Large Coastal Plain Research Articles

Hydrological drought and floodplain disconnectivity: quantifying flow thresholds in a large coastal plain river

ABSTRACT Hydrological drought and river disconnectivity worsen from dams and human activities. This research investigates how dam constructions exacerbate hydrological drought and river disconnectivity in Apalachicola River system. Data from two hydrological stations (1928-2022), a hydrographic survey, high-resolution LiDAR, and a triangular irregular network (TIN) were used to identify three connectivity flow (141.5 m3/s, 169.8 m3/s, and 226.4 m3/s) categories, each impacting floodplain differently. A relative elevation model (REM) was also developed for floodplain connectivity assessment. Findings show a significant increase in the frequency, duration, and magnitude of connectivity low flow events from pre-dam to post-dam periods, especially at the 169.8 m³/s connectivity flow threshold. A 20th percentile variable threshold shows increasing hydrological drought. Drought increases have led to decreased connectivity between tributaries and the main river system, and an increase in the moderately disconnected floodplain area from 37.1% to 46.0%. Adaptive management strategies are needed to address drought-induced floodplain disconnectivity

Predicting compound coastal inundation in 2100 by considering the joint probabilities of landfalling tropical cyclones and sea-level rise

In the twenty-first century, the effects of sea-level rise (SLR) and more intense tropical cyclones (TCs) are increasing compound coastal inundation worldwide. To facilitate the adaptation efforts being made by coastal communities, here, we use a coastal surge-wave model together with a novel statistical approach to incorporate the six joint probability density functions (PDFs) of five landfall TC parameters and SLR values, instead of the traditional five-parameter approach, which considers the five PDFs of TCs with prescribed SLR values as boundary conditions. The five-parameter approach determines the 1% annual chance of coastal inundation by conducting numerous sets of surge-wave simulations, each for a different SLR scenario, for the future TC ensemble. The six-parameter approach, however, uses a future TC and SLR ensemble to conduct only one set of surge-wave simulations without the subjective selection of an SLR scenario, and is much less uncertain and much more efficient. In this paper, we focus on the 1% risk of inundation in a large coastal flood plain in southwest Florida by incorporating intensifying TCs and accelerating SLR under a representative concentration pathway 8.5 climate scenario in 2100. The 1% risk of inundation determined by the six-parameter approach is comparable to that obtained from the traditional approach forced with the expected SLR value in 2100. The total inundation volume, total inundation area, average inundation height, and maximum inundation height are expected to dramatically increase by (5.7, 2.4, 2.6, and 2.5) times, respectively, compared to their 1982–2009 values. The coastal inundations caused by TCs and SLR are found to interact nonlinearly over the coastal flood plain. Near the coast, TCs account for 70%–80% of the total 1% inundation risk for 1 m of SLR and 30%–70% for 2 m of SLR. Therefore, future inundation analyses must consider TCs and their nonlinear interaction with SLR-induced inundation. These findings will inform local communities and help them to develop coastal adaptation plans.

Mechanisms Driving Decadal Changes in the Carbonate System of a Coastal Plain Estuary

AbstractUnderstanding decadal changes in the coastal carbonate system is essential for predicting how the health of these waters responds to anthropogenic drivers, such as changing atmospheric conditions and riverine inputs. However, studies that quantify the relative impacts of these drivers are lacking. In this study, the primary drivers of decadal trends in the surface carbonate system, and the spatiotemporal variability in these trends, are identified for a large coastal plain estuary: the Chesapeake Bay. Experiments using a coupled three‐dimensional hydrodynamic‐biogeochemical model highlight that, over the past three decades, the changes in the surface carbonate system of Chesapeake Bay have strong seasonal and spatial variability. The greatest surface pH and aragonite saturation state (ΩAR) reductions have occurred in the summer in the middle (mesohaline) Bay: −0.24 and −0.9 per 30 years, respectively, with increases in atmospheric CO2 and reductions in nitrate loading both being primary drivers. Reductions in nitrate loading have a strong seasonal influence on the carbonate system, with the most pronounced decadal decreases in pH and ΩAR occurring during the summer when primary production is strongly dependent on nutrient availability. Increases in riverine total alkalinity and dissolved inorganic carbon have raised surface pH in the upper oligohaline Bay, while other drivers such as atmospheric warming and input of acidified ocean water through the Bay mouth have had comparatively minor impacts on the estuarine carbonate system. This work has significant implications for estuarine ecosystem services, which are typically most sensitive to surface acidification in the spring and summer seasons.

