Relationships between wetland fragmentation and recent hydrologic changes in a Deltaic Coast

Patterns in coastal wetland loss in the northern Gulf of Mexico were examined using aerial imagery from 1955–56 and 1978. Five qualitative types of wetland changes are evident: (1) spoil bank-parallel pond formation, (2) pond formation with apparent random distribution for the smallest ponds, but very clumped distribution for larger ponds, (3) semi- or complete impoundment resulting in open water formation, (4) cutting off of stream channels upstream of where a spoil bank crosses a natural channel, and (5) erosion at the land-water interface. Only ponds <20 ha formed and disappeared in the interval, and it is clear that wetland breakup, not erosion at the pond-lake edge, is the dominant form of wetland-to-open water conversion. Canals and their spoil banks are spatially related to wetland-to-water conversion which is evident up to 2 km away from those man-made features. The indirect impacts of canals and spoil banks vary regionally, for example, with sediment compaction rates that increase with increasing sediment deposition. These results are consistent with the hypothesis that canals and spoil banks are a major factor driving wetland loss rates because they change wetland hydrology.     

Pattern and process of land loss in the Mississippi Delta: A Spatial and temporal analysis of wetland habitat change

An earlier investigation (Turner 1997) concluded that most of the coastal wetland loss in Louisiana was caused by the effects of canal dredging, that loss was near zero in the absence of canals, and that land loss had decreased to near zero by the late 1990s. This analysis was based on a 15-min quadrangle (approximately 68,000 ha) scale that is too large to isolate processes responsible for small-scale wetland loss and too small to capture those responsible for large-scale loss. We conducted a further evaluation of the relationship between direct loss due to canal dredging and all other loss from 1933–1990 using a spatial scale of 4,100 ha that accurately captures local land-loss processes. Regressions of other wetland loss on canal area (i.e., direct loss) for the Birdfoot, Terrebonne, and Calcasieu basins were not significant. Positive relationships were found for the Breton (r²=0.675), Barataria (r²=0.47), and Mermentau (r²=0.35) basins, indicating that the extent of canals is significantly related to wetland loss in these basins. A significant negative relationship (r²=0.36) was found for the Atchafalaya coastal basin which had statistically lower loss rates than the other basins as a whole. The Atchafalaya area receives direct inflow of about one third of the Mississippi discharge. When the data were combined for all basins, 9.2% of the variation in other wetland loss was attributable to canals. All significant regressions intercepted the y-axis at positive loss values indicating that some loss occurred in the absence of canals. Wetland loss did not differ significantly from the coast inland or between marsh type. We agree with Turner that canals are an important agent in causing wetland loss in coastal Louisiana, but strongly disagree that they are responsible for the vast majority of this loss. We conclude that wetland loss in the Mississippi delta is an ongoing complex process involving several interacting factors and that efforts to create and restore Louisiana’s coastal wetlands must emphasize riverine inputs of freshwater and sediments.     

Input of Nutritionally Rich Organic Matter from the Mississippi River to the Louisiana Coastal Zone

Isotopes have often been used to discern riverine subsidies to coastal food chains, but there are few direct measurements of nutritional quality of riverine particulates. We tested for nutritionally enriched organic matter in the Mississippi River suspended sediment and evidence for its delivery to Louisiana coastal sediments by measuring enzymatically hydrolysable amino acids (EHAA). Riverine suspended sediments contained EHAA concentrations of up to 5 mg g^?1, higher than reported in any coastal sediment. Pigment concentrations indicated that EHAA in some river samples were dominated by phytoplankton, but many samples contained significant non-algal EHAA. Coastal sediments showed EHAA concentrations lower than riverine sediments but still higher than most reported shelf values. Incubation of riverine sediment showed losses of 28–34% of their EHAA over 6 days, similar to differences found between riverine and coastal sediments. EHAA concentrations decreased more rapidly than total nitrogen, indicating the relative lability of this pool of material in the studied region. These EHAA-enriched materials provide fuel for various coastal biota whose composition likely depends on factors such as disturbance regimes.     

