Geochronology and sedimentology of the lower Passaic River,New Jersey

In an attempt to characterize localized rates of sediment accretion, 10 sediment cores were collected from the lower reach of the Passaic River, a major tributary of Newark Bay, New Jersey. Sediments were assayed for ²¹⁰Pb activity at predetermined depths and the rate of sediment accretion (cm yr^?1) was estimated from the least squares regression of the log of unsupported activity versus depth. Sediment accretion rates, derived from ²¹⁰Pb measurements (R_Pb) were used to predict the depth interval within the core containing sediments deposited around 1954; subsequent ¹³⁷Cs analyses were focused on this depth interval. Sediment accretion rates derived from ¹³⁷Cs measurements (R_Cs) were extrapolated from the depth of the 1954 horizon. Lead-210 derived sediment accretion rates in cores collected from a sediment bench extending along the inside bend on the southern shore of a meander in the river, ranged from 4.1 cm yr^?1 to 10.2 cm yr^?1 and averaged 6.8 cm yr^?1. The R_Cs estimates for cores from this area ranged from 3.8 cm yr^?1 to 8.9 cm yr^?1 and averaged 6.6 cm yr^?1. The R_Cs for cores collected in a more hydrologically dynamic reach of the river upstream of the sediment bench, were only 0.41 cm yr^?1 and 0.66 cm yr^?1. The results of this investigation indicate that this reach of the lower Passaic River is an area of high sediment accumulation, retaining much of the sediment load deposited from upstream and downstream sources. The rates of sediment accretion in the lower Passaic River are among the highest reported anywhere in the Newark Bay estuary.     

Accretion rates and sediment accumulation in Rhode Island salt marshes

In order to test the assumption that accretion rates of intertidal salt marshes are approximately equal to rates of sea-level rise along the Rhode Island coast,²¹⁰Pb analyses were carried out and accretion rates calculated using constant flux and constant activity models applied to sediment cores collected from lowSpartina alterniflora marshes at four sites from the head to the mouth of Narragansett Bay. A core was also collected from a highSpartina patens marsh at one site. Additional low marsh cores from a tidal river entering the bay and a coastal lagoon on Block Island Sound were also analyzed. Accretion rates for all cores were also calculated from copper concentration data assuming that anthropogenic copper increases began at all sites between 1865 and 1885. Bulk density and weight-loss-on-ignition of the sediments were measured in order to assess the relative importance of inorganic and organic accumulation. During the past 60 yr, accretion rates at the eight low marsh sites averaged 0.43±0.13 cm yr^?1 (0.25 to 0.60 cm yr^?1) based on the constant flux model, 0.40±0.15 cm yr^?1 (0.15 to 0.58 cm yr^?1) based on the constant activity model, and 0.44±0.11 cm yr^?1 (0.30 to 0.59 cm yr^?1) based on copper concentration data, with no apparent trend down-bay. High marsh rates were 0.24±0.02 (constant flux), 0.25±0.01 (constant activity), and 0.47±0.04 (copper concentration data). The cores showing closest agreement between the three methods are those for which the excess²¹⁰Pb inventories are consistent with atmospheric inputs. These rates compare to a tide gauge record from the mouth of the bay that shows an average sea-level rise of 0.26±0.02 cm yr^?1 from 1931 to 1986. Low marshes in this area appear to accrete at rates 1.5–1.7 times greater than local relative sea-level rise, while the high marsh accretion rate is equal to the rise in sea level. The variability among the low marsh sites suggests that marshes may not be poised at mean water level to within better than ±several cm on time scales of decades. Inorganic and organic dry solids each contributed about 9% by volume to low marsh accretion, while organic dry solids contributed 11% and inorganic 4% to high marsh accretion. Water/pore space accounted for the majority of accretion in both low and high marshes. If water associated with the organic component is considered, organic matter accounts for an average of 91% of low marsh and 96% of high marsh accretion. A dramatic increase in the organic content at a depth of 60 to 90 cm in the cores from Narragansett Bay appears to mark the start of marsh development on prograding sand flats.     

