Effects of weir management on marsh loss,Marsh Island,Louisiana, USA

Weirs are low-level dams traditionally used in Louisiana's coastal marshes to improve habitat for ducks and furbearers. Currently, some workers hope that weirs may reduce marsh loss, whereas others fear that weirs may accelerate marsh loss. Parts of Marsh Island, Louisiana, have been weir-managed since 1958 to improve duck and furbearer habitat. Using aerial photographs, marsh loss that occurred between 1957 and 1983 in a 2922-ha weir-managed area was compared to that in a 2365-ha unmanaged area. Marsh loss was 0.38%/yr in the weir-managed area, and 0.35%/yr in the unmanaged area. Because marsh loss in the two areas differed less than 0.19%/yr, it was concluded that weirs did not affect marsh loss. The increase in open water between 1957 and 1983 did not result from the expansion of lakes or bayous. Rather, solid marsh converted to broken marsh, and the amount of vegetation within previously existing broken marsh decreased. Solid marsh farthest from large lakes and bayous, and adjacent to existing broken marsh, seemed more likely to break up. Marsh Island has few canals; therefore, marsh loss resulted primarily from natural processes. Weirs may have different effects under different hydrological conditions; additional studies are needed before generalizations regarding weirs and marsh loss can be made.     

Backfilling canals to mitigate Wetland dredging in Louisiana coastal marshes

Returning canal spoil banks into canals, or backfilling, is used in Louisiana marshes to mitigate damage caused by dredging for oil and gas extraction. We evaluated 33 canals backfilled through July 1984 to assess the success of habitat restoration. We determined restoration success by examining canal depth, vegetation recolonization, and regraded spoil bank soils after backfilling. Restoration success depended on: marsh type, canal location, canal age, marsh soil characteristics, the presence or absence of a plug at the canal mouth, whether mitigation was on- or off-site, and dredge operator performance.Backfilling reduced median canal depth from 2.4 to 1.1 m, restored marsh vegetation on the backfilled spoil bank, but did not restore emergent marsh vegetation in the canal because of the lack of sufficient spoil material to fill the canal and time. Median percentage of cover of marsh vegetation on the canal spoil banks was 51.6%. Median percentage of cover in the canal was 0.7%. The organic matter and water content of spoil bank soils were restored to values intermediate between spoil bank levels and predredging marsh conditions.The average percentage of cover of marsh vegetation on backfilled spoil banks was highest in intermediate marshes (68.6%) and lowest in fresh (34.7%) and salt marshes (33.9%). Average canal depth was greatest in intermediate marshes (1.50 m) and least in fresh marshes (0.85 m). Canals backfilled in the Chenier Plain of western Louisiana were shallower (average depth = 0.61 m) than in the eastern Deltaic Plain (mean depth range = 1.08 to 1.30 m), probably because of differences in sediment type, lower subsidence rate, and lower tidal exchange in the Chenier Plain. Canals backfilled in marshes with more organic soils were deeper, probably as a result of greater loss of spoil volume caused by oxidation of soil organic matter. Canals ten or more years old at the time of backfilling had shallower depths after backfilling. Depths varied widely among canals backfilled within ten years of dredging. Canal size showed no relationship to canal depth or amount of vegetation reestablished. Plugged canals contained more marsh reestablished in the canal and much greater chance of colonization by submerged aquatic vegetation compared with unplugged canals. Dredge operator skill was important in leveling spoil banks to allow vegetation reestablishment. Wide variation in dredge performance led to differing success of vegetation restoration.Complete reestablishment of the vegetation was not a necessary condition for successful restoration. In addition to providing vegetation reestablishment, backfilling canals resulted in shallow water areas with higher habitat value for benthos, fish, and waterfowl than unfilled canals. Spoil bank removal also may help restore water flow patterns over the marsh surface. Increased backfilling for wetland mitigation and restoration is recommended.     