Dedication: Dr. Scott W. Nixon (1943–2012)

The availability of suspended sediments will be a dominant factor influencing the stability of tidal wetlands as sea levels rise. Watershed-derived sediments are a critical source of material supporting accretion in many tidal wetlands, and recent declines in wetland extent in several large river delta systems have been attributed in part to declines in sediment delivery. Little attention has been given, however, to changes in sediment supply outside of large river deltas. In this study, significant declines in suspended sediment concentrations (SSCs) over time were observed for 25 of 61 rivers examined that drain to the East and Gulf Coasts of the USA. Declines in fluvial SSC were significantly correlated with increasing water retention behind dams, indicating that human activities play a role in declining sediment delivery. There was a regional pattern to changes in fluvial sediment, and declines in SSCs were also significantly related to rates of relative sea level rise (RSLR) along the coast, such that wetlands experiencing greater RSLR also tend to be receiving less fluvial sediment. Tidal wetlands in the Mid-Atlantic, Mississippi River Delta, and Texas Gulf especially may become increasingly vulnerable due to rapid RSLR and reductions in sediment. These results also indicate that past rates of marsh accretion may not be indicative of potential future accretion due to changes in sediment availability. Declining watershed sediment delivery to the coastal zone will limit the ability of tidal marshes to keep pace with rising sea levels in some coastal systems.     

Nutrient changes in the Mississippi River and system responses on the adjacent continental shelf

The Mississippi River system ranks among the world's top 10 rivers in freshwater and sediment inputs to the coastal ocean. The river contributes 90% of the freshwater loading to the Gulf of Mexico, and terminates amidst one of the United States' most productive fisheries regions and the location of the largest zone of hypoxia, in the western Atlantic Ocean. Significant increases in riverine nutrient concentrations and loadings of nitrate and phosphorus and decreases in silicate have occurred this century, and have accelerated since 1950. Consequently, major alterations have occurred in the probable nutrient limitation and overall stoichiometric nutrient balance in the adjacent continental shelf system. Changes in the nutrient balances and reduction in riverine silica loading to, the continental shelf appear to have led to phytoplankton species shifts offshore and to an increase in primary production. The phytoplankton community response, as indicated by long-term changes in biological uptake of silicate and accumulation of biologically bound silica in sediments, has shown how the system has responded to changes in riverine nutrient loadings. Indeed, the accumulation of biologically bound silica in sediments beneath the Mississippi River plume increased during the past two decades, presumably in response to, increased nitrogen loading. The duration, size, and severity of hypoxia has probably increased as a consequence of the increased primary production. Management alternatives directed at water pollution issues within the Mississippi River watershed may have unintended and contrasting impacts on the coastal waters of the northern Gulf of Mexico.     

Dispersal of river sediments in coastal seas: Six contrasting cases

The fate of sediment seaward of river mouths involves at least four stages: supply via plumes; initial deposition; resuspension and transport by marine processes; and long-term net accumulation. The processes that operate at each stage, and relative roles of each stage in governing the long-term accumulation patterns, vary appreciably with river regime and coastal ocean environment. To illustrate the diversity and illuminate the process of dispersal, information is synthesized for six systems: Amazon, Changjiang, Mississippi, Columbia, Purari, and Huanghe. These systems differ markedly in terms of water discharge, sediment discharge, and coastal energy regime and much of the diversity of dispersal patterns is attributed to these differences as well as to the temporal sequencing of river discharge relative to oceanographic transport processes. Although the sediment: water ratio of the discharge of the Mississippi River is 70 times less than that of the Huanghe, both of these systems exhibit rapid deposition and accumulation of sediments near the river mouths. In contrast, sediments dispersed by, the other four systems are transported greater distances from the mouths by oceanographic processes, and are accumulating over relatively wide areas.     