Pollution records from sediments of three lakes in New York State

Sediment core segments from Sylvan Lake, Lake Champlain and Lake Canadarago were dated radiometrically with ²¹⁰Pb and ¹³⁷Cs. Their respective sedimentation rates were determined to be 0.11, 0.14 and 0.52 g cm^?2 yr^?1. For the two lakes of lower sedimentation the variations of selected elemental abundances as function of depth were analyzed. Two groupings were found: Al, K, Ti, Rb and Zr were correlated among themselves but reflected different variations in the input of terrigenous erosion material to the lakes. The Cu, Zn and Pb correlated among themselves showed similar depth dependence with increasing concentrations toward the top which can be attributed to cultural pollution. Recent ‘excess’ fluxes to the sediments above the natural contribution by clastic material were derived for the location of the cores, which for Cu, Zn and Pb amounted to 3.8, 24 and 16 μg cm^?2 yr^?1 respectively for Sylvan Lake and 4.9, 20 and 16 μg cm^?2 yr^?1 for Lake Champlain. The corresponding ²¹⁰Pb flux was 3.3 and 2.3 dpm cm^?2 yr^?1, respectively for the two lakes.Approximate residence times in the water column were obtained for trace metals at the Lake Champlain location. Short residence times estimated for Pb (< 0.15 yr) and Cu (< 0.4 yr) indicate fast removal, whereas those for Zn (1.0 ± 0.3 yr) and Cr (2.0 ± 0.5 yr) appeared to be dominated by the water residence time.     

A 2,500-year history of anoxia and eutrophication in Chesapeake Bay

Ongoing monitoring programs and historical data are not sufficient to establish anthropogenic effects on the ecology of Chesapeake Bay. However, stratigraphic records preserved in the sediments can be used to reconstruct both prehistoric and historic sedimentation and water conditions of the bay, including anoxia and eutrophication. Pollen, diatoms, total organic carbon (TOC), nitrogen, total sulfur, and an estimate of the degree of pyritization of iron (DOP) are being used as paleoecological indicators in dated sediment cores for the purpose of reconstructing a long-term environmental history of the bay. Analysis of the data indicates that sedimentation rates, anoxic conditions, and eutrophication have increased in the Chesapeake Bay since the time of European settlement. For example, since initial land clearance around 1760, sedimentation rates have increased from as low as 0.02 cm yr^?1 to an average 0.22 cm yr^?1, and TOC from 0.14 mg cm^?2 yr^?1 to a high 4.96 mg cm^?2 yr^?1. Diatom community structure shows a steady decrease in overall diversity since 1760 and the centric:pennate ratio has increased significantly since 1940.     

Lead-210 sediment geochronology in a changing coastal environment

Sediment accumulation rate studies utilizing excess ²¹⁰Pb and ¹³⁷Cs were conducted as part of recent investigations of biogeochemical cycling at a single site in Cape Lookout Bight, a rapidly changing coastal basin on the Outer Banks of North Carolina (U.S.A.). Cores three meters in length reveal a depositional history for the bight interior characterized by a gradual transition in texture from coarse-grained to fine-grained material over the period 1946–1979. This transition is controlled by progressive enclosure of the bight by an active northerly migrating recurved spit. The textural gradation is periodically interrupted by layers of well-sorted sand associated with major storm events. Lead-210 data indicate that the upper meter of the sediment has accumulated at a rate of 3.35 to 4.71 g · cm^?2 · yr^?1 or approximately 8.4 to 11.8 cm · yr^?1 (at ø = 0.84). Below 120 cm depth, dilution of clay and silt by low activity sand necessitates correction of the ²¹⁰Pb profile in order to establish a geochronology. Grain size ²¹⁰Pb distribution measurements at three depths reveal that the specific activity (dpm · g^?1) of clay is 3.2 times that of silt and 24.7 times that of sand. Corrections of bulk sediment excess ²¹⁰Pb activities based on these measurements lead to dates for textural changes which are consistent with charted changes in basin morphology and major storm events.Sixteen ¹³⁷Cs measurements between 33–241 cm depth reveal a peak activity at 105–115 cm and indicate a minimum sedimentation rate of approximately 2.7 g · cm^?2 · yr^?1.     

Sediment accumulation rates in Conowingo reservoir as determined by man-made and natural radionuclides

The Susquehanna River is the major contributor to sediment loadings in the Chesapeake Bay. Because many environmental contaminants are associated with suspended particulates, the degree of particle retention within the reservoirs of the lower Susquehanna River is an important consideration in evaluating contaminant loadings to the Chesapeake Bay. Profiles of weapons-test Cs-137, nuclear power plant-related Cs-134 and Cs-137, and naturally-derived Pb-120 were used to estimate rates of sediment accretion in the conowingo Reservioir, an impoundment of the Susquehanna River along the Maryland-Pennsylvania border. Net accretion rates ranged from about 2 cm yr^?1 downstream of a nuclear power plant cooling discharge to a high of about 7 cm yr^?1 at the mount of an incoming creek. Slight, but consistent, increases in the annual rate of accretion since the creation of the reservoir in 1928 are apparent. The current net average annual sediment load reatined by the reservoir is estimated to be 0.4×10⁶ to 1.5 × 10⁶ metric tons yr^?1. The retained sediment load represents about 8–23% of the long-time average sediment input to the reservoir.     

