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Large, low-lying rivers have a variety of societal uses and show a wide range of disturbances. One such example, the Apalachicola River in the Florida Panhandle, experienced anthropogenic activities that altered its hydrologic, geomorphic, and ecologic condition. The river's sand bars enlarged more than threefold in area because of dredge disposal and localized incision from anthropogenic activities during a navigation project. Once dredging ended, the river experienced passive recovery through the slow regrowth of riparian vegetation, with sand bars reducing in area ~18% from 2005 to 2015. The outcomes of tree-line change analyses from 1999 to 2019 indicate a higher recovery rate in the first decade (1999-2010), migrating fastest in locations where vegetation islands stabilize sediments. To help accelerate vegetation regrowth, during the growing seasons of 2018 and 2019, we planted willow stakes at various positions and microhabitats covering ~3.6 ha of three enlarged sand bars near Blountstown, FL. Our mapping shows some synergies between passive recovery and active restoration as tree-line expansion had the fastest rates between 2019 and 2021. Our study gives insights and guidance into passive recovery and restoration projects involving plantings on large, low-lying rivers. [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]

As escalating environmental pressures threaten the world's river systems, understanding the driving factors influencing their geomorphological changes is of critical global importance. This study illuminates the complex interaction between natural and anthropogenic factors that drove geomorphological changes along the upper Apalachicola River (UAR) from 1960 to 2010. This study utilized LiDAR point cloud data and hydrographic surveys conducted between 1960 and 2010 to identify the primary factors driving riverbed changes. A triangular irregular network (TIN) was used to convert the combined datasets from each survey, following coordinate transformation, into a 1.5 m-resolution digital elevation model (DEM) raster. The DEM of difference (DEMoD) was computed by subtracting the 1960 DEM from that of 2010, and the total sediment loss and gain were then determined by multiplying the DEMoD by the DEM resolution. A random forest (RF) regression model was used to determine the most influential factors contributing to riverbed deformation. The results revealed substantial differences in sediment loss and gain across the river. The UAR, divided into three segments, experienced sediment loss of about −6.56 × 106 m3 between 1960 and 2010, with the farthest upstream section (RM 106 to 98) losing −3.14 × 106 m3, which is ~48 % of the sediment losses in the study area. Some sections of the channel have deepened significantly, with a pronounced depth difference of −17.20 m between RM 92 and 94, potentially caused by the presence of limestone and the enlargement of a sinkhole or spring on the riverbed. The RF model identified the dam (>28 %) as the most crucial anthropogenic factor in riverbed deformation, followed by the presence of dikes (~17—18 %), then historical dredging (~7—8 %), highlighting the substantial impact of artificial structures on riverbed deformation. In contrast, rock removal, while potentially impactful in certain locations, had negligible effects (>0.5 %) at the system scale. This study highlights the interaction of natural and anthropogenic factors in river geomorphology and the potential for machine learning to identify these dynamics and inform data-driven river management and conservation strategies. • Novel examination of factors behind riverbed deformation using machine learning technique • The study reveals that the dam is the prime driving factor in riverbed deformation. • Greater impact of wing-dikes vs. dredging due to cascading effects on erosion and deposition • Rock removal found to have negligible effects at the system scale. • Anomalous elevation change suggests sinkhole or spring expansion on the riverbed. [ABSTRACT FROM AUTHOR]

The Apalachicola River, which begins at the confluence of the Chattahoochee and Flint rivers near the Georgia-Florida State line, has multiple human impacts. Water inputs declined due to upstream irrigation and urbanization in Georgia. Sediment trapped by numerous small to large dams, including construction of Jim Woodruff Dam in 1954 near the Apalachicola-Chattahoochee-Flint (ACF) confluence has increased degradation. Shortly thereafter, the river was modified for a navigation project, with 29.6 × 10 6 m 3 dredged between 1957 and 2002 from the Apalachicola alone. This study investigates how historic dredging coincides with the modern morphology of the channel and how historic dredging, disposal, and other activities have modified the floodplain landforms and sediments. This analysis of the navigation impacts in the middle Apalachicola River (River Miles 40 to 65) ties spatial and temporal variations of dredging, field-derived bathymetry, historic maps, patterns of floodplain disposal of dredge spoil from LiDAR imagery, and modern point bar channel change of the Apalachicola River. Floodplain mounds of coarse material, built from out-of-bank disposal constitute > 800,000 m 3 in the study area. Approximately 7.7 × 10 6 m 3 of sediment was dredged within the study reach, roughly 11% of the volume dredged remains on the floodplain. Sand bars were disposal sites thus their increased area of 263% is partly tied to this practice. Thus, the legacy of dredging affects the modern sedimentology and morphology of the floodplain and channel. Findings show that a failed navigation project could have been pre-empted with better geomorphic, geologic and hydrologic study and suggest that vegetative restoration of point bars would help in narrowing and stabilizing this dynamic system. [ABSTRACT FROM AUTHOR]