Reducing the Effects of Dredged Material Levees on Coastal Marsh Function: Sediment Deposition and Nekton Utilization

Dredged material levees in coastal Louisiana are normally associated with pipeline canals or, more frequently, canals dredged through the wetlands to allow access to drilling locations for mineral extraction. The hydrologic impact on marshes behind the levee is of concern to coastal resource managers because of the potential impact on sediment transport and deposition, and the effect on estuarine organism access to valuable nursery habitat. This study examined the effects of gaps in dredged material levees, compared to continuous levees and natural channel banks, on these two aspects of marsh function. Field studies for sediment deposition were conducted biweekly for a year, and nekton samples were collected in spring and fall. Variation in nekton density among study arears and landscape types was great in part because of the inherent sampling gear issues and in part because of differences in characteristics among areas. Nekton densities were generally greater in natural compared to leveed and gapped landscapes. Differences in landscape type did not explain patterns in sediment deposition. The gaps examined appear to be too restrictive of marsh flooding to provide efficient movements of floodwaters onto the marsh during moderate flooding events. The “trapping” effect of the levees increases sediment deposition during extreme events. Gapping material levees may be an effective method of partially restoring upper marsh connection to nekton, but this method may work best in lower elevation marshes where nekton use is greater.     

Effect of river sediment on phosphorus chemistry of similarly aged natural and created wetlands in the Atchafalaya Delta, Louisiana, USA

The goal of wetland creation is to produce an artificial wetland that functions as a natural wetland. Studies comparing created wetlands to similarly aged natural wetlands provide important information about creation techniques and their improvement so as to attain that goal. We hypothesized that differences in sediment phosphorus accretion, deposition, and chemistry between created and natural wetlands in the Atchafalaya Delta, Louisiana, USA were a function of creation technique and natural river processes. Sediment deposition was determined with feldspar marker horizons located in created and natural wetlands belonging to three age classes (<3, 5-10, and 15-20 yr old). Phosphorus fractions were measured in these deposited sediments and in suspended and bedload sediment from the Atchafalaya River. Bedload sediment had significantly lower iron- and aluminum-bound, reductant-soluble, and total phosphorus than suspended sediment due to its high sand percentage. This result indicates that wetlands artificially created in the Atchafalaya Delta using bedload sediment will initially differ from natural wetlands of the same age. Even so, similarities between the mudflat stratum of the <1- to 3-yr-old created wetland and the mudflat stratum of the 15- to 20-yr-old natural wetland support the contention that created wetlands in the Atchafalaya Delta can develop natural characteristics through the deposition of river suspended sediment. Differences between three created wetland strata, the 15- to 20-yr-old willow stratum and the <1- to 3-yr-old willow and mixed marsh strata, and their natural counterparts were linked to design elements of the created wetlands that prevented the direct deposition of the river's suspended sediment.     

Bird Use of Restoration and Reference Marshes Within the Barn Island Wildlife Management Area, Stonington, Connecticut, USA

/ Tidal marshes have been actively restored in Connecticut for nearly 20 years, but evaluations of these projects are typically based solely on observations of vegetation change. A formerly impounded valley marsh at the Barn Island Wildlife Management Area is a notable exception; previous research at this site has also included assessments of primary productivity, macroinvertebrates, and use by fishes. To determine the effects of marsh restoration on higher trophic levels, we monitored bird use at five sites within the Barn Island complex, including both restoration and reference marshes. Use by summer bird populations within fixed plots was monitored over two years at all sites. Our principal focus was Impoundment One, a previously impounded valley marsh reopened to full tidal exchange in 1982. This restoration site supported a greater abundance of wetland birds than our other sites, indicating that it is at least equivalent to reference marshes within the same system for this ecological function. Moreover, the species richness of birds and their frequency of occurrence at Impoundment One was greater than at 11 other estuarine marshes in southeastern Connecticut surveyed in a related investigation. A second marsh, under restoration for approximately ten years, appears to be developing in a similar fashion. These results complement previous studies on vegetation, macroinvertebrates, and fish use in this system to show that, over time, the reintroduction of tidal flooding can effectively restore important ecological functions to previously impounded tidal marshes.KEY WORDS: Estuarine; Tidal marsh; Wetland birds; Restoration     