Reconstruction of Recent Sedimentary Processes in a Carbonate Platform (Gulf of Batabano,Cuba) Using Environmental Radiotracers

Recent (past 100 years) sedimentary processes in the highly dynamic Gulf of Batabano (Cuba, Caribbean Sea) were investigated through the analyses of environmental radionuclides (e.g., ²¹⁰Pb, ²²⁶Ra, ¹³⁷Cs, ^239,240Pu, and ¹⁴C) in nine sediment cores. We evaluated the mean mass accumulation rates (MARs) and the surface mixed layers (SMLs) in each sediment core. Based on these results, three sedimentary environments were identified in the study region. In the central zone, the sediments were mainly composed of carbonate transported from the southern area and showed elevated mass accumulation rates (MAR, 0.11–0.23 g cm^?2 year^?1) and relatively deep surface mixed layers (SML, 14–16 cm). The southwestern zone was characterized by lower MAR (0.05–0.08 g cm^?2 year^?1) and thinner SML (7–8 cm). In both areas, the long sediment mixing times in the SMLs (of 45–61 years) smoothed out the sedimentary records. The coastline sedimentary environments were characterized by higher MAR (0.30–0.57 g cm^?2 year^?1) and the sedimentary records displayed clear signatures of extreme climatic events such as the intensive rains in 1999 reported for La Coloma and the hurricanes Lili and Isodore in 2002. Our study shows that the application of the ²¹⁰Pb sediment dating method in dynamic costal zones is a challenging task but still may provide important information regarding sedimentation and mixing processes in the ecosystem.     

Cyclic sedimentation produced by fluctuations in meltwater discharge, tides and marine productivity in an Alaskan fjord

Glacimarine sediment deposited in the fjord adjacent to Muir Glacier in south-eastern Alaska consists of rhythmically laminated muds, stratified sandy mud, sand and gravelly mud facies. Cyclicity is recorded by gravelly mud facies deposited during winter by ice-rafting, black mud laminae formed by spring plankton blooms and variations in tidal rhythmite thickness and texture produced by the interaction of meltwater discharges and tidal currents in the macrotidal fjord. Regular cyclicity in laminae thickness is tested statistically by Fourier transform and can be attributed to a lunar tidal cycle control in the five cores collected up to 6 km from the sediment source. Cores close to the source can have additional laminae as a result of discharge fluctuations, and distal cores may lack full cycles because of variability in the plume path and attenuation with distance. Cyclic variations in sediment texture are recorded in magnetic susceptibility (MS) profiles of the cores. High MS values are produced by turbidite sand beds or by stratified sandy mud deposited by overflow plumes during peak summer meltwater discharge. Low values reflect muddy intervals deposited during periods of low meltwater discharge, such as during autumn and winter. Sediment accumulation rates measured by ²¹⁰Pb dating range from 82 cm year^–1, 2 km from the sediment source at the head of the fjord, to 16 cm year^–1, 6 km away. These rates are within the same range as average sediment accumulation rates determined from cyclic seasonal markers within the cores. These data show that, with careful documentation, annual cycles of glacimarine sediment accumulation can be detected within marine cores. Cores collected from the distal portion of the basin were deposited during the transition of Muir Glacier from a tidewater terminus ending in deep water to a terrestrial glacier with an ice-contact delta deposited in front of the terminus. This transition is recorded by a coarsening-upward sedimentary sequence formed by turbidite sands originating from the prograding delta above fine-grained, laminated basin fill deposited by turbid overflow plumes.     