Floodplain response to varied flows in a large coastal plain river

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.

Seasonal Variability of the CO2 System in a Large Coastal Plain Estuary

AbstractThe Chesapeake Bay, a large coastal plain estuary, has been studied extensively in terms of its water quality, and yet, comparatively less is known about its carbonate system. Here we present discrete observations of dissolved inorganic carbon (DIC) and total alkalinity from four seasonal cruises in 2016–2017. These new observations are used to characterize the regional CO2 system and to construct a DIC budget of the mainstem. In all seasons, elevated DIC concentrations were observed at the mouth of the bay associated with inflowing Atlantic Ocean waters, while minimum concentrations of DIC were associated with fresher waters at the head of the bay. Significant spatial variability of the partial pressure of CO2 was observed throughout the mainstem, with net uptake of atmospheric CO2 during each season in the upper mainstem and weak seasonal outgassing of CO2 near the outflow to the Atlantic Ocean. During the time frame of this study, the Chesapeake Bay mainstem was (1) net autotrophic in the mixed layer (net community production of 0.31‐mol C m−2·year−1) and net heterotrophic throughout the water column (net community production of −0.48‐mol C m−2·year−1), (2) a sink of 0.38‐mol C m−2·year−1 for atmospheric CO2, and (3) significantly seasonally and spatially variable with respect to biologically driven changes in DIC.

Comparison of a wood sampler for macroinvertebrate bioassessment of non-wadeable streams in the southeastern coastal plain

Aquatic macroinvertebrates are often the subject of biomonitoring efforts because they are ubiquitous in most systems and are known to be suitable indicators of water quality. The vast majority of biomonitoring efforts that use macroinvertebrates are tailored towards small, wadeable streams, whereas methods for sampling macroinvertebrates in non-wadeable streams (i.e., large rivers) are often less available. We compared the use of a wood sampler to collect aquatic macroinvertebrates with two more commonly used samplers (Hester-Dendy and leaf packs) to assess its suitability for sampling non-wadeable systems in the Georgia Coastal Plain. All three types of samplers were deployed at three sites on each of the Savannah and Ogeechee River for approximately 30 days during the fall of 2014. Mean, standard deviation, and variance components were examined from 53 common bioassessment metrics and 2 multi-metric indices to assess for potential differences in colonizing macroinvertebrates between the samplers. Only 9 of the metrics examined scored significantly different between sampling devices, however, assemblages colonizing the three sampling devices differed (PERMANOVA; F14,37 =1.6078, P = 0.001). Results suggest assemblage differences were primarily driven by the proportions of taxa collected by each device, rather than by different taxa colonizing devices altogether. Estimates of variance components attributed a large percentage (i.e., >20%) of the variability encountered to differences between sites, rather than devices. Our study shows that wood samplers are comparable to other devices commonly used for collecting macroinvertebrates from non-wadeable streams in the Georgia Coastal Plain. Furthermore, wood samplers can provide a suitable alternative for bioassessment, although some differences in the proportions of taxa collected with alternative devices should be expected. Determining the precision and overall efficiency of alternative sampling devices is an important step towards developing standard operating procedures for the bioassessment of large coastal plain rivers.

Modelling Holocene analogues of coastal plain estuaries reveals the magnitude of sea-level threat

Hydrodynamic modelling of Australia’s lower Murray River demonstrates the response of a large coastal plain estuary to the mid-Holocene (7,000–6,000 yr BP) sea-level highstand. The approximately two metre higher-than-present sea level during the highstand forced the estuarine limit upstream generating an extensive central basin environment extending more than 200 kilometres from the river mouth (143 kilometres upstream of the modern tidal limit). The geomorphic history of the region does not conform to conventional estuarine facies models as, for much of the Holocene, the lower Murray River acted as a landward, gorge-confined extension of the Murray estuary. The incredibly low relief of this coastal plain system drove significant saline incursion and limited current velocities across the estuary facilitating deposition of a laminated silt-clay sequence which our results suggest may be regionally extensive. Variations to discharge, barrier morphology, or the estuary’s bathymetry result in minimal change to the estuarine palaeo-environment. The shift to the present-day fresher water distribution in the Murray estuary requires a fall in sea level to present-day conditions. The dominance of sea level as the controlling factor on this estuarine palaeo-environment highlights the significant potential impact of climate change induced sea-level rise to coastal plain estuaries.