In situ push-pull method to determine ground water denitrification in riparian zones

To quantify ground water denitrification in discrete locations of riparian aquifers, we modified and evaluated an in situ method based on conservative tracers and 15N-enriched nitrate. Ground water was "pushed" (i.e., injected) into a mini-piezometer and then "pulled" (i.e., extracted) from the same mini-piezometer after an incubation period. This push-pull method was applied in replicate mini-piezometers at two Rhode Island riparian sites, one fresh water and one brackish water. Conservative tracer pretests were conducted to determine incubation periods, ranging from 5 to 120 h, to optimize recovery of introduced plumes. For nitrate push-pull tests, we used two conservative tracers, sulfur hexafluoride and bromide, to provide insight into plume recovery. The two conservative tracers behaved similarly. The dosing solutions were amended with 15N-enriched nitrate that enabled us to quantify the mass of denitrification gases generated during the incubation period. The in situ push-pull method detected substantial denitrification rates at a site where we had previously observed high denitrification rates. At our brackish site, we found high rates of ground water denitrification in marsh locations and minimal denitrification in soils fringing the marsh. The push-pull method can provide useful insights into spatial and temporal patterns of denitrification in riparian zones. The method is robust and results are not seriously affected by dilution or degassing from ground water to soil air. In conjunction with measurements of ground water flow-paths, this method holds promise for evaluating the influence of site and management factors on the ground water nitrate removal capacity of riparian zones.     

Selenium distribution and fluxes in intertidal wetlands, San Francisco Bay, California

Selenium (Se) concentrations exceeding ecological guidelines for sediments and suspended particulate matter (SPM) have been observed in the northern reach of the San Francisco Bay estuary. Longterm availability of elevated Se in wetland sediments depends in part on the fluxes controlling Se distribution. The relative contribution of sedimentary vs. post-depositional Se fluxes in two San Francisco Bay intertidal wetlands was estimated. Selenium concentrations on surface wetland sediments were compared with levels on SPM, and with previously established background levels in San Francisco Bay sediments. Sediment Se fluxes to the wetlands were measured directly using sediment traps. Although dissolved Se concentrations are higher than particulate Se concentrations in San Francisco Bay water, sediment input into the system provides the major flux of Se. Strong correlation between Se and C on SPM (r2 = 0.81) indicates the importance of organic particulate deposition. Dependence on sediment texture was qualitatively established by measuring Se on particle-size separates. Normalization to Al showed that 65% of Se spatial variability is related to sediment texture. Selenium is further enriched in the marsh via post-depositional inputs, probably due to in situ adsorption from overlying water and chemical reduction. According to sediment flux measurements, enrichment in the marsh is equivalent to 20 to 25% of the particulate Se flux, thereby defining the marsh as a Se sink. These findings highlight the need for more intensive monitoring of SPM as the major source of Se to intertidal wetlands.     

In Situ Burning Restores the Ecological Function and Structure of an Oil-Impacted Coastal Marsh

As the use of in situ burning for oil spill remediation in coastal wetlands accelerates, the capacity of this procedure to restore the ecological structure and function of oil-impacted wetlands becomes increasingly important. Thus, our research focused on evaluating the functional and structural recovery of a coastal marsh in South Louisiana to an in situ burn following a Hurricane Katrina-induced oil spill. Permanent sampling plots were set up to monitor marsh recovery in the oiled and burned areas as well as non-oiled and non-burned (reference) marshes. Plots were monitored for species composition, stem density, above- and belowground productivity, marsh resiliency, soil chemistry, soil residual oil, and organic matter decomposition. The burn removed the majority of the oil from the marsh, and structurally the marsh recovered rapidly. Plant biomass and species composition returned to control levels within 9 months; however, species richness remained somewhat lower in the oiled and burned areas compared to the reference areas. Recovery of ecological function was also rapid following the in situ burn. Aboveground and belowground plant productivity recovered within one growing season, and although decomposition rates were initially higher in the oiled areas, over time they became equivalent to those in reference sites. Also, marsh resiliency, i.e., the rate of recovery from our applied disturbances, was not affected by the in situ burn. We conclude that in situ burning is an effective way to remove oil and allow ecosystem recovery in coastal marshes.     