Sedimentation rate variations and anthropogenic metal fluxes into Lake Constance sediments

Sedimentation rates were determined by²¹⁰Pb in three sediment cores from the main basin of Lake Constance. Rates vary from 0.094 to 0.133 g cm^?2y^?1, in agreement with previous determinations. A constant radionuclide flux (CR) model reveals long-term, quasi-synchronous fluctuations of sedimentation rate on the basin-wide scale. On average, the rate of sedimentation remains much the same from the beginning of this century indicating little effect of human activity in the watershed on fine-silt sediment supply. The anthropogenic fluxes of Zn, Pb, and Cd in the three cores are compared and the application of²¹⁰Pb as a heavy metal tracer in Lake Constance is examined. It seems to be a very good tracer for Pb, moderately good for Zn and Cu, and not useful for Cd.     

Determination of 137Cs activities in surface sediments and derived sediment accumulation rates in Thessaloniki Gulf, Greece

Surficial and core samples collected from the eastern Thessaloniki Gulf, located in the NW Aegean Sea, were analyzed for their sedimentation rate and inventories of ¹³⁷Cs and ²¹⁰Pb. The study of the spatial radionuclide dispersion in the specific region is essential for the assessment of marine pollution levels. The sedimentation rates were calculated from the vertical distribution of ¹³⁷Cs and excess ²¹⁰Pb in the sediment cores. The spatial distribution of ¹³⁷Cs was studied with respect to sediment characteristics such as the grain size of the sample and the organic carbon content. The activity concentrations were measured by means of gamma ray spectrometry using HPGe detector for ¹³⁷Cs and gross alpha counting using alpha counter for ²¹⁰Pb. The average sedimentation rate along the sediment cores is characterized by a consistent pattern and varies from 0.18?±?0.02 to 0.22?±?0.03?cm?year^?1. The ¹³⁷Cs inventories varied from 205?±?15 to 602?±?39?Bq?m^?2, while the ²¹⁰Pb inventories of the studied cores exhibited average value of 140?±?9?Bq?m^?2. Elevated ¹³⁷Cs activities were observed compared to certain Mediterranean marine areas; however, they were lower than the reported values in the Black Sea.     

Determination of recent deposition rates in Lake Constance with radioisotopic methods

Sedimentation rates were determined with the ²¹⁰Pb method in eight sediment cores from Lake Constance. The rate of deposition in the main basin (Obersee) varies from about 0.06 g cm^?2 y^?1 in the central part to 0.13 g cm^?2 y^?1 in the eastern part of the lake and then increases rapidly towards the Rhine delta. In the central lake area the rate of deposition has been approximately constant since 1900, and dating with the ²¹⁰Pb method is in good agreement with sedimentological observations. In the Konstanzer Trichter area, the deposition rate has been increasing since about 1955 as a result of eutrophication and subsequent high carbonate production. Dating with ¹³⁷Cs is fairly accurate for sediments deposited at a high rate, but is questionable for slowly accumulating ones. A positive correlation of ²¹⁰Pb fluxes and sedimentation rates indicates that ²¹⁰Pb flux into sediments follows the distribution pattern of solids. ²¹⁰Pb profiles in four sediment cores interpreted in terms of a constant flux model display synchronous fluctuations of the sedimentation rate; however, their relation to long-range particulate input variations remains to be proved. Sedimentation rates determined with the ²¹⁰Pb method were used to calculate recent nutrient and heavy metal fluxes. Anthropogenic fluxes of Zn and Pb are in the same range of magnitude as in other polluted areas in Europe and America.     

Estimating selenium removal by sedimentation from the Great Salt Lake,Utah

The mass of Se deposited annually to sediment in the Great Salt Lake (GSL) was estimated to determine the significance of sedimentation as a permanent Se removal mechanism. Lake sediment cores were used to qualitatively delineate sedimentation regions (very high to very low), estimate mass accumulation rates (MARs) and determine sediment Se concentrations. Sedimentation regions were defined by comparison of isopach contours of Holocene sediment thicknesses to linear sedimentation rates determined via analysis of ²¹⁰Pb, ²²⁶Ra, ⁷Be and ¹³⁷Cs activity in 20 short cores (<5 cm), yielding quantifiable results in 13 cores. MARs were developed via analysis of the same radioisotopes in eight long cores (>10 cm). These MARs in the upper 1–2 cm of each long core ranged from 0.019 to 0.105 g_sed/cm²/a. Surface sediment Se concentrations in the upper 1 or 2 cm of each long core ranged from 0.79 to 2.47 mg/kg. Representative MARs and Se concentrations were used to develop mean annual Se removal by sedimentation in the corresponding sedimentation region. The spatially integrated Se sedimentation rate was estimated to be 624 kg/a within a range of uncertainty between 285 and 960 kg/a. Comparison to annual Se loading and other potential removal processes suggests burial by sedimentation is not the primary removal process for Se from the GSL.     