晚更新世江苏海岸带沉积分布模拟研究

中国边缘海大陆架在晚更新世时期曾是海岸平原,在古长江、古黄河泥沙填充下形成了陆架堆积体,并在全新世发育了南黄海辐射沙脊群、废黄河三角洲和长江三角洲。根据点状的地质钻孔分析和重建,对南黄海-江苏海岸的沉积体系的分布和变化机制尚不明了。作为动力机制探讨,基于气候-海面-沉积系统,根据气候水文学、沉积学原理以及泥沙沉积面的动态高程计算,构建了气候冰川驱动-东黄海地海系统响应-河流沉积建造的数值模式,模拟了14万年、4万年和1万年不同时间尺度江苏海岸线和长江三角洲沉积的变化过程和分布,进而对冰川气候、构造沉降、沉积压实等复杂效应下的海面特征、陆源泥沙沉积和海岸线进行分析。模拟结果与地质钻孔资料揭示的层序和埋深能够进行对比。     

Wetland Plant Community Responses to the Interactive Effects of Simulated Nutrient and Sediment Loading: Implications for Coastal Restoration Using Mississippi River Diversions

The Mississippi River Delta Complex (MRDC) has experienced extensive wetland loss in the last century due, in part, to flood control levees that have isolated the lower Mississippi River and its sediment resource from adjacent wetlands. Reconnecting the River to these wetlands through diversions is being used and proposed on a larger scale for the future, to reduce wetland loss rates. However, some currently operating diversions (e.g., Caernarvon and Davis Pond) have been implicated in causing negative impacts on wetland ecosystem structure and function due to increased nutrient loads in diverted Mississippi River water combined with insufficient sediment delivery. Initial assessments of these concerns were carried out in a greenhouse setting where six nutrient enrichment treatment levels (control, NO₃, NH₄, PO₄, SO₄, and Combo [NO₃?+?NH₄?+?PO₄?+?SO₄]) were applied with and without sediment addition to 60 marsh sods from a Sagittaria lancifolia-dominated oligohaline wetland at rates simulating the Davis Pond Diversion of the Mississippi River. After 25 months, independent enrichment with N (regardless of form) and sediment was generally beneficial to wetland structure and function, while SO₄ enrichment had the opposite effect, regardless of sediment addition. Simultaneous application of N and P (i.e., the Combo treatment level) ameliorated the negative impacts of SO₄-loading, but the concurrent application of sediment did not, likely because the loading rate was based on a diversion that was designed to deliver water and not to maximize sediment input. Nonetheless, sediment input is critical to the sustainability of MRDC wetlands experiencing high rates of deterioration. Thus, optimizing future diversions to maximize sediment delivery, along with continued surveillance of negative nutrient effects, are recommended management decisions.     

Heavy metals in sediments from San Antonio Bay and the northwest Gulf of Mexico

Sediments from San Antonio Bay, the northwest Gulf of Mexico, and the Mississippi River Delta were acid leached and analyzed for Fe, Mn, Pb, Zn, Cd, Cu and Ni by atomic absorption spectrophotometry. In order to account for differences in sediment clay, carbonate, and organic matter content, metal concentrations were normalized to Fe. Significant linear correlations of metals to Fe were obtained for unpolluted sediments and deviations from these “natural” statistical populations were found for areas thought to have metal input caused by man. San Antonio Bay sediments show little evidence of metal pollution despite 70 years of shell dredging in the bay. However, the San Antonio-Guadalupe River system, the bay's prime sediment source, has 10% to 50% higher than natural levels of Pb, Cd and Cu. Sediments from a 1500 km² area of the Mississippi River Delta have Pb and Cd concentrations 10% to 100% higher than expected levels. The vertical distribution of Pb and Cd in these sediments suggests that inputs have occurred during the past 30 to 40 years. We find no indication of metal pollution in other areas of the Delta or along the continental shelf of the northwest Gulf of Mexico.     