Facies and paleoenvironments in Paraíba do Sul Deltaic Complex area, North of Rio de Janeiro state, Brazil

The Paraíba do Sul Deltaic Complex (PSDC) is a large Quaternary coastal plain located along Rio de Janeiro's northern coast and forms part of the onshore portion of Campos Basin. The main studies of the PSDC focused on modeling the paleoenvironmental evolution and were based on paleogeographic reconstructions, surface sampling, radiocarbon datings (14C), drillcores from shallow wells (<40 m depth) and results of ground penetrating radar (GPR) surveys. The paucity of deep subsurface data, in particular continuous drillcores, has led to divergent interpretations of not only the age, paleoenvironmental conditions and stratigraphic position of the PSDC deposits, but also of the underlying sedimentary units. In order to improve the knowledge about the evolution of PSDC, this study aims the stratigraphic characterization and paleoenvironmental interpretation by facies analyzes of a deep drillcore (410 m) recovered at the central area of PSDC. Fourteen sedimentary facies, grouped in six facies associations related to a wide variety of depositional paleoenvironments have been identified. The depositional history of the PSDC began with a transgressive event that led to the drowning of an incised valley cutting the continental deposits of the São Tomé Member, which is represented by the distal alluvial fan (DAF) and fluvial (F-I) facies associations in the analyzed drillcore. The valley infill is marked by two Pleistocene depositional sequences representing estuarine (FE-I facies association) and estuarine to open marine (FE-II facies association) paleoenvironments deposited in response to high-frequency base-level oscillations within a major transgressive sequence after the Marine Isotope Stage 6 (Penultimate Marine Transgression - 123.000 years B.P.). This transgression was followed by a regressive phase (normal to forced) that promoted the development of extensive Pleistocene beach-ridge terraces (BR facies association), which are still partially preserved at the surface of the study area. Marine and lagoonal deposits associated with the transgressive period occurred after the Marine Isotope Stage 2 (Last Marine Transgression - 5.100 years B.P.) documented in the previous studies, were not found in the analyzed drillcore, suggesting that these deposits may have discontinuously preserved within the study area or that they were eroded by Holocene fluvial systems represented by F-II facies association.

Contribution of HVSR measures for seismic microzonation studies

The HVSR method applied to seismic noise can be a very useful technique to map the site effects of the territory, to identify the thickness of the soft covering and so the depth of the seismic bedrock. The case of the urban area of Oliveri is presented. Because of its high seismic hazard this area has been subject of first level seismic microzonation. The town lies on a large coastal plain made of mixed fluvial/marine sediments, overlapping a deformed substrate. In order to identify points on the area probably suffering of relevant site effects and to define a preliminary Vs subsurface model, 23 HVSR measurements were performed. A clustering technique of continuous signals has been used to optimize the calculation of the HVSR curves and 42 reliable peaks, in the frequency range 0.1-20 Hz, have been identified. A second clustering technique has been applied to the set of 42 vectors, containing coordinates, frequency and amplitude of each peak, to identify subsets attributed to the same seismic discontinuities. Three main clusters have been identified. The two characterized by lower frequencies have been considered in the HVSR data inversion, as stratigraphic peaks probably caused by the seismic bedrock. Finally, the morphology of the top of the seismic bedrock has been mapped. The deepening of the seismic bedrock below the mouth of the Elicona Torrent suggests the possible presence of a buried paleo-valley.

Nitrate trends in groundwater of the Campania region (southern Italy).