The Atchafalaya River in south-central Louisiana is of geomorphic interest because of the large volumes of flow and sediment transported and its role as a major floodway for the lower Mississippi River, notwithstanding its potential to become an avulsion pathway at Old River. Since its origin in the sixteenth century, it has undergone many transformations and currently receives flow from the Red River and partial flow of the Mississippi River through multiple pathways. Flow increased progressively beginning in the mid-1800s and continued until concerns of diversion spurred placement of engineering structures to regulate flows at the juncture between the Mississippi and Atchafalaya rivers in 1963. Prior studies, that looked at reaches or six to seven rangelines or cross sections on hydrographic surveys document areal, width, and depth increases in the uppermost 80 km (50 mi) of the Atchafalaya River from 1880 to 1974. This study assesses how channel geometry of the Atchafalaya River has changed with flow variations since the 1880s by evaluating instantaneous discharge measurements that document change at superior temporal resolution and differences in thalweg values from coupled modern hydrographic surveys which document change at better spatial resolution than prior works. While structures have regulated and limited flow into the Atchafalaya since 1963, the highest values of discharge, cross-sectional area, and width at Simmesport (upstream) follow river regulation. Associated geometry data at Simmesport and Morgan City (downstream) document that mean bed and thalweg elevations have risen. Comparison of thalweg elevations on hydrographic surveys from 1963 and 2006 show pronounced local variability in bed elevation change but also reaches dominated by degradation (including the Red River and uppermost section of the Atchafalaya) near the Old River diversion. Aggradation occurs in the lower Atchafalaya, a zone where along-channel lakes are filling, flow and sediment quantities are changing in part caused by the constructed Wax Lake Outlet, and deltas have been growing in Atchafalaya Bay. Findings are relevant to river engineering and flood management, especially navigation, and help in better understanding the evolution of large distributaries and the role of human impacts. [ABSTRACT FROM AUTHOR]

Old River is one of the most important river junctures in North America. It connects three large, navigable rivers the Mississippi, Atchafalaya and Red which support economies and environments critical to United States of America. Starting in the 1800s, the Atchafalaya River began receiving more flow through this juncture, and by the mid-1900s, an avulsion was likely so the Old River Control Project was authorized to prevent diversion of the Mississippi River to the Atchafalaya. This paper examines the historical changes of Lower Old River, the main juncture prior to 1963 before the flow became controlled by structures, including its flow changes, human modifications, and whether and how it was changing as the Atchafalaya received more flow. In contrast to some reports that suggest that bidirectional flow in Old River was just a few years in duration, occasional flow from the Red River is documented: starting in the 1880s and persisting for several decades through 1945. This paper identifies three transitions in the transformation from bidirectional to unidirectional flow, the first of unknown cause in the late 1910s, the second likely instigated by the flood of record in 1927, and the third immediately following a local artificial cut-off in 1944-1945. Also, changes in flow and geometry in Lower Old River are related to changes in the adjacent Mississippi and Atchafalaya Rivers. [ABSTRACT FROM AUTHOR]