Effect of hydrologic management on marsh surface sediment deposition in coastal Louisiana

High rates of coastal land loss in Louisiana have prompted efforts to maintain or restore coastal wetland habitats, and structural management of marsh hydrology is one of a number of approaches that has been adopted. The aim of this study was to determine the effect of hydrologic management measures on marsh-surface sediment deposition in the Mississippi deltaic plain. Four impoundments, ranging in size from 50 ha to 177 ha and similarly sized control sites were included in the study. At each site, marsh-surface sediment traps were collected approximately biweekly for 3.5 yr to measure changes in sediment deposition. There was no significant difference in sediment deposition between any of the impoundment and control sites during the premanagement period. The results show no significant difference in marsh-surface sediment deposition between management year 1992–1993 and management year 1994–1995, but management year 1993–1994 had significantly lower sediment deposition than either the first or the last year of the study. Management year 1992–1993 showed the highest sediment deposition when it was assessed across all basins, areas, and sites, and this high rate of deposition is accounted for by the impact of Hurricane Andrew. For all management years there were significant differences between impoundment and control sites, with control sites showing higher rates of marsh surface sediment deposition than impoundments. No clear pattern was identified concerning the influence of various types of hydrologic management on rates of sediment deposition. This reduction in sediment deposition indicated that the areas under management in this study are receiving insufficient inorganic sediment deposition to keep pace with sea-level rise.     

Compensatory effects of boat wake and dredge spoil disposal on assemblages of macroinvertebrates

Estuarine assemblages are exposed to multiple disturbances that overlap in time and space. Along the Atlantic Intracoastal Waterway (east coast, United States), two disturbances that frequently co-occur are the production of wake by boats and the disposal of sediment dredged from boat channels. Boat wake generally coarsens sediments by eroding finer particles while deposition of dredge spoil decreases mean grain size. If previously demonstrated effects of boat wake on infauna are due to coarsening of grain size, deposition of dredge spoil on wake affected sites may, through compensatory effects, prevent an effect of wake from being detected. Epifaunal assemblages associated with seagrass blades that are more likely to be structured by hydrodynamic forces than granulometry may instead be affected by boat wake irrespective of the previous deposition of fine materials. To test these hypotheses, in fauna and epifauna were sampled in patchy seagrass habitat at sites with and without boat wake that were affected by historic deposition of dredge spoil and at sites without wake that had not received dredge spoil. Sediment granulometry and infaunal assemblages differed between sites with and without dredge spoit but not between spoil affected sites differing in exposure to wake. Epifaunal assemblages differed between sites with and without wake irrespective of sediment granulometry. The effect of wake on epifauna was primarily due to lesser abundances of the gastropodBittiolum varium and the slipper limpet,Crepidula fornicata, at wake exposed sites. These results suggest that because of their opposing effects on sediment granulometry, boat-wake and sediment disposal may have compensatory effects on infaunal assemblages. The detection of an effect of wake on epifauna despite the absence of a sedimentological effect of the disturbance shows that ecological impacts do not necessarily mirror physical effects and should be considered separately when adopting strategies of management.     

Application of discriminant analysis and manova to grain-size data on a study of the distribution and movement of dredged sediment

A dredged material disposal operation was monitored at a location in Lake Erie, 8 km offshore at Ashtabula, Ohio, in 1975. Approximately 200 sediment cores were collected from 12 experimental and 4 control locations before and after dredging and analyzed for the grain-size distribution and related heavy-metal content. The dredged sediments were similar to those from the lake bottom at the disposal and control sites. Because of this similarity, it was extremely difficult to distinguish between the dredged material and the lake bottom sediments without tagging the material with dyes or radioactive isotopes. A sequence consisting of a linear discriminant analysis followed by a univariate and multivariate analysis of variance was successfully applied to discriminate between the dredged and the original lake sediments. Results indicate that 4 months after the disposal operation some stations had returned to predisposal conditions, a probable result of currents stripping dredged material off the lake bottom. The analysis of variance indicated that the clay-size fraction was responsible for initial changes in the grain-size distribution. Storm induced scouring caused an eventual return of the grain-size distribution to predisposal conditions. In support of this observation, the concentrations of iron and zinc, which were statistically correlated to the clay size fraction, also exhibited the same trends.     

