Macrobenthic communities from wetland impoundments and adjacent open marsh habitats in South Carolina

Forty-eight core and grab samples were taken from two impoundments and an adjacent tidal creek and salt marsh during each of six sampling periods (January, June and November 1983; and January, April and July 1984). Habitats sampled within the impoundments included the perimeter ditch and shallow vegeted areas dominated byRuppia maritima, Spartina alterniflora, andScirpus robustus. The adjacent tidal creek bottom and low marsh ofS. alterniflora were sampled for comparison with the impoundment sites. Major differences in faunal composition and density of macrobenthic invertebrates were observed between habitats in this study. Macrobenthic density was highest (475 individuals 0.05 m^?2) at the impoundment site dominated byScripus robustus, where oligochaetes were abundant. The open marsh site had a density of 254 individuals 0.05 m^?2. Among unvegetated sites, density for all sampling periods was higher in Chainey Creek than in the perimeter ditches of the impoundments. The total number of taxa was highest for the open marsh and tidal creek sites. The impoundments contained vegetated sites which were inhabited by fewer species than nonimpounded sites, while the perimeter ditch sites were comparatively depauperate. Cluster and nodal analyses identified four broad assemblages based on habitat: 1) an open marsh assemblage, 2) a creek assemblage, 3) a eurytopic assemblage, and 4) an impoundment assemblage. The separation of faunal assemblages by sampling site rather than sampling period suggests that physical differences between habitats were important factors determining distribution patterns.     

Benthic macrofaunal communities in partially impounded salt marshes in Delaware: Comparisons with natural marshes and responses to sediment exposure

This two-part study examined the benthic macrofaunal community in Delaware salt marsh impoundments having partial tidal restriction. The first part compared abundance, diversity, and taxonomic composition in three habitat types in impoundments—creeks, vegetated creek banks, and ponds—to those found in natural marshes. Impoundment effects were present but were habitat-specific. Abundances were higher in natural marsh creeks than in impoundment creeks, and diversities were higher in impoundment ponds than in natural marsh ponds. Vegetated bank communities in impoundments were about 50% insects and arachnids and 50% oligochaetes, while natural bank communities were primarily oligochaetes and the polychaeteManayunkia aestuarina. This is likely due to the decrease in flooding of the vegetated high marsh caused by partial impoundment. Pond and creek community composition also showed impoundment effects: in comparison with natural marshes, impoundments had higher proportions of the burrowing anemoneNematostella vectensis, nemerteans, andTubificoides sp. oligochaetes and lower proportions of the oligochaeteClitellio arenarius. The second part of the study compared benthic macrofauna in an impoundment before, during, and after the water level was lowered so that some bottom sediments were exposed and some covered with just a few centimeters of water for several weeks. During this event, macrofaunal abundances were reduced and the community shifted from being dominated by annelids, anemones, and nemerteans toward one dominated by annelids and insects. About 6 wk after reflooding, persistent effects of this disturbance were still suggested by greatly increased abundances and 96% dominance by one species of oligochaete,Paranais litoralis. Impoundment management plans calling for periods of sediment exposure or very low water may want to consider the potential for strong and persistent effects on the macrofaunal community.     

Effects of impoundment on vertical accretion of coastal marsh

Vertical accretion of impounded marsh and adjacent natural marsh at four sites in southwestern Louisiana was estimated in 1994 by determining the depth of a stratum containing¹³⁷Cs deposited in 1963. With relative marsh elevation, soil bulk density, organic matter content, and organic and mineral matter accumulation rates were used to describe soil formation. Three sites were impounded in 1956 and one site in 1951. Impounded marshes had lower marsh surface elevation than natural marshes because of hydrologic isolation from tidal sediment subsidies and substrate oxidation during forced drying. The elevation of natural marshes ranged from 12 cm to 42 cm higher than the elevation of the impounded marshes in 1963 and from 20 cm to 32 cm higher in 1994. Vertical accretion between 1963 and 1994 ranged from 9 cm to 28 cm in impounded marsh and from 15 cm to 21.5 cm in natural marsh. Only in impounded marsh that remained permanently flooded was accretion greater than in natural marsh.     

