Search

Floodplain characteristics contribute to the natural regeneration of vegetation and help facilitate their ecosystem recovery but vary within and between river systems and more so in a river system recovering from anthropogenic disturbances. Here, we explored the floodplain characteristics that contribute to the woody vegetation regeneration on point bars of the middle reach of the Apalachicola River in Florida, USA, which has been passively recovering for over two decades after dredging and disposal activities ended. Woody vegetation regeneration on point bars was analysed using 1999 and 2019 aerial imagery and divided into zones and sections. Data were extracted from a digitized geodatabase regarding several floodplain characteristics with potential to influence vegetation regenetation. Later, we analysed noncollinear floodplain characteristics using a random forest (RF) regression model. The top characteristics that contributed to the RF model include vegetation density, distance‐to‐seeding source and bend curvature. Findings suggest that high vegetation density contributes substantially to woody vegetation regeneration. The negative relationship between distance‐to‐seeding source and vegetation regeneration shows the importance of seeding availability. The bend curvature corresponds with the highest regeneration in the lowermost section of point bars. Our results signify the importance of maintaining healthy vegetated corridors as a buffer to reinforce the vegetation community and potential seed source for passive recovery. These findings regarding floodplain characteristics can be applied to future active restoration of anthropogenically disturbed floodplains. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The natural flood pulse maintains river-floodplain ecosystems through the exchange of freshwater resources between the main-stem and floodplain habitats. Few prior studies have quantified the relationship between flows and floodplain response including estimating inundation area, floodwater volumes, and slough connectivity. Floodplain modeling typically uses the flow-stage height relationship at river gauge stations. In this study, we compared a relative elevation model (REM) and the Hydrologic Engineering Center River Analysis System (HEC-RAS 1D) model using events from 2015 and 2016 covering a range of flows from the 1st to the 99th percentile for the Apalachicola River, Florida. Because digital elevation models (DEM) from LiDAR (light detection and ranging) data lack details of riverbed topography, we compared a LiDAR-alone and LiDAR-sonar combined DEM to assess their differences. Estimates from the REM and HEC-RAS models were compared to maps based on Landsat imagery-derived water and vegetation indices. In this river, we found a non-linear relationship between the inundated area and flow, increasing markedly through the 90th flow percentile after which increases are minimal. Inundated areas from both REM and HEC-RAS models were similar for all selected flow levels except at the 74th percentile (708 m3/s) flow at which the REM produced 11% higher inundated area than HEC-RAS. Near the median flows, major sloughs were fully connected with backswamps and low-lying patches being inundated. At the higher flows, only a few anthropogenic features were exposed. Floodplain inundation estimates from Landsat performed poorly, detecting 9% with the modified normalized difference water index (mNDWI) and 41% with the open water likelihood index (OWL). These estimates were much lower than the HEC-RAS model (96% flooded), largely because the satellite is unable to penetrate dense forests and examine the floodplain surface, and the Landsat pixel size is twice the width of floodplain sloughs. The LiDAR-sonar combined DEM produced a higher floodwater volume estimate with the HEC-RAS model than using LiDAR-alone. The difference of 1,368,000 m3 at the 1st percentile (142 m3/s) and 2,825,000 m3 at the 89th percentile (1133 m3/s) demonstrate the limitation of using a LiDAR-alone DEM, which cannot penetrate the water surface, and the importance of surveying floodplains using sonar. Modeling results under predict historical wetting of the floodplain because Corps dredging made the main channel approximately 13% wider from its historical width in 1941. Further, in the past few decades, droughts and low flows have become more common because of varied upstream water uses, resulting in less inundation than in the past. Frequent high flows are required to maintain river-floodplain connectivity, floodplain forests, and other hydroecological functions in the Apalachicola River floodplain. Our findings present a basis to assess the legacy of past and ongoing disturbances, inform potential policy decisions for water and floodplain management, and provide a baseline for further research. • Increasing flow levels produced a non-linear relationship with inundation area. • Moderate flows maintained slough connectivity enabling mainstem-floodplain exchanges. • Landsat-derived indices underestimated inundation compared with REM and HEC-RAS. • LiDAR-sonar combined DEMs better quantify floodplain storage than LiDAR alone. • REM models provide sound initial results that help assess HEC-RAS performance. [ABSTRACT FROM AUTHOR]