珠江三角洲近岸海洋地质环境与地质灾害调查主要进展及成果

通过遥感、水文和水化学、综合物探、地质取样、钻探以及静力触探等多种调查手段与方法,首次在我国重点经济区——珠江三角洲的近岸开展了1∶10万比例尺海洋地质环境与地质灾害综合调查,对调查区水域的海水化学要素、环境质量、水动力、地形地貌、沉积物、浅地层结构、潜在地质灾害以及海底工程地质条件进行了综合分析与研究,对海洋地质环境进行了综合评价与预警,并对该区海洋开发和工程建设提出了建议。     

Sediment dynamics in the lowermost Mississippi River

There is much to be gained from investigating sediment dynamics in the lower Mississippi system, the largest river in terms of discharge and sediment load in North America. Such work can improve conceptual knowledge concerning downstream changes at the lower end of large river systems and can be applied to manage sediment diversions for wetland restoration in south Louisiana. Suspended sediment dynamics in the lowermost Mississippi River system in Louisiana are characterized using three approaches: (1) temporal changes in discharge-suspended sediment relationships showing interannual variations and the effects of floods over short timescales; (2) empirical relationships between discharge and suspended sediment variables at various locations; and (3) downstream changes in discharge-suspended sediment relationships. Interpretation of this data set is enhanced with other secondary data regarding processes, morphology, and bed materials.

Upstream, near Old River, LA, empirical relationships show nonlinearity, particularly in fine sediments, with decreased concentrations at highest discharges. During high discharge years, suspended sediment concentration peaks precede discharge crests by 40–85 days. The lead generally decreases with decreasing discharge maxima so that in low discharge years sediment peaks and discharge crests closely coincide in time. Downstream, near Belle Chasse, LA, fine bottom materials are resuspended and the timing of sediment peaks and discharge crests is coincident, regardless of flow magnitude. Conceptually, results suggest caution when generalizing about the relative timing of the sediment wave and flood wave and their downstream progression. These phenomena are influenced by local bed material and hydraulic conditions, and depend on the causative factors of sediment peaks. From an applied perspective, diversions should be managed differently depending upon where they are constructed along the river and upon the magnitude of the annual maximum flow. During high discharge years, when concerns for navigation and water supply are minimal, flow should be diverted on the rising limb upstream, near Old River, and during the discharge crest downstream near New Orleans.     

环渤海湿地沉积物球囊霉素相关土壤蛋白分布特征及其对湿地土壤风化的影响

【研究目的】球囊霉素作为丛枝菌根真菌（AMF）分泌的一种难降解土壤蛋白,广泛分布于陆地生态系统中,是长期碳贮的重要组成。当前鲜有研究涉及其在滨海湿地中的分布。基于此,本文对中国环渤海主要滨海湿地表层沉积物中球囊霉素相关土壤蛋白（GRSP）的空间分布进行表征,探讨不同湿地生境下GRSP分布及其对沉积物风化的指示意义。【研究方法】本文选取辽河三角洲、北大港和黄河三角洲湿地作为研究区,对166个表层样（0~5 cm）和4个柱状样（0~35 cm）的GRSP、粒度以及常量元素进行测定,并计算化学蚀变系数（CIA）。【研究结果】研究区GRSP在空间上的动态变化受植被类型影响显著,范围在0.06~11.31 mg/g,均值为（2.35±0.16） mg/g;沉积物以粉砂质砂和砂为主,CIA值分布范围为44.79~69.59,部分区域达到中等化学风化;CIA与GRSP呈显著相关（R=~0.49,p<0.01）,总体上CIA随GRSP的增加呈现先增加后减少的趋势。【结论】GRSP在滨海湿地沉积物中的分布受到生境差异性的影响,其与CIA的相关性表征AMF及其代谢产物在地质体风化过程中发挥了潜在的生态功能。创新点：揭示了球囊霉素相关土壤蛋白与地质过程的互馈作用;阐明了丛枝菌根真菌潜在的生态功能。     

Evidence of regional subsidence and associated interior wetland loss induced by hydrocarbon production, Gulf Coast region, USA