Plant Community Composition and Biomass in Gulf Coast Chenier Plain Marshes: Responses to Winter Burning and Structural Marsh Management

Many marshes in the Gulf Coast Chenier Plain, USA, are managed through a combination of fall or winter burning and structural marsh management (i.e., levees and water control structures; hereafter SMM). The goals of winter burning and SMM include improvement of waterfowl and furbearer habitat, maintenance of historic isohaline lines, and creation and maintenance of emergent wetlands. Although management practices are intended to influence the plant community, effects of these practices on primary productivity have not been investigated. Marsh processes, such as vertical accretion and nutrient cycles, which depend on primary productivity may be affected directly or indirectly by winter burning or SMM. We compared Chenier Plain plant community characteristics (species composition and above- and belowground biomass) in experimentally burned and unburned control plots within impounded and unimpounded marshes at 7 months (1996), 19 months (1997), and 31 months (1998) after burning. Burning and SMM did not affect number of plant species or species composition in our experiment. For all three years combined, burned plots had higher live above-ground biomass than did unburned plots. Total above-ground and dead above-ground biomasses were reduced in burned plots for two and three years, respectively, compared to those in unburned control plots. During all three years, belowground biomass was lower in impounded than in unimpounded marshes but did not differ between burn treatments. Our results clearly indicate that current marsh management practices influence marsh primary productivity and may impact other marsh processes, such as vertical accretion, that are dependent on organic matter accumulation and decay.     

Nonpoint Source Loading Rates From Selected Land Uses1

ABSTRACT: Loading rates derived from monitoring natural runoff from selected land uses are compared. Land uses selected for evaluation are construction sites, barnyards, and agriculture (dairying). Runoff volumes, sediment, and nutrient fractions were monitored and expressed as areal loadings for comparison purposes. Sediment yield and total phosphorus (total P) loss was directly proportional to runoff (m³/ha). In decreasing order, the loadings for sediment and total P were as follows: construction site > barnyard > general dairying. Runoff from the barnyard area was approximately 10 times higher in soluble phosphorus and ammonium nitrogen than the other land uses under investigation. Areal loss for nitrate nitrogen was highest from the construction site and was attributed to the higher volume of runoff per unit area. Results show that barnyards in a dairying watershed are potentially a major source of sediment and nutrients, especially those dissolved fractions which have the potential for immediate water quality impacts. Relative to general agricultural land, urban construction sites also appear to be a major source of sediment and nutrients. As with barnyard sites, however, the effect of such sites on water quality likely depends on proximity to surface water bodies and other watershed characteristics affecting delivery ratios of contaminants.     

SEDIMENT RESUSPENSION AND DRAWDOWN IN A WATER SUPPLY RESERVOIR1

ABSTRACT: The magnitudes and patterns of sediment resuspension are assessed in Cannonsville Reservoir, New York, to quantify and characterize this internal source of sediment. The assessment is based on analyses of sediment trap collections from 10 sites over the spring to fall interval of two years. Temporal and spatial patterns in sediment deposition are demonstrated to be driven by resuspension/redeposition processes. Sediment that had been resuspended and redeposited represented 80 to 96 percent, on average, of the depositing solids collected along the main axis of the lake. About 90 percent of the redeposited sediment was inorganic. Increased resuspension caused by drawdown of the reservoir surface and fall turnover resulted in 10 to 50‐fold increases in deposition rates compared to levels observed when the reservoir was full and strongly thermally stratified. Elevated levels of redeposition from resuspension in the reservoir have been driven by both higher water column concentrations of suspended solids and settling velocities. Recurring longitudinal and lateral gradients in resuspension are delineated, establishing that resuspended solids are transported from the riverine to the lacustrine zone and from near‐shore to pelagic areas. Resuspension is demonstrated to cause increases in inanimate particle (tripton) concentrations. Higher tripton levels have been observed in years with greater drawdown. Water quality impacts of the resuspension phenomenon are considered.     