The Environmental Protection Agency of the Campania region in Italy (ARPAC) manages a groundwater quality monitoring network. For almost all the polluted waters, the key parameter driving the classification is the concentration of nitrate; hence, the Campania region, in coherence with the EU regulations, outlined the vulnerable areas and undertook remediation policies. The best groundwater quality is recorded for carbonate aquifers of the Apennine chain; on the contrary, the Tyrrhenian coastal plains are affected by severe contamination, with a locally very contaminated groundwater of the shallow and also the deeper aquifers. The study is especially focused on a large coastal plain of Campania region, where nitrate concentration sometimes exceeds 200mg/L. The study, based on almost 200 sampling points for the whole region during the period 2003-2015 (approx two samples per year), verified the effectiveness of the groundwater monitoring network, the present distribution of nitrate in groundwater, and the evolution of nitrate trends at different scales: regional, groundwater body, and single well, using spatial and time series statistical approaches. Significant variations in contamination evolution within the study area have been observed and the correlation with land use has been highlighted.

Seasonal variability of the inorganic carbon system in a large coastal plain estuary

Abstract. Carbonate geochemistry research in large estuarine systems is limited. More work is needed to understand how changes in land-use activity influence watershed export of organic and inorganic carbon, acids, and nutrients to the coastal ocean. To investigate the seasonal variation of the inorganic carbon system in the Delaware Estuary, one of the largest estuaries along the US east coast, dissolved inorganic carbon (DIC), total alkalinity (TA), and pH were measured along the estuary from June 2013 to April 2015. In addition, DIC, TA, and pH were periodically measured from March to October 2015 in the nontidal freshwater Delaware, Schuylkill, and Christina rivers over a range of discharge conditions. There were strong negative relationships between river TA and discharge, suggesting that changes in HCO3− concentrations reflect dilution of weathering products in the drainage basin. The ratio of DIC to TA, an understudied but important property, was high (1.11) during high discharge and low (0.94) during low discharge, reflecting additional DIC input in the form of carbon dioxide (CO2), most likely from terrestrial organic matter decomposition, rather than bicarbonate (HCO3−) inputs due to drainage basin weathering processes. This is also a result of CO2 loss to the atmosphere due to rapid water transit during the wet season. Our data further show that elevated DIC in the Schuylkill River is substantially different than that in the Delaware River. Thus, tributary contributions must be considered when attributing estuarine DIC sources to the internal carbon cycle versus external processes such as drainage basin mineralogy, weathering intensity, and discharge patterns. Long-term records in the Delaware and Schuylkill rivers indicate shifts toward higher alkalinity in estuarine waters over time, as has been found in other estuaries worldwide. Annual DIC input flux to the estuary and export flux to the coastal ocean are estimated to be 15.7 ± 8.2 × 109 mol C yr−1 and 16.5 ± 10.6 × 109 mol C yr−1, respectively, while net DIC production within the estuary including inputs from intertidal marshes is estimated to be 5.1 × 109 mol C yr−1. The small difference between riverine input and export flux suggests that, in the case of the Delaware Estuary and perhaps other large coastal systems with long freshwater residence times, the majority of the DIC produced in the estuary by biological processes is exchanged with the atmosphere rather than exported to the sea.

Status of the imperiled Alabama Sturgeon (Scaphirhynchus suttkusiWilliams and Clemmer, 1991)

Summary The Alabama Sturgeon (ALS) Scaphirhynchus suttkusi has the smallest maximum size (78 cm FL), most restricted native range (the Mobile Basin, restricted to large coastal plain rivers), and the lowest abundance of any North American sturgeon. This riverine species is currently restricted to 524 RKM in the lower Cahaba and Alabama rivers in south Alabama. A commercial fishery existed in the 1890s, and it was still ‘not uncommon’ in Alabama in the 1930s. However, by the 1980s there was a sharp decrease in reports and confirmed collections of ALS. The recent collapse is due to loss and fragmentation of free-flowing riverine habitat and blockage of migratory routes due to construction of dams. Since 1997, state and federal agencies have collected only six individuals while expending over 3000 personnel-da in effort. Propagation efforts have proven unsuccessful to date. The last sighting of an ALS (not captured) was by Alabama Department of Conservation and Natural Resources (ADCNR) fisheries biologists in the Alabama River in April 2009. The last confirmed collection of an ALS occurred in April 2007. This specimen was implanted with a sonic tag, providing valuable habitat and movement data. Very little is known about life history or population structure of this species. Controversy surrounded designation of the ALS as federally endangered in the U.S. under the Endangered Species Act, with a business-oriented coalition challenging the listing through the federal circuit court of appeals. Recovery efforts are focused on obtaining individuals for propagation and providing fish passage through navigation locks on the Alabama River, although this option may not be viable for the ALS. The outlook for the ALS is bleak unless adequate number of individuals can be collected for propagation, access can be provided to historical spawning grounds in the lower Cahaba River, and flow and habitat can be restored to meet life history requirements. However, preliminary results from environmental DNA (eDNA) analysis in 2015 detected the presence of ALS in the Alabama River and may guide future collection efforts.