Mixing rates in Shagawa Lake,Minnesota, sediments as determined from 106Ru profiles

Rates of surficial sediment mixing and sediment burial are measured in Shagawa Lake, Minnesota, using radionuclide tracers. Based on ¹⁰⁶Ru profiles in 16 cores, mixing rates average 13 cm²/yr within the upper 9 cm of sediment. Two other nuclides, ²¹⁰Pb and ¹³⁷Cs, provide further evidence for calculation of mixing rates. In addition, ²¹⁰Pb profiles estimate sediment burial rates (about 0.4 cm/yr). Mixing estimates are shown to be fairly insensitive both to uncertainties in sediment burial rates and to temporal patterns of ¹⁰⁶Ru deposition.     

Recent accretion in mangrove ecosystems based on137Cs and210Pb

Accretion rates were measured in fringe and basin mangrove forests in river and tidally dominated sites in Terminos Lagoon, Mexico, and a basin mangrove forest in Rookery Bay, Florida, USA. Accretion rates were determined using the radionuclides²¹⁰Pb and¹³⁷Cs. Consolidation-corrected accretion rates for the Rookery Bay cores, ranged from 1.4 to 1.7 mm yr^?1, with an average rate of 1.6 mm yr^?1. Rates at the Mexico sites ranged from 1.0 to 4.4 mm yr^?1, with an average of 2.4 mm yr^?1. Determination of rates in these mangrove forests was greatly affected by the consolidation corrections which decreased the apparent accretion rate by over 50% in one case. Accretion rates at basin sites compare favorably with a reported 1.4 to 1.6 mm yr^?1 rate of sea-level rise, indicating little or no subsidence at inland locations. Accretion rates in fringe sites are generally greater than basin sites, indicating greater subsidence rates in these sediments over longer time intervals.     

High-resolution210Pb dating of Lake Constance sediments: Stable lead in Lake Constance

Two sediment cores (BO90/13b and BO90/17b) from Lake Constance were investigated by-spectrometry for²¹⁰Pb,¹³⁴Cs,¹³⁷Cs,²⁴¹Am,²³⁴Th, and other members of the²³⁸U decay chain. The sediments were dated using the constant-flux model for²¹⁰Pb, and accumulation rates were determined. These range from 0.04 to 0.65g/cm ²/yr (BO90/13b) and 0.04 to 0.8g/cm ²/yr (BO90/17b), respectively. The mean accumulation rate amounts to 0.16g/cm ²/yr for both cores. The cores had already been dated by lamination counting and reconstruction of high-water events at the Limnological Institute at Constance, so that a very precise time scale was available. Both ages derived are in agreement within statistical error up to 1900, which means dating with the constant-flux model for²¹⁰Pb was confirmed up to that age. The position of the maxima of bomb cesium and americium confirm the stratigraphic and²¹⁰Pb datings. With²⁴¹Am a further radioactive isotope is available, which can, due to the half-life of²⁴¹Pu (t _1/2=14.4yr) be detected now by-spectrometry and can serve as an additional time indicator, the maximum being dated at 1963. By applying the various time scales, the depth profiles of stable lead and zinc of core BO90/13b were dated. Both heavy metals show a very significant maximum located beneath the layer of the maxima of bomb cesium and americium, showing that these maxima are older than those of the bomb isotopes. It is remarkable in this context that the maximum of zinc concentration occurs a little later than that of stable lead. Similar concentration profiles are observable in core BO90/17b and other, older sediment cores (CS6-CS10) on a transect across the lake. In contrast to a former assumption, the depth profile of stable lead in Lake Constance sediments does not reflect the anthropogenic gasoline lead emissions into the atmosphere for Germany, their maximum being dated at 1971.     