Recovery Trajectories After In Situ Burning of an Oiled Wetland in Coastal Louisiana,USA

The high degree of physical disturbance associated with conventional response options to oil spills in wetlands is driving the investigation of alternative cleanup methodologies. In March 1995, a spill of gas condensate in a brackish marsh at Rockefeller Wildlife Refuge in southwestern Louisiana was remediated through the use of in situ burning. An assessment of vegetation recovery was initiated in three treatment marshes: (1) oil-impacted and burned, (2) oil impacted and unburned, and (3) a nonoiled unburned reference. We compared percent cover, stem density, and biomass in the treatment marshes to define ecological recovery of the marsh vegetation and soil hydrocarbon content to determine the efficacy of in situ burning as a cleanup technique. Burning led to a rapid decrease in soil hydrocarbon concentrations in the impacted-and-burned marsh to background levels by the end of the first growing season. Although a management fire accidentally burned the oil-impacted-and-unburned and reference marshes in December 1995, stem density, live biomass, and total percent cover values in the oil-impacted-and-burned marsh were equivalent to those in the other treatment marshes after three years. In addition, plant community composition within the oil-impacted-and-burned marsh was similar to the codominant mix of the grasses Distichlis spicata (salt grass) and Spartina patens (wire grass) characteristic of the surrounding marsh after the same time period. Rapid recovery of the oil-impacted-and-unburned marsh was likely due to lower initial hydrocarbon exposure. Water levels inundating the soil surface of this grass-dominated marsh and the timing of the in situ burn early in the growing season were important factors contributing to the rapid recovery of this wetland. The results of this in situ burn evaluation support the conclusion that burning, under the proper conditions, can be relied upon as an effective cleanup response to hydrocarbon spills in herbaceous wetlands.     

Salt Marsh Diking and Restoration: Biogeochemical Implications of Altered Wetland Hydrology

Experimental short-term desalination and drainage of salt marsh cores in greenhouse microcosms caused Spartina production to increase after one growing season, reflecting decreased salt stress and sulfide toxicity. However, production thereafter declined, likely due to pyrite oxidation and acidification in drained treatments and sulfide accumulation in waterlogged treatments. A survey of longer-term (decadal) effects of diking on peat composition of Cape Cod, Massachusetts, USA, marshes revealed acidification, Fe(II) mobilization, and decreased organic content in drained sites. Despite the aerobic decomposition of organic matter, abundant nutrients remained as sorbed NH₄ and mineral-bound PO₄. In diked, seasonally waterlogged sites, porewater alkalinity, sulfide, ammonium and orthophosphate were much lower, and organic solids higher, than in adjacent natural marsh. Seawater was added to cores from diked marshes to study the effects of tidal restoration. Salination of the drained peat increased porewater pH, alkalinity, ammonium, orthophosphate, Fe, and Al; copious ammonium N, and Fe(II) for sulfide precipitation favored Spartina growth. Salination of diked–waterlogged peat increased sulfate reduction and caused 6–8 cm of sediment subsidence. The resulting increase in porewater sulfides and waterlogging decreased vigor of transplanted Spartina alterniflora. Results indicate that seawater restoration should proceed cautiously to avoid nutrient loading of surface waters in drained sites or sulfide toxicity in diked–waterlogged marshes.     