Analysis of remote images, elevation surveys, stratigraphic cross-sections, and hydrocarbon production data demonstrates that extensive areas of wetland loss in the northern Gulf Coast region of the United States were associated with large-volume fluid production from mature petroleum fields. Interior wetland losses at many sites in coastal Louisiana and Texas are attributed largely to accelerated land subsidence and fault reactivation induced by decreased reservoir pressures as a result of rapid or prolonged extraction of gas, oil, and associated brines. Evidence that moderately-deep hydrocarbon production has induced land-surface subsidence and reactivated faults that intersect the surface include: (1) close temporal and spatial correlation of fluid production with surficial changes including rapid subsidence of wetland sediments near producing fields, (2) measurable offsets of shallow strata across the zones of wetland loss, (3) large reductions in subsurface pressures where subsidence rates are high, (4) coincidence of orientation and direction of displacement between surface fault traces and faults that bound the reservoirs, and (5) accelerated subsidence rates near producing fields compared to subsidence rates in surrounding areas or compared to geological rates of subsidence. Based on historical trends, subsidence rates in the Gulf Coast region near producing fields most likely will decrease in the future because most petroleum fields are nearly depleted. Alternatively, continued extraction of conventional energy resources as well as potential production of alternative energy resources (geopressured-geothermal fluids) in the Gulf Coast region could increase subsidence and land losses and also contribute to inundation of areas of higher elevation.     

Accretion and canal impacts in a rapidly subsiding wetland II. Feldspar marker horizon technique

Recent (6–12 month) marsh sediment accretion and accumulation rates were measured with feldspar marker horizons in the vicinity of natural waterways and man-made canals with spoil banks in the rapidly subsiding environment of coastal Louisiana. Annual accretion rates in aSpartina alterniflora salt marsh in the Mississippi deltaic plain averaged 6 mm in marsh adjacent to canals compared to 10 mm in marsh adjacent to natural waterways. The rates, however, were not statistically significantly different. The average rate of sediment accretion in the same salt marsh region for a transect perpendicular to a canal (13 mm yr^?1) was significantly greater than the rate measured for a transect perpendicular to a natural waterway (7 mm yr^?1). Measurements of soil bulk density and organic matter content from the two transects were also different. This spatial variability in accretion rates is probably related to (1) spoil bank influences on local hydrology; and (2) a locally high rate of sediment input from lateral erosion associated with pond enlargement. In a brackishSpartina patens marsh on Louisiana’s Chenier plain, vertical accretion rates were the same along natural and canal waterways (3–4 mm yr^?1) in a hydrologically restricted marsh region. However, the accretion rates for both waterways were significantly lower than the rates along a nonhydrologically restricted natural waterway nearby (11 mm yr^?1). The vertical accretion of matter displayed semi-annual differences in the brackish marsh environment.     

Response of macrobenthic communities to restoration efforts in a New England estuary

Changes in macrobenthic communities were studied over a 3.5 yr period following restoration activities in Alewife Cove, a small estuary located in southeastern Connecticut, U.S. Development around this estuary had resulted in reduced freshwater and tidal inflows, loss of critical habitats such as salt marshes, and eutrophication. Early in 1988 the lower reach of the estuary was dredged to increase tidal flushing and enhance environmental quality. Following dredging, tidal range within the Cove increased from 52 to 83 cm. Due to erosion within the Cove's lower channel and sediment migration into the Cove, tidal flows and ranges approached pre-dredge levels by 1991. Despite these changes, the percentage of silt/clay in the surface sediments in the middle and upper basins of the Cove declined by 30–45% over the study period. Changes in infaunal community structure in the lower reach following dredging were not great, primarily comprising shifts in the relative abundances of species typical of sandier versus muddier sediments. Directional changes in community structure were most evident in the middle and upper basins, away from the dredged area. Infaunal species richness increased significantly, with many species previously found only in the sand habitats of the lower reach establishing populations in the middle and upper basins. There was a significant decrease in the summer abundances of the pollution indicator polychaeteCapitella capitata throughout much of the middle and upper basins. Restoration efforts in Alewife Cove centering on altered bydrology resulted in selected positive changes. Increases in tidal flow altered environmental conditons in the middle and upper basins where shifts in infaunal community structure indicated decreases in organic loading of sediments over 2–3 yr. Continued changes in the physical dynamics of the lower reach reduced tidal flow, arresting the positive ecological changes that were beginning to occur. This type of restoration approach of small estuaries can have positive results, but there may be a lag in the ecological response of the system. Macrobenthic communities, in particular summer abundance patterns of selected species, provided an integrated assessment of ecological changes in the Cove.     