Contemporary Deposition and Long-Term Accumulation of Sediment and Nutrients by Tidal Freshwater Forested Wetlands Impacted by Sea Level Rise

Contemporary deposition (artificial marker horizon, 3.5 years) and long-term accumulation rates (²¹⁰Pb profiles, ~150 years) of sediment and associated carbon (C), nitrogen (N), and phosphorus (P) were measured in wetlands along the tidal Savannah and Waccamaw rivers in the southeastern USA. Four sites along each river spanned an upstream-to-downstream salinification gradient, from upriver tidal freshwater forested wetland (TFFW), through moderately and highly salt-impacted forested wetlands, to oligohaline marsh downriver. Contemporary deposition rates (sediment, C, N, and P) were greatest in oligohaline marsh and lowest in TFFW along both rivers. Greater rates of deposition in oligohaline and salt-stressed forested wetlands were associated with a shift to greater clay and metal content that is likely associated with a change from low availability of watershed-derived sediment to TFFW and to greater availability of a coastal sediment source to oligohaline wetlands. Long-term accumulation rates along the Waccamaw River had the opposite spatial pattern compared to contemporary deposition, with greater rates in TFFW that declined to oligohaline marsh. Long-term sediment and elemental mass accumulation rates also were 3–9× lower than contemporary deposition rates. In comparison to other studies, sediment and associated nutrient accumulation in TFFW are lower than downriver/estuarine freshwater, oligohaline, and salt marshes, suggesting a reduced capacity for surface sedimentation (short-term) as well as shallow soil processes (long-term sedimentation) to offset sea level rise in TFFW. Nonetheless, their potentially large spatial extent suggests that TFFW have a large impact on the transport and fate of sediment and nutrients in tidal rivers and estuaries.     

Patterns of sediment accumulation in the tidal marshes of Maine

One year’s measurements of surficial sedimentation rates (1986–1987) for 26 Maine marsh sites were made over marker horizons of brick dust. Observed sediment accumulation rates, from 0 to 13 mm yr^?1, were compared with marsh morphology, local relative sea-level rise rate, mean tidal range, and ice rafting activity. Marshes with four different morphologies (back-barrier, fluvial, bluff-toe, and transitional) showed distinctly different sediment accumulation rates. In general, back-barrier marshes had the highest accumulation rates and blufftoe marshes had the lowest rates, with intermediate values for transitional and fluvial marshes. No causal relationship between modern marsh sediment accumulation rate and relative sea-level rise rate (from tide gauge records) was observed. Marsh accretionary balance (sediment accumulation rate minus relative sea-level rise rate) did not correlate with mean tidal range for this meso- to macro-tidal area. Estimates of ice-rafted debris on marsh sites ranged from 0% to >100% of measured surficial sedimentation rates, indicating that ice transport of sediment may make a significant contribution to surficial sedimentation on Maine salt marshes.     

Natural fisheries,marsh management,and mariculture: Complexity and conflict in Louisiana

Most of Louisiana’s economically important saltwater fishes and crustaceans spawn in the Gulf of Mexico, but their young must use the vast coastal marsh as their nursery. Marsh management in Louisiana usually consists of emplacement of levees and water-control structures in the marsh. These structures significantly reduce fisheries production and offshore recruitment. In addition, in 1987 private entities were authorized to use 32,380 ha of the marsh for experimental mariculture, which (if successful) will likewise lead to greatly reduced natural fisheries production and offshore recruitment. Private interests also nearly succeeded in legalizing entrapment and eventual harvest of the wild fisheries from 100,000 ha, and 20,000 ha, of the marsh in 1991 and 1992, respectively. The effects of all these threats to natural fisheries production are further complicated by Louisiana's confused legal situation regarding coastal land ownership.     

