few metres different but some areas show aggradation of 6 m and others show degradation of nearly 10 m. The moving average of 1 river mile of measurements shows that the upper $20 RM, or 30 km, is typically 1-2 m below zero, indicating degradation, and another pronounced dip of similar magnitude below zero is near RM 27 to 33 (Figure9a).…”

Biogeomorphology, quo vadis? On processes, time, and space in biogeomorphology

“…In this sense, Mossa et al . (2020) point out that the anthropogenic impact usually covers the entire length of river corridors and, as shown by Corenblit et al . (2020), may shift biogeomorphic system state.…”

“…high-mountain and polar environments, deserts, hillslopes, rivers and wetlands, salt marshes and deltas). In this sense, Mossa et al (2020) point out that the anthropogenic impact usually covers the entire length of river corridors and, as shown by Corenblit et al (2020), may shift biogeomorphic system state. Hence, when it comes to NbS in river management, it is essential to understand how human activities may dominate, and may even be considered as a key zoogeomorphic agent, placed within the biogeomorphic systems of the Anthropocene.…”

“…At the small scale, work is done in understanding the role of vegetation, and particularly animals, on hydrodynamics and sediment dynamics (DeAnna and Wohl, 2019;Dong et al, 2019;Grenfell et al, 2019;Gurnell et al, 2019;Mason et al, 2019;Rice et al, 2019;Jerin and Phillips, 2020). At larger spatial and temporal scales, GIS and remote sensing are increasingly used to monitor and evaluate the evolution of fluvial riparian environments (Gurnell et al, 2019;Kleinhans et al, 2019;Corenblit et al, 2020;Mossa et al, 2020). A particular focus is set on river bars and how their dynamics are affected by plant and ecological traits.…”