Change in fluvial systems has interested geomorphologists for several decades, but awareness of change dates back much further to the ancient Greeks. This paper introduces seven articles that focus on aspects of temporal and/or spatial change in rivers in the central and southeastern United States of America. Timescales examined span from the late Pleistocene through historical and into modem times. Spatial scales investigated range from small watersheds to the juncture between two of the larger rivers in North America. Natural drivers of change discussed include sea-level rise which followed the last glacial maxima and influenced drainage connections, avulsions, and lateral migration which influenced channel position and caused stream capture; and transitions across geomorphic provinces with differing bedrock and structure. Human-induced drivers of change include land use alterations such as settlement and logging which influenced sediment input and channel form, artificial cut-offs, removal of log rafts, dredging, and artificial channels and floodways which changed flow, sedimentation, and geometry, and the introduction of contaminants which influenced sediment quality. Collectively, these papers fill voids in the literature on topics and places where there has been little prior investigation. [ABSTRACT FROM AUTHOR]

This study uses oxbow lakes to ascertain the extent and nature of change in a river's form, focusing on the effects of post-1950s floodplain gravel extraction on the geomorphology of the Leaf River in southeastern Mississippi, USA. The river and oxbow lakes of a 5 km reach were examined for the effects of local and upstream floodplain and in-channel gravel mining. Data from 12 cross-sections suggests that the present-day river is 35% wider and 12% shallower than the cross-sectional geometry in cutoffs that pre-date 1955. Field measurements reveal that the Leaf River was relatively stable from at least 1911 to the early 1940s, but has since undergone an average of 2.65 m of degradation in the study reach. Survey data from a nearby USGS stream gage show that degradation began abruptly in the mid 1970s, most likely as a result of in-stream mining and pit avulsions on a major tributary that deprived downstream areas of bed load and created profile adjustments. [ABSTRACT FROM AUTHOR]

A number of large rivers across the world are dredged for navigation. Dredging removes sediment from the channel, creates spoil mounds in the adjoining floodplain, and disposes sediment along the channel margin on a bar, or in open water. Retrospective studies that examine the placement and fate of this disposed material can provide guidance for other localities where dredging is practiced, and better elucidate the impacts associated with disposal at different types of sites. Many studies exist for impacts and benefits of dredging in the coastal environment, particularly to ecosystems and water quality. Few studies have examined disposal site selection and the fate of dredge spoil placed along a river corridor from a geomorphic and retrospective perspective. The Apalachicola River in Florida was dredged from the 1950s for several decades, with disposal on sand bars, in open water on the channel margins, and on the floodplain. Numerous large and small floodplain dredge-spoil mounds are a legacy from this navigation project. One of the larger mounds, disposal site 40 (DS 40), which locals call "Sand Mountain," stands ~22 m high and extends ~10 ha in area. This disposal site originated in an artificial cutoff created in 1968–69 and grew to hold >625,000 m3 of sediment. Because this section of river was a problem for navigation when dredging was being conducted, detailed historical survey sheets with topographic and bathymetric data exist. The purpose of this paper was to analyze the local floodplain and channel geospatially, coupled with dredging and disposal data, to interpret the geomorphic changes of DS 40 and the adjoining river. Poor placement has resulted in this and other dredge spoil mounds returning sediment to the river through lateral erosion and mass wasting. Intermittent sediment pulses have altered channel morphology adjacent to the spoil mound with the channel becoming wider and shallower than originally, mean bed and thalweg elevations increasing by ~1.5 to 2 m, and width-depth ratio increasing three-fold. Although dredging of this river has not been conducted for nearly two decades, study of historical dredge spoil mounds in this reach suggest poor placement of spoil can continue to stress and alter a river in the decades that follow. Based on our findings, dredging downstream of distributaries will have poor outcomes. Dredge spoil placement in cut-off channels close to the river can be problematic because the river will regain sinuosity. Further, sites on the outer bend of meanders are more subject to erosion, and sites immediately upstream of problem or shallow areas will feed sediment to the problem reach. Findings of this study have implications to other settings, such as intensively mined rivers where unused unconsolidated material is often left on floodplains. • Dredging downstream of distributaries is difficult due to riverbed elevation increases. • Artificial cutoffs regain sinuosity making nearby disposal sites susceptible to erosion. • Outer banks are inferior disposal sites compared to inner bends of meanders. • Sediment from eroding disposal sites affects many channel-form measures for decades. • Placement of the dredge material upstream of shallow areas exacerbates navigation. [ABSTRACT FROM AUTHOR]