Temporal and spatial variation in intertidal sedimentation rates

Sediment deposition and erosion rates are reported for an intertidal zone in the Burry Inlet, South Wales. Measured deposition rates over the salt marsh are compared with deposition rates calculated from observed suspended sediment concentrations. Notably, it is concluded that residual turbulence at slack water should not be discounted when calculating deposition rates. Grain-size distributions of suspended sediments over the marsh surface, during flood and ebb tides, contrast with the grain-size distribution of deposited marsh sediments, the latter being significantly coarser. These data in conjunction with mass budget calculations are used to relate total annual deposition and sediment supply by tidal action during settled meteorological periods. The analysis suggests that episodic storm-induced sediment transport is probably an important mechanism for introducing coarse sediment on to the marsh surface. Finally, it is noted that seasonal reworking of the sandy non-cohesive sediments may be related to variations in the intensity of wave-breaking throughout the year.     

Microbial nitrogen removal in a developing suburban estuary along the South Carolina coast

The conversion of undisturbed coastal regions to commercial and suburban developments may pose a threat to surface and groundwater quality by introducing nitrate-nitrogen (NO₃ ^?-N) from runoff of land-applied wastewater and fertilizers. Microbial denitrification is an important NO₃ ^?-N removal mechanism in coastal sediments. The objective of this study was to compare denitrification and nitrate conversion rates in coastal sediments from a golf course, suburban site, undeveloped marsh, and nonmarsh area near rapidly developing Hilton Head Island, South Carolina. Nitrous oxide was measured using gas chromatography and nitrate and ammonium concentrations were measured using a flow injection autoanalyzer in microcosms spiked, with 50 μg NO₃ ^?-N gdw^?1. The two marsh sites had the greatest ammonium production, which was correlated with fine sediment particle size and higher background sediment nitrate and surface water sulfate concentrations. The golf course swale had greatest denitrification rates, which were correlated with higher total carbon and organic nitrogen in sediments. Nitrate was consumed in golf course sediments to a greater extent than in the undeveloped marsh and upland freshwater sites, suggesting that the undeveloped sites and receiving estuaries may be more susceptible to nitrate contamination than the golf course swale and marsh under nonstorm conditions. Construction of swales and vegetated buffers using sediments with high organic carbon content as best management practices may aid in removing nitrate and other contaminants from runoff prior to its transport to the receiving marsh and estuary.     

Salinity history of coastal marshes reconstructed from diatom remains

Sediment cores were collected from three Louisiana coastal marsh ponds, dated with radioisotopes, and analyzed for diatom remains to determine if long-term salinity changes were evident in the sediment record. A diatom-based salinity index formulated from a statistical comparison of available salinity data and changing diatom assemblages demonstrated that diatom remains appear to preserve salinity signals in coastal brackish and salt marsh environments. The salinity index was applied to sediment cores spanning the late 1600s to the 1990s and provided a more complete record of salinity than field data, which were temporally and spatially incomplete. The salinity reconstructions indicated that salinity has increased at two sites and decreased at a third since the early 1900s. The salinity changes are less than 1‰ per decade in all cases, and may be due to natural variability as depicted by the wide range of salinities observed between the late 1600s and 1900. Salinity regimes may be very localized (<2 km from a hydrologic source), indicating single-site studies may not be applicable to regional inferences. This study demonstrates that diatoms can be used to reconstruct past salinity in coastal marsh environments and can provide a useful tool with which to study the changing hydrology of river-influenced ecosystems.     

Metal accumulation in surface salt marsh sediments of the Bay of Fundy,Canada

We measured the amount of arsenic, chromium, copper, lead, nickel, vanadium, and zinc accumulated over a five-year period from 1997 to 2002 in surface sediments of seven salt marshes along the New Brunswick coast of the Bay of Fundy, Canada. Study sites extended from outer to inner Bay, spanning a gradient in tidal range (6–12 m) and mean sediment deposition rate (0.27–1.76 cm yr⁻¹). In each study site, metal concentrations were measured in low and high marsh areas. Concentrations of chromium, nickel, and zinc appear to be within their natural range, while arsenic, lead, and vanadium are enriched in some sites. Calculated sediment metal loadings rates showed variability among marsh sites that closely followed sediment deposition patterns, suggesting sediment deposition rate is the driving factor of short-term metal accumulation in Fundy marshes. The value of salt marshes as a sink for metals may be enhanced by high sedimentation rates.     