Sedimentation patterns across a Coastal Plain floodplain: The importance of hydrogeomorphic influences and cross-floodplain connectivity

The floodplains of large Coastal Plain rivers in the southeastern U.S. are important long-term storage sites for alluvial sediment and nutrients. Yet considerable uncertainty surrounds sediment dynamics on many large river floodplains and, in particular, the local scale factors that affect the flux of sediment and nutrients from rivers onto their floodplains and their subsequent deposition. This research quantifies short-term rates of sediment deposition from 2012 to 2014 on floodplain sites at Congaree National Park using feldspar pads. Sediment deposition rates ranged from 0.1 to 15.6cm (median=1.46cm) and were closely associated with inundation frequency and geomorphic position. Cross-floodplain distributary channels served as particularly important conduits for moving sediment onto the floodplain. Physical and chemical analyses of soil samples demonstrated that the most flood-exposed sites had higher major nutrient and micronutrient levels (especially of phosphorus) and more diverse nutrient compositions. This research advances current understandings of lateral floodplain connectivity by demonstrating the complex effects of regional hydrology and local floodplain environmental characteristics on the supply of sediment and nutrients.

Seismic Stratigraphy of Pleistocene Deltaic Deposits in Bahía Blanca Estuary, Argentina

The Bahía Blanca estuary (Argentina) has a morphological configuration resulting from hydrological and sedimentary processes related to Late Quaternary sea level changes. This estuarine system occupies a large coastal plain with a dense net of tidal channels, low-altitude islands and large intertidal flats. Little is known about the sedimentary units of the marine subbottom. Therefore, a stratigraphical analysis of the northern coast of Bahía Blanca estuary was carried out using high resolution seismic (3.5 kHz) in order to: i) define Quaternary sequences, ii) describe sedimentary structures, and iii) determine the paleoenvironmental conditions of sedimentation. The seismic stratigraphic data collected and their correlation with drilling lithological data show five seismic sequences (S1, S2, S3, S4 and S5), of which S1-S2 were found to be associated with a continental paleoenvironment of Miocene-Pleistocene age. Sequences S3 and S4, whose lithology and seismic facies (paleochannel structures and prograding reflection configurations), were defined on these materials, to evidence the development of an ancient deltaic environment which was part of a large Pleistocene drainage system. The S5 sequence was formed during the Holocene transgressive-regressive process and complete the seismostratigraphic column defined in the present study.

Plio‐Pleistocene landscape and vegetation reconstruction of the coastal area of the Tjörnes Peninsula, Northern Iceland

Marine and continental deposits from the Tjörnes area in northern Iceland were studied to obtain their pollen/spore content. Six Pollen Zones (PZ) were defined in the Early Pliocene Tjörnes beds and the Early Pleistocene Breidavík Group. The pollen is most diverse during the deposition of the lowest Tapes Zone (PZ 1) and the lower part of the overlying Mactra Zone (PZ 2). Local pollen from marshland, levee and foothill forests was deposited on a large coastal plain. The pollen spectrum reflects transgression and deepening during the second part of the Mactra Zone (PZ 3) and the lower part of the Serripes Zone (PZ 4). Gymnosperm pollen derived from the higher inland plateau increases in PZ 3. This background pollen was of minor importance during periods with an extensive coastal plain (PZ 1, 2, 4, 6). PZ 5 did not yield sufficient pollen for analysis. The pollen analysis allowed refinement of the sea‐level variations based on sedimentology and molluscs. Pollen of warmth‐demanding plants is recorded throughout the Tjörnes beds and the Early Pleistocene interglacial deposits. Warmth‐loving species indicate summers 8°C warmer than today during deposition of the Tapes Zone, and at least 5°C warmer during the rest of the Tjörnes beds. The Pliocene vegetation of Iceland matches well that of the present‐day western European maritime temperate climate. The drastic cooling at the onset of the Quaternary led to a marked vegetation impoverishment, already noticeable in the Early Pleistocene Breidavík Group.