Resource protection for waterbirds in Chesapeake bay

Many living resources in the Chesapeake Bay estuary have deteriorated over the past 50 years. As a result, many governmental committees, task forces, and management plans have been established. Most of the recommendations for implementing a bay cleanup focus on reducing sediments and nutrient flow into the watershed. We emphasize that habitat requirements other than water quality are necessary for the recovery of much of the bay's avian wildlife, and we use a waterbird example as illustration. Some of these needs are: (1) protection of fast-eroding islands, or creation of new ones by dredge deposition to improve nesting habitat for American black ducks(Anas rubripes), great blue herons(Ardea herodias), and other associated wading birds; (2) conservation of remaining brackish marshes, especially near riparian areas, for feeding black ducks, wading birds, and wood ducks(Aix sponsa); (3) establishment of sanctuaries in open-water, littoral zones to protect feeding and/or roosting areas for diving ducks such as canvasbacks(Aythya valisineria) and redheads(Aythya americana), and for bald eagles(Haliaeetus leucocephalus); and (4) limitation of disturbance by boaters around nesting islands and open-water feeding areas. Land (or water) protection measures for waterbirds need to include units at several different spatial scales, ranging from “points” (e.g., a colony site) to large-area resources (e.g., a marsh or tributary for feeding). Planning to conserve large areas of both land and water can be achieved following a biosphere reserve model. Existing interagency committees in the Chesapeake Bay Program could be more effective in developing such a model for wildlife and fisheries resources.     

Louisiana wetland loss: A regional water management approach to the problem

Loss of Louisiana's coastal wetlands has reached catastrophic proportions. The loss rate is approximately 150 km²/yr (100 acres/day) and is increasing exponentially. Total wetland loss since the turn of the century has been almost 0.5 million ha (1.1 million acres) and represents an area larger than Rhode Island. The physical cause of the problem lies in man's attempts to control the Mississippi River's flooding, while enhancing navigation and extracting minerals. Levee systems and control structures confine sediments that once nourished the wetlands to the river channel. As a consequence, the ultimate sediment deposition is in deep Gulf waters off the Louisiana coast. The lack of sediment input to the interdistributary wetlands results in an accretion deficit. Natural and human-induced subsidence exceeds accretion so that the wetlands sink below sea level and convert to water. The solution is to provide a thin veneer of sediment (approximately 0.6 cm/yr; an average of 1450 g m^?2 yr^?1) over the coastal marshes and swamps and thus prevent the submergence of vegetation. The sediment source is the Mississippi River system. Calculations show that 9.2% of the river's annual suspended sediment load would be required to sustain the deltaic plain wetlands. It should be distributed during the six high-water months (December–June) through as disaggregated a network as possible. The problem is one of distribution: how can the maximum acres of marsh be nourished with the least cost? At present, the river is managed through federal policy for the benefit of navigation and flood control. A new policy structure, recognizing the new role for the river-sediment distribution, is recommended.     

An evaluation of the impact of persistent water level changes on the areal extent of Georgian Bay/North Channel marshlands

A conceptual mathematical model has recently been devised to assist environmental managers in predicting the impact on coastal marsh areas of long-term changes in water levels. The model considers such impact solely in terms of the geometry of the confining basin, the change in ambient water level, and the maximum depth for which bottom-rooted emergent vegetation is present. This model is applied to 17 shoreline marshes of various shapes in the Georgian Bay/North Channel region of the Great Lakes.Model outputs of predicted maximum and minimum marsh area subsequent to changes in long-term levels are compared to marsh areas measured from available historical air photos dating from 1935 to 1985. The results of such comparisons indicate that such a geometric model, despite its neglect of the biological complexities of marsh ecology, can serve as a valuable tool for assessing the range of impacts of both natural and man-made changes in long-term ambient water levels on shoreline marshes.     