Patterns of sediment deposition in subsiding coastal salt marshes,Terrebonne Bay,Louisiana: The role of winter storms

High rates of wetland loss in the Mississippi deltaic plain have been attributed to a combination of insufficient marsh sedimentation and relative sea-level rise rates of over 1.2 cm yr^?1. This study examines contemporary patterns of sediment delivery to the marsh surface by evaluating the contribution of individual marsh flooding events. Strong meteorological effects on water level in Terrebonne Bay often mask the usual microtidal fluctuations in water level and cause flood events to be of unpredictable frequency and duration. Sediment deposited on the marsh surface was collected weekly at two sites. Preliminary results allow the relative contributions of tidal and storm inundations to be calculated. Maximum sedimentation is associated with strong southerly winds both causing increased flooding and mobilizing sediment from open bay areas. Sediment deposition is limited by the availability of suspended sediment and the opportunity for its transport onto the marsh surface.     

Impacts of sea-level rise on deltas in the Gulf of Mexico and the Mediterranean: The importance of pulsing events to sustainability

In deltas, subsidence leads to a relative sea-level rise (RSLR) that is often much greater than eustatic rise alone. Because of high RSLR, deltaic wetlands will be affected early by an acceleration of eustatic sea-level rise. If there is sufficient vertical accretion, wetlands can continue to exist with RSLR; however, lack of sediment input eventually leads to excessive water logging and plant death. Areas with low tidal range, such as the Mediterranean and Gulf of Mexico, are especially vulnerable to rising water levels because the elevational growth range of coastal vegetation is related to tide range. Reduction of suspended sediments in rivers and prevention of wetland flooding by river dikes and impoundments have reduced sediment input to Mediterranean and Gulf of Mexico deltaic wetlands. This sediment deficit will become more important with an acceleration in sea-level rise from global warming. Most sediment input occurs during strong pulsing events such as river floods and storms, and management policies and decisions are especially designed to protect against such events. Management approaches must be reoriented to take advantage of pulsing events to nourish marsh surfaces with sediments. We hypothesize that deltas can be managed to withstand significant rates of sea-level rise by taking advantage of pulsing events leading to high sediment input, and that this type of management approach will enhance ecosystem functioning.     

Marsh-water column interactions in two Louisiana estuaries. I. Sediment dynamics

Throughflow marsh flumes were used to measure total sediment exchanges (TSS) between the marshes and water column of two Louisiana estuaries. One, the Barataria Basin estuary, is isolated from significant riverine sediment input. There were significant (p<0.05) imports of 33.9 to 443 mg TSS m^?2 h^?1 at the Barataria Basin brackish marsh (BM) site. The Barataria Basin saltmarsh (SM) site exported TSS in two summer samplings, but significant uptake was measured in April (166 mg m^?2 h^?1) and November (45 mg m^?2 h^?1) during a winter frontal passage event. The other estuary, Fourleague Bay, receives large sediment inputs from the Atchafalaya River, and TSS imports of 22.5 to 118.5 mg m^?2 h^?1 were measured at the BM site here. We calculated sediment accumulation from fluxes quantified in marsh flumes using site-specific sedimentological data and flooding regimes at each site. Water level records from May 1987 to April 1989 showed an extended period of unusually low flooding frequencies. As a result, calculated accretion rates were low, with monthly rates of 0.02 to 0.11 mm and ?0.06 to 0.06 mm at the Barataria BM and SM sites, respectively, and ?0.18 to 0.08 mm at the Fourleague Bay marsh flume site. Actual net sediment deposition, determined by feldspar marker horizon analysis, was 0.7–1.6 mm mo^?1 at the Barataria SM and 0.2–1.3 mm mo^?1 at the Fourleague Bay BM. Even the highest calculated accretion rates, based on flume measurements, were half to one order of magnitude lower than actual measured sediment deposition. This discrepancy was probably because: 1) most sedimentation occurs during episodic events, such as Hurricane Gilbert in September 1988, which deposited 3.5–15.5 mm of sediment on the Barataria Basin saltmarsh, or 2) most vertical accretion in Louisiana marshes occurs via deposition of in situ organic matter rather than by influx of allochthonous sediment. Our results affirm the variability of short-term sediment transport and depositional processes, the close coupling of meteorologic forcing and flooding regime to sediment dynamics, and the importance of understanding these interrelated mechanisms in the context of longer term measurements.     