Dynamics of estuarine phytoplankton assemblages in Mex Bay,Alexandria (Egypt): Influence of salinity gradients

ex Bay, west of Alexandria (Egypt) is a large, shallow and turbid coastal plain estuary of socio-economic importance. Along its salinity gradient (3.30 to 39.64), seven sampling stations were chosen to cover the different habitats. Regionally, four water masses could be defined; the Mediterranean water mass of salinity >38.5, mixed Mediterranean of 30 to 38.5, diluted of 10 to 30 and drain water mass of salinity 20 µm cells dominated (62.73% of total Chl. a) in May, both fraction size (50.1% and 49.9%, respectively) in July, and the 20 µm size with dinoflagellate abundances. The distribution patterns of biomass showed relatively high Chl. a (average 15.68 µg l -1 , 83.73% 38.5, respectively. Values of the diversity index fluctuated between 0.34 nats (May) and 2.24 nats (September), linked with species diversity and blooming.

Reconstruction of Mid-Holocene sedimentary environments in the central part of the Thessaloniki Plain (Greece), based on microfaunal identification, magnetic susceptibility and grain-size analyses

The study aims to estimate the relative contributions of the two drainage basins of the Aliakmon and Axios rivers which, since the Mid-Holocene, have been responsible for building the largest deltaic area in Greece. Sediments from five cores located in the central part of the Thessaloniki Plain have been studied for their environmental changes using paleontological and sedimentological methods. Chronostratigraphical evidence was obtained from 14C AMS dating of marine shells, peat and organic sediment samples. During the Holocene marine transgression, this large coastal plain was a shallow marine bay reaching approx. 35 km inland circa the 4th millennium BC, from which the sea subsequently regressed to the east. Around the middle of the 3rd millennium BC, strong fluvial deposition of Aliakmon, to the east, and of Axios, to the north, occurred and was responsible of a gradual change to lagoonal and limnic environmental conditions. Around the 5th Century BC, a freshwater lake occupied the westernmost part of the plain. Microfaunal identification, together with magnetic susceptibility measurements, and grain-size analysis reveal three main environments of sediment deposition that reflect combinations of both concentrated and dispersed sources of magnetic/source minerals. Using remote sensing and a combination of spectral bands (LANDSAT TM imagery), we identify former fluvial levees and a freshwater lake, and give a spatial interpretation of the rivers' influences in building this deltaic complex. The mechanisms of edification of the plain as well as the roles played by Aliakmon and Axios sedimentation are described.

Spatial Relations Between Floodplain Environments and Land Use – Land Cover of a Large Lowland Tropical River Valley: Pánuco Basin, México

Large lowland river valleys include a variety of floodplain environments that represent opportunities and constraints for human activities. This study integrates extensive field observations and geomorphic data with analysis of satellite remote sensing data to examine spatial relations between land use/land cover (LULC) and floodplain environments in the lower Pánuco basin of eastern Mexico. The floodplain of the lower Pánuco basin was delineated by combining a digital elevation model with a satellite image of a large flood event. The LULC was classified by combining a hybrid classification strategy with image stratification, applied to 15-m-resolution ASTER data. A geomorphic classification of floodplain environments was performed using a dry-stage image (ASTER data) and a 1993 Landsat image acquired during a large flood event. Accuracy assessment was based on aerial photographs (1:38,000), global positioning satellite ground-truthing, and a Landsat 7ETM(+) image from 2000, which resulted in an overall accuracy of 82.9% and a KHAT of 79.8% for the LULC classification. The geomorphic classification yielded 83.5% overall accuracy, whereas the KHAT was 81.5%. LULC analysis was performed for the entire floodplain and individually within four valley segments. The analysis indicates that the study area is primarily utilized for grazing and farming. Agriculture is primarily associated with coarse-grained (sandy/silty) natural levee and point bar units close to the river channel, whereas cattle grazing occurs in distal and lower-lying reaches dominated by cohesive fine-grained (clayey) deposits, such as backswamps. In the Pánuco valley, wetlands and lakes occur within backswamp environments, whereas in the Moctezuma segments, wetlands and lakes are associated with relict channels. This study reveals considerable variation in LULC related to spatial differences in floodplain environments and illustrates the importance of considering older anthropogenic influences on the landscape. The research design should be applicable for other large lowland coastal plain river valleys where agriculture is a major component of the floodplain landscape.