Accretion of Nutrients and Sediment by a Constructed Stormwater Treatment Wetland in the Lake Tahoe Basin

This unique study evaluates the cumulative 16‐year lifetime performance of a wetland retention basin designed to treat stormwater runoff. Sediment cores were extracted prior to basin excavation and restoration to evaluate accretion rates and the origin of materials, retention characteristics of fine particulate matter, and overall pollutant removal efficiency. The sediment and organic layers together accreted 3.2 cm of depth per year, and 7.0 kg/m²/yr of inorganic material. Average annual accretion rates in g/m²/yr were as follows: C, 280; N, 17.7; P, 3.74; S, 3.80; Fe, 194; Mn, 2.68; Ca, 30.8; Mg, 30.7; K, 12.2; Na, 2.54; Zn, 0.858; Cu, 0.203; and B, 0.03. The accretion of C, N, P and sediment was comparable to nonwastewater treatment wetlands, overall, and relatively efficient for stormwater treatment wetlands. Comparison of particle size distribution between sediment cores and suspended solids in stormwater runoff indicated the wetland was effective at removing fine particles, with sediment cores containing 25% clay and 56% silt. A majority of the accretion of most metals and P could be attributed to efficient trapping of allochthonous material, while over half the accretion of C and N could be attributed to accumulation of autochthonous organic matter. Stormwater treatment was shown to be effective when physical properties of a retention basin are combined with the biological processes of a wetland, although sediment accretion can be relatively rapid.     

A COMPARISON OF SURFACE RUNOFF AND SEDIMENT YIELDS FROM LOW- AND HIGH-SEVERITY SITE PREPARATION BURNS1

ABSTRACI:. Slash burning is a common site preparation technique used after timber harvest throughout the Southeastern United States. Little quantitative information exists on the hydrologic response to burn severity. This study compared the effects of low-severity and high-severity burns on runoff and sediment yields during rainfall simulation and during natural rainfall in the Southern Appalachian Mountains. Fire severity was largely determined by moisture conditions of the forest floor prior to ignition. Runoff and sediment yield variability was high between plots within the same treatment area due to differences in forest floor characteristics and infiltration rates. Conditions of high-severity resulted when burning was conducted with relatively dry fuels. Sediment yields were 40-times greater for the high-severity treatment areas than the low-severity treatment areas.     

Facilitating Sediment Budget Construction for Land Management Applications

Sediment budgets describe the production, transport, deposition and export of sediment in a catchment, and thus provide information useful for planning soil conservation, restoration and monitoring programmes and for evaluating existing and future environmental impacts. Five examples from New Zealand illustrate a variety of sediment budgeting approaches and their use in achieving goals relevant to land-use planning and management. Appropriate sediment budgeting strategies can be selected for a particular application only through careful consideration of the kinds of decisions that will follow from budgeting results. Sediment budget projects can be designed to incorporate the close co-operation between technical experts and clients that results in the most useful sediment budgets.     

Salt marsh vegetation change in response to tidal restriction

Vegetation change in response to restriction of the normal tidal prism of six Connecticut salt marshes is documented. Tidal flow at the study sites was restricted with tide gates and associated causeways and dikes for purposes of flood protection, mosquito control, and/or salt hay farming. One study site has been under a regime of reduced tidal flow since colonial times, while the duration of restriction at the other sites ranges from less than ten years to several decades. The data indicate that with tidal restriction there is a substantial reduction in soil water salinity, lowering of the water table level, as well as a relative drop in the marsh surface elevation. These factors are considered to favor the establishment and spread ofPhragmites australis (common reed grass) and other less salt-tolerant species, with an attendant loss ofSpartina-dominated marsh. Based on detailed vegetation mapping of the study sites, a generalized scheme is presented to describe the sequence of vegetation change from typicalSpartina- toPhragmites-dominated marshes. The restoration of thesePhragmites systems is feasible following the reintroduction of tidal flow. At several sites dominated byPhragmites, tidal flow was reintroduced after two decades of continuous restriction, resulting in a marked reduction inPhragmites height and the reestablishment of typical salt marsh vegetation along creekbanks. It is suggested that large-scale restoration efforts be initiated in order that these degraded systems once again assume their roles within the salt marsh-estuarine ecosystem.