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.     

Stream response to subsidence from underground coal mining in central Utah

 Short-term geomorphic and hydrologic effects of subsidence induced by longwall mining under Burnout Creek, Utah were evaluated. During the year after longwall mining, 0.3–1.5 m of subsidence was measured near impacted reaches of the mountain stream channel. The major channel changes that occurred in a 700-m reach of Burnout Creek that was subsided from 1992 to 1993 were: (1) increase in lengths of cascades and to a lesser extent glides; (2) increases in pool length, numbers and volumes; (3) increase in median particle diameter of bed sediment in pools; and (4) some constriction in channel geometry. Most of the changes appeared short-lived, with channel recovery approaching pre-mining conditions by 1994. In a 300-m reach of the South Fork that was subsided from 1993 to 1994, only channel constriction was observed, although any impacts on pool morphology may have been confounded by heavy grazing in the riparian reaches during the dry summer of 1994. Similar near-channel sedimentation and loss of pool volume between 1993 and 1994 were noted throughout Burnout Creek and in adjacent, unmined James Creek. Subsidence during the 3-year period had no effect on baseflows or near-channel landslides. Received: 10 April 1998 · Accepted: 21 September 1998     

Spatial and isotopic analysis of watershed soil loss and reservoir sediment accumulation rates in Lake Anna, Virginia, USA

Soil erosion and associated sedimentation are a threat to the sustainable use of surface water resources through the loss of volume storage capacity and conveyance of pollutants to receiving water bodies. The RUSLE2 empirical model and isotopic sediment core analyses were used to evaluate watershed erosion and reservoir sediment accumulation rates for Lake Anna, in Central Virginia. A sediment flux rate of 66,000 Mg/year was estimated from the upper basin and land use was determined to be the primary factor contributing to soil erosion. Barren lands and agricultural activities were estimated to contribute the most sediment (>20 Mg/ha/year), whereas forested and herbaceous landscapes were less likely to erode (<0.3 Mg/ha/year). Eleven separate ²¹⁰Pb-based estimates of sediment accumulation were used to construct reservoir-scale sedimentation rates. Sedimentation rates in the upper reaches of the reservoir were variable, ranging from 2.3 to 100 Mg/ha/year, with a median rate of 8.4 Mg/ha/year. Historical sedimentation showed an increase in annual accumulation from 1972 to present. Based on these data the reservoir has experienced a 2% loss of volume storage capacity since impoundment in 1972. Results from this study indicate that Lake Anna is not currently experiencing excessive sedimentation and erosion problems. However, the predominance of highly erosive soils (soil erodibility factor >0.30) within the watershed makes this system highly vulnerable to future anthropogenic stressors.     

Recruitment and Habitat Use of Early Life Stage Tarpon (<Emphasis Type="Italic">Megalops atlanticus</Emphasis>) in South Carolina Estuaries

Our study was designed to examine early life stage tarpon (Megalops atlanticus) recruitment, habitat use, and residency in coastal environments near the northern limit of their distribution in the western Atlantic Ocean. We employed a multi-faceted approach to (1) collect ingressing larval tarpon on nighttime flood tides at multiple sites, (2) document larval and juvenile tarpon use of natural high marsh pools, and (3) examine juvenile tarpon movement and behavior in managed marsh impoundments, all in the North Inlet-Winyah Bay estuarine system of South Carolina, USA. The timing of recruitment (June through November) and size of larvae (mean ± standard deviation = 23 ± 3 mm standard length [SL]) during estuarine ingress was similar to that reported from other subtropical locations in the region. Soon after recruiting into the system, larval and small juvenile tarpon (47 ± 25 mm SL) co-occurred in high marsh pools from July to November, and large juveniles (201 ± 34 mm SL) were also present in marsh impoundments during this same time period. An increase in tarpon length over time during their residency in high marsh pools and the relatively large size they attain in marsh impoundments indicate these environments may function as favorable nursery habitats. As water temperatures decreased during October and November, juvenile tarpon emigrated from these estuarine habitats. Tarpon appear to use a variety of estuarine habitats in coastal South Carolina from summer through late fall during their early life stage development. The fate of these individuals after they leave estuarine habitats at the onset of winter in this region is currently unknown.     