Palaeoenvironmental control on sediment composition and provenance in the late Quaternary deltaic successions: a case study from the Po delta area (Northern Italy)

Accumulation of continental, deltaic and shallow‐marine sediments in the Po River coastal plain preserves a record of the Late Quaternary sea‐level changes and shoreline migrations. The palaeoenvironmental evolution of this area and the changes in composition and provenance of sediments have been investigated through integrated sedimentological, micropalaeontological (mainly foraminifers) and geochemical analyses of core S1, from the southern part of the Po River delta, within a chronological framework supported by radiocarbon dating and correlations with adjacent core sequences. Eleven lithofacies, grouped into five facies associations, and four palaeontological assemblages provide the basis to define the palaeoenvironmental reconstruction of this succession consisting, from the base to the top, of: (i) continental sediments accumulated during the Late Pleistocene; (ii) back‐barrier sediments marking the onset of Holocene sea‐level rise; (iii) transgressive sands deposited during the rapid landward migration of a barrier‐lagoon system; (iv) shallow‐marine and prodelta sediments with faunal associations indicating a gradual approach to the Po River mouth; and (v) sub‐recent delta front sands that form a considerable portion of the present coastal plain. Bulk chemical composition of sediments shows remarkable relationships with palaeoenvironments and locally improves facies characterizations. For example, they reveal carbonate leaching that emphasizes the occurrence of palaeosols in continental deposits or record enrichments in loss on ignition, S and Br, diagnostic of organic‐rich layers in back‐shore sediments. Selected geochemical elements (e.g. Mg and Ni) are particularly effective for the recognition of sediment provenances from the three main source areas observed in the subsurface deposits of the Po River coastal plain (e.g. Apenninic rivers, North Adriatic rivers and Po River). An Apenninic provenance is observed in continental and back‐barrier sediments. A North Adriatic provenance characterizes the transgressive sands and the shallow‐marine deposits; a significant Po River provenance is recorded in sediments related to the onset of the prodelta environment, confirmed by foraminiferal assemblages indicating remarkable increase in fluvial influxes. Copyright © 2006 John Wiley & Sons, Ltd.     

Lower Mississippi River tributaries: contributions to the collective science concerning the “Father of Waters”

The geological and geomorphic information preserved in the tributary valleys of the lower Mississippi River (LMR) contributes to our understanding of the lower valley's Quaternary geological history. Prominent Pleistocene terraces are preserved in the tributary valleys. Fisk first formulated his four terraces framework on the Red River. Caution needs to be followed in projecting the Red River terraces across the entire Lower Mississippi Valley (LMV). The tributary system cannot be assumed to operate in a synchronized fashion in response to changes in climate and base level. To compare the collective contribution of the tributaries of the LMR, the streams are described in terms of. (1) their characteristics, (2) geomorphic development, (3) process and response of the tributaries to and from the LMR, and (4) engineering investigations and implications. The characteristics of the tributaries are a direct function of their drainage basin size and geology. The tributary system drains portions of six physiographic provinces. Synoptically, the tributaries can be viewed as two groups: the eastern and western tributaries. All of the eastern tributaries are intra-regional, i.e., they drain only one physiographic province, the Coastal Plain, and therefore, have a restricted sediment source. Generally, the eastern tributaries are more numerous and shorter than the western tributaries. The longer western tributaries drain outside the Coastal Plain. The extra-regional nature of the western tributaries adds to the variability of discharge and sediment types. The sediment record of the tributaries reflects response to the trunk Mississippi. During glacial outwash flushes, many of the tributaries were alluvially drowned, producing alluvial cones expressed as flattened longitudinal profiles. More recently, a number of tributaries in the state of Mississippi have experienced episodes of accelerated channel erosion. The effects of navigation and flood control modification of the Mississippi River on the tributaries have not been fully studied. Therefore, fluvial geomorphic research in the tributaries is essential to understanding ways to mitigate the adverse effects of river engineering, thereby designing engineering works in balance with the alluvial architecture and processes of the stream system.