Small-scale spatial variations of natural radionuclide and trace metal distributions in sediments from the Hudson River estuary

Multiple sediment cores were collected in June 1994 in the turbidity maximum zone of the Hudson River estuary off Manhattan, New York. Results from X-radiography of the sediments and measurements of natural radionuclides (²³⁴Th,⁷Be, and²¹⁰Pb) and trace metals (Ag, Cd, Cu, Pb, and Zn) show significant spatial variability of sediment composition and structure and patchy distributions of radionuclides activities and trace metal concentrations in this small area (0.6 km × 0.5 km). Radionuclide and trace metal analyses confirm prior work (Olsen et al. 1978; Olsen et al. 1981; Hirschberg et al. 1996) that show the western margin area of the river acts as a repository of these chemical constituents at least for the short-term period (0.5–1 yr), and the mid-channel area is not a depositional area for sediments and associated chemical constituents.⁷Be profiles reveal short-term sediment deposition rates ranging from 6 cm yr^?1 to 26 cm yr^?1 in the western margin area. Significant spatial variations in excess²³⁴Th and⁷Be inventories (up to a factor of 10 and 5 for²³⁴Th and⁷Be, respectively) are found in the western margin depositional area, although the inventories are balanced, on average, with in situ production in water column and atmospheric supply. The spatial variation of surficial excess²¹⁰Pb and trace metal concentrations in depositional areas of the western margin are ≤10% for Ag, Cu, Pb, and Zn and 29% for Cd. However, the variations in the transition zone range from 28% to 93%. This variability is likely related to variations in tidal current velocity, bottom shear stress, and river channel morphology.     

210Pb balance in Long Island Sound

A material balance is constructed for excess ²¹⁰Pb (relative to ²²⁶Ra) as a test of the retentivity of Long Island Sound for a reactive heavy metal. Excess ²¹⁰Pb is supplied to Long Island Sound chiefly by direct atmospheric deposition [1 ± 0.2(dis·min^?1)cm^?2·yr^?1]. Rivers supply less than 20% of the atmospheric flux, and other inputs, from open ocean waters, ²²⁶Ra decay, groundwater seepage, and sewage discharge, appear to be negligible. The total input of excess ²¹⁰Pb represents approximately the flux required to maintain the inventory of excess ²¹⁰Pb measured in sediment cores from central Long Island Sound; that is, excess ²¹⁰Pb is lost from Long Island Sound chiefly by radioactive decay. The retention of excess ²¹⁰Pb within Long Island Sound is achieved in two steps: a rapid removal of soluble ²¹⁰Pb onto suspended particles and the ongoing entrapment of particles in the basin by the residual bottom-water influx from the east.     

Silica distribution in interstitial waters and sediments from the southeastern pacific

Analyses for silica in the interstitial water of five cores from the southeast Pacific are presented. Silica is enriched in these interstitial waters resulting in a vertical flux of silica of between 10 and 50 μmol cm^?2 yr^?1 from the sediment into the overlaying seawater. This flux is generated by the dissolution of biogenic silica, the dissolution of which is increased in areas of bottom water turbulence. The Si, Al and calculated opal (Leinen, 1977) contents of the bulk sediment of these cores are also presented. Small scale variations over depth intervals of tens of centimetres are present as a result of chaning conditions of sedimentation.     

On the formation of perylene in recent sediments: kinetic models

The concentration of perylene increases regularly with depth in a radiometrically dated sediment core from Mountain Pond, Coburn Mountain, Maine, U.S.A. This concentration profile was fit to simple reaction kinetic models which indicate that perylene forms in this core either by a first order reaction characterized by a rate constant of 0.012 yr^?1 or by a second order reaction characterized by a rate constant of 0.0013 nmole^?1g yr^?1.     

Accretion and canal impacts in a rapidly subsiding wetland. I.137Cs and210Pb techniques

The influence of canals on vertical marsh accretion, including mineral sediment and organic matter accumulation, was evaluated at three locations along the Louisiana coast representing different geographic regions. The isotopes²¹⁰Pb and¹⁵⁷Cs were used to determine vertical accretion along transects representing a canal and a control site. Rapid rates of vertical accretion were measured at all sites and ranged from 0.47 cm yr^?1 to 0.90 cm yr^?1. Results indicated that there was no measurable effect of canals on marsh accretionary processes. In general, greater variation in vertical accretion, including mineral sediment deposition and organic matter accumulation, was observed between geographical regions than between canal and control sites within a region. Statistical analysis of data suggest that any difference between canal and control site would be less than 0.20 cm yr^?1. Such a change in marsh surface-water level relationships as a result of any canal influence on marsh accretionary processes would be less than reported eustatic sea-level rise for the Gulf of Mexico. Results suggest that any change in the marsh surface-water level relationship could be the influence of canals on local hydrology, resulting in increased water level rather than any appreciable reduction in accretionary processes. Such changes in hydrology under certain conditions could stress vegetation, resulting in marsh deterioration.