Sustainability of a Tidal Freshwater Marsh Exposed to a Long-term Hydrologic Barrier and Sea Level Rise

A 115-year-old railroad levee bisecting a tidal freshwater marsh perpendicular to the Patuxent River (Maryland) channel has created a northern, upstream marsh and a southern, downstream marsh. The main purpose of this study was to determine how this levee may affect the ability of the marsh system to gain elevation and to determine the levee’s impact on the marsh’s long-term sustainability to local relative sea level rise (RSLR). Previously unpublished data from 1989 to 1992 showed that suspended solids and short-term sediment deposition were greater in the south marsh compared to the north marsh; wetland surface elevation change data (1999 to 2009) showed significantly higher elevation gain in the south marsh compared to the north (6?±?2 vs. 0?±?2 mm year^?1, respectively). However, marsh surface accretion (2007 to 2009) showed no significant differences between north and south marshes (23?±?8 and 26?±?7 mm year^?1, respectively), and showed that shallow subsidence was an important process in both marshes. A strong seasonal effect was evident for both accretion and elevation change, with significant gains during the growing season and elevation loss during the non-growing season. Sediment transport, deposition and accretion decreased along the intertidal gradient, although no clear patterns in elevation change were recorded. Given the range in local RSLR rates in the Chesapeake Bay (2.9 to 5.8 mm year^?1), only the south marsh is keeping pace with sea level at the present time. Although one would expect the north marsh to benefit from high accretion of abundant riverine sediments, these results suggest that long-term elevation gain is a more nuanced process involving more than riverine sediments. Overall, other factors such as infrequent episodic coastal events may be important in allowing the south marsh to keep pace with sea level rise. Finally, caution should be exercised when using data sets spanning only a couple of years to estimate wetland sustainability as they may not be representative of long-term cumulative effects. Two years of data do not seem to be enough to establish long-term elevation change rates at Jug Bay, but instead a decadal time frame is more appropriate.     

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.     

Sediment Accretion in Tidal Freshwater Forests and Oligohaline Marshes of the Waccamaw and Savannah Rivers,USA

Sediment accretion was measured at four sites in varying stages of forest-to-marsh succession along a fresh-to-oligohaline gradient on the Waccamaw River and its tributary Turkey Creek (Coastal Plain watersheds, South Carolina) and the Savannah River (Piedmont watershed, South Carolina and Georgia). Sites included tidal freshwater forests, moderately salt-impacted forests at the freshwater–oligohaline transition, highly salt-impacted forests, and oligohaline marshes. Sediment accretion was measured by use of feldspar marker pads for 2.5 year; accessory information on wetland inundation, canopy litterfall, herbaceous production, and soil characteristics were also collected. Sediment accretion ranged from 4.5 mm year^?1 at moderately salt-impacted forest on the Savannah River to 19.1 mm year^?1 at its relict, highly salt-impacted forest downstream. Oligohaline marsh sediment accretion was 1.5–2.5 times greater than in tidal freshwater forests. Overall, there was no significant difference in accretion rate between rivers with contrasting sediment loads. Accretion was significantly higher in hollows than on hummocks in tidal freshwater forests. Organic sediment accretion was similar to autochthonous litter production at all sites, but inorganic sediment constituted the majority of accretion at both marshes and the Savannah River highly salt-impacted forest. A strong correlation between inorganic sediment accumulation and autochthonous litter production indicated a positive feedback between herbaceous plant production and allochthonous sediment deposition. The similarity in rates of sediment accretion and sea level rise in tidal freshwater forests indicates that these habitats may become permanently inundated if the rate of sea level rise increases.