Austro‐boreal disjunctions: a remarkable biogeographical pattern illustrated by Nan Ao Island, Guangdong, China

AbstractAim To document disjunct, insular populations of animals separated from similar conspecifics distributed to the north, inland and upland, and compare this pattern with that historically described for northern Florida and the Florida Keys islands.Location Nan Ao Island is on the Tropic of Cancer 117° E in the South China Sea, eastern Guangdong Province, China. There are archipelagos of islets extending south‐east, and several coastal islets around Nan Ao.Methods Five field surveys were made in 1999–2001. Voucher specimens were tagged, catalogued, and deposited in major museum collections. The comparable Floridian disjunct pattern was researched and described in overview.Results Five species – a toad, two lizards and two snakes – collected on Nan Ao and three satellite islands were added to the two previously documented species – a lizard and a snake – that fit the austro‐boreal disjunct pattern. One lizard and one snake are minor range extensions of a hundred or few km. One snake's Nan Ao population is disjunct by about 450 km and one lizard's by about 550 km. The small, semi‐fossorial, short‐legged toad is the most notable, separated from the main area of its range by about 800 km. This is similar to the greatest distance between north Floridian and Keys disjunct populations.Main conclusions (1) Seven Nan Ao species of terrestrial vertebrates are isolated and disjunct from conspecifics found to the north, upland and inland; this compares to five species showing a similar north Florida to Florida Keys pattern. (2) The Nan Ao species have no conspecifics known from the intervening area whereas four of the five Floridian species have conspecific, but dissimilar, populations in the gap. (3) Nan Ao and its satellite islets were high points on a large coastal plain at glacial maximum. (4) Holocene sea level rise and concomitant global warming may have isolated the Nan Ao relicts that had continuous distributions at glacial maximum. (5) The intervening mainland conspecific populations may have been replaced by tropical species that moved north along the mainland but failed to colonize the islands. (6) If so, significant adaptation to a warmer climate must have occurred to enable survival of the Nan Ao relicts to the present.

Local- and watershed-scale controls on the spatial variability of natural levee deposits in a large fine-grained floodplain: Lower Pánuco Basin, Mexico

This study examines spatial variations in natural levee deposits within the lower reaches of a large coastal plain drainage system. The Pánuco basin (98,227 km 2) drains east-central Mexico, and is an excellent setting to examine the influence of watershed and local controls on the morphology and sedimentology of natural levees. Although many fluvial systems in the U.S. Gulf Coastal Plain have been investigated, the rivers in the Mexican Gulf Coastal Plain have received comparatively little attention. Lateral and downstream characteristics of natural levee morphology and sediment texture are considered within the context of meandering river floodplain deposits. Data sources include total-stations surveying, sediment samples of surficial levee deposits, topographic maps (1:50,000), and aerial photographs (1:40,000). The slope of natural levees average 0.0049 m/m, whereas the texture ( D 84) of levee deposits averages 0.12 mm. Natural levee characteristics vary due to local- and watershed-scale controls. The lateral reduction in levee height displays a curvilinear pattern that coincides with an abrupt change in sediment texture. The downstream pattern of natural levee texture exhibits the influence of local-scale perturbations superimposed upon a larger watershed-scale trend. Disruption to the fining trend, either by tributary inputs of sediment or reworking of Tertiary valley deposits, is retained for a limited distance. The influence of the channel planform geometry on levee morphology is examined by consideration of the radius of curvature ( R c) of meander bends, and is inversely related to natural levee width. This suggests that the planform geometry of river channels exerts a control on the dispersal of flood sediments, and is responsible for considerable local variability in the floodplain topography. The average width of natural levees increases with drainage area, from an average of 747 m in the Moctezuma to an average of 894 m in the Pánuco. However, in the lower reaches of the Pánuco valley the width of natural levees rapidly decreases, which is associated with fining of the suspended sediment load. Thus, the reduction in natural levee width signifies an abrupt change in the directionality of cause–effect relationships at the watershed-scale. Findings from this study elucidate linkages between meandering river channels and floodplains for a large lowland alluvial valley.