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

Abstract

Lowland coastal plain rivers are important for navigation and ecosystems, having unique concerns regarding water and sediment management. The Apalachicola River in Florida, the lower end of the ACF (Apalachicola–Chattahoochee–Flint) system, had historical modifications beginning in the 1800s. Most impacts are attributable to the Apalachicola Navigation Project in the latter half of the 20th century, which included dredging, disposal, rock and snag removal, artificial cutoffs, and wing‐dike building. As one of few studies using paired hydrographic surveys over a long reach and time (170 km, 50 years), we defined four zones of bed elevation change between 1960 and 2010 based on the cumulative thalweg elevation plots. The river has four bed change zones: (1) uppermost, marked degradation from river mile (RM) 106 to 85 because of dam construction; (2) middle, minimal degradation from RM 85 to 34; (3) artificial cutoff zone, highest degradation, from RM 34 to 27; and (4) lowermost zone, aggradation from RM 27 to Apalachicola Bay because of upstream bed and bank sediment disturbances. Artificial cutoffs and lithology influence lateral migration, with rates being highest in cutoff zones and least in areas with some bedrock upstream and finer sediments downstream. Length changes from 1941 to 2019 are stable, except for the middle reach, which lengthened nearly 10%, and the lower non‐tidal reach, where length shortened with cutoffs then regained in subsequent decades. Minimizing disturbance has led to 18% of the sand bar area revegetating since a decade of dredging ended. This framework helps in understanding historic and ongoing human activities, notably upstream water consumption, connectivity changes, sediment inputs, and climate change, that will affect the water and sediment management of the river, its floodplain, and bay, thus varying approaches for restoration.

Lowland rivers: Geomorphology, human impacts, and management

“…Additionally, the upstream basin is impounded by numerous dams. Utilizing a range of geomorphic data types, including historic aerial photographs, paired hydrographic surveys, high resolution geospatial data (LiDAR [light detection and ranging]), and field observations, among others, Mossa and Chen (2022) document the temporal and spatial adjustment of the lower Apalachicola River. Upstream reaches that underwent shortening by meander neck cutoffs degraded by some ~2 m, with some deep bedrock‐controlled pools scouring by 7.3 m. And much of this sediment was evidently transported and deposited in the lowermost deposition zone, as subaerial deltaic progradation ensued and created new coastal wetlands.…”

“…Dredging and channel bed aggregate mining is an increasingly important topic with a myriad range of fluvial impacts (i.e., Park et al, 2020). The study by Mossa and Chen (2022) provides a timescale for fluvial adjustment, which is crucial in the design of effective restoration and management, and assessment of further causal impacts being imposed upon the system.…”

“…The 10 special issue contributions herein reviewed concern important fundamental and applied topics within lowland river science, specifically highlighting three key themes. These include ‘natural’ river channel dynamics within settings that vary from the standard body of knowledge (Guha et al, 2022; Rogério et al, 2022; Sinha et al, 2022), geomorphic response to channel engineering (Mossa & Chen, 2022; Marteau et al, 2022; Tang et al, 2022; Wu et al, 2022), and flood basin management in response to environmental change (Keim et al, 2022; Phillips, 2022; Valentine et al, 2022). The categories are not mutually exclusive, and most contributions have linkages across multiple themes.…”

Floodplain characteristics affect woody vegetation regeneration on point bars of a coastal plain river recovering from anthropogenic disturbances

“…Anthropogenic activities, largely dredging and disposal of sediments, have influenced the river‐floodplain systems of the middle Apalachicola River, a low‐relief meandering river in the southeastern US coastal plain. Dredging and disposal, a process that removes or shifts sediments from the river bottom to deepen the river channel for navigation and dispose sediment on point bars, channel margins and the floodplain, have substantially impacted woody cover and point bars of the river floodplain (Light et al, 2006; Mossa et al, 2020; Mossa & Chen, 2022; Price et al, 2006). After dredging ceased in 2002 because of the high maintenance expense coupled with limited use from the barge traffic and the Restoration Act, the river has experienced passive recovery manifested by the shrinking point bars due to the woody vegetation regeneration (Mossa et al, 2020; Mossa & Chen, 2022).…”

“…Dredging and disposal, a process that removes or shifts sediments from the river bottom to deepen the river channel for navigation and dispose sediment on point bars, channel margins and the floodplain, have substantially impacted woody cover and point bars of the river floodplain (Light et al, 2006; Mossa et al, 2020; Mossa & Chen, 2022; Price et al, 2006). After dredging ceased in 2002 because of the high maintenance expense coupled with limited use from the barge traffic and the Restoration Act, the river has experienced passive recovery manifested by the shrinking point bars due to the woody vegetation regeneration (Mossa et al, 2020; Mossa & Chen, 2022). Designing and implementing restoration projects that can accelerate the woody vegetation regeneration and succession process without understanding how much, how quickly and in what ways a river recovers are a challenge.…”