Characterizing land surface erosion from cesium-137 profiles in lake and reservoir sediments

Recognition of the threat to the sustainable use of the earth's resources posed by soil erosion and associated off-site sedimentation has generated an increasing need for reliable information on global rates of soil loss. Existing methods of assessing rates of soil loss across large areas possess many limitations and there is a need to explore alternative approaches to characterizing land surface erosion at the regional and global scale. The downcore profiles of 137Cs activity available for numerous lakes and reservoirs located in different areas of the world can be used to provide information on land surface erosion within the upstream catchments. The rate of decline of 137Cs activity toward the surface of the sediment deposited in a lake or reservoir can be used to estimate the rate of surface lowering associated with eroding areas within the upstream catchment, and the concentration of 137Cs in recently deposited sediment provides a basis for estimating the relative importance of surface and channel, gully, and/or subsurface erosion as a source of the deposited sediment. The approach has been tested using 137Cs data from several lakes and reservoirs in southern England and China, spanning a wide range of specific suspended sediment yield. The results obtained are consistent with other independent evidence of erosion rates and sediment sources within the lake and reservoir catchments and confirm the validity of the overall approach. The approach appears to offer valuable potential for characterizing land surface erosion, particularly in terms of its ability to provide information on the rate of surface lowering associated with the eroding areas, rather than an average rate of lowering for the entire catchment surface.     

Legacy Effects of Colonial Millponds on Floodplain Sedimentation,Bank Erosion,and Channel Morphology,Mid‐Atlantic,USA1

Abstract: Many rivers and streams of the Mid‐Atlantic Region, United States (U.S.) have been altered by postcolonial floodplain sedimentation (legacy sediment) associated with numerous milldams. Little Conestoga Creek, Pennsylvania, a tributary to the Susquehanna River and the Chesapeake Bay, is one of these streams. Floodplain sedimentation rates, bank erosion rates, and channel morphology were measured annually during 2004‐2007 at five sites along a 28‐km length of Little Conestoga Creek with nine colonial era milldams (one dam was still in place in 2007). This study was part of a larger cooperative effort to quantify floodplain sedimentation, bank erosion, and channel morphology in a high sediment yielding region of the Chesapeake Bay watershed. Data from the five sites were used to estimate the annual volume and mass of sediment stored on the floodplain and eroded from the banks for 14 segments along the 28‐km length of creek. A bank and floodplain reach based sediment budget (sediment budget) was constructed for the 28 km by summing the net volume of sediment deposited and eroded from each segment. Mean floodplain sedimentation rates for Little Conestoga Creek were variable, with erosion at one upstream site (?5 mm/year) to deposition at the other four sites (highest = 11 mm/year) despite over a meter of floodplain aggradation from postcolonial sedimentation. Mean bank erosion rates range between 29 and 163 mm/year among the five sites. Bank height increased 1 m for every 10.6 m of channel width, from upstream to downstream (R² = 0.79, p < 0.0001) resulting in progressively lowered hydraulic connectivity between the channel and the floodplain. Floodplain sedimentation and bank erosion rates also appear to be affected by the proximity of the segments to one existing milldam, which promotes deposition upstream and scouring downstream. The floodplain and bank along the 28‐km reach produced a net mean sediment loss of 5,634 Mg/year for 2004‐2007, indicating that bank erosion was exceeding floodplain sedimentation. In particular, the three segments between the existing dam and the confluence with the Conestoga River (32% of the studied reach) account for 97% of the measured net sediment budget. Future research directed at understanding channel equilibria should facilitate efforts to reduce the sediment impacts of dam removal and legacy sediment.     

RESERVOIR SEDIMENTATION RATES LINKED TO LONG-TERM CHANGES IN AGRICULTURAL LANI) USE1

ABSTRACT: Long-term land use and reservoir sedimentation were quantified and linked in a small agricultural reservoir-watershed system without having historical data. Land use was determined from a time sequence of aerial photographs, and reservoir sedimentation was determined from cores with ¹³⁷Cs dating techniques. They were linked by relating sediment deposition to potential sediment production which was determined by the Universal Soil Loss Equation and by SCS estimates for gullied land. Sediment cores were collected from Tecumseh Lake, a 55-ha reservoir with a 1,189-ha agricultural watershed, constructed in 1934 in central Oklahoma. Reservoir sediment deposition decreased from an average of 5,933 Mg/yr from 1934 to 1954, to 3,179 Mg/yr from 1954 to 1962, and finally to 1,017 Mg/yr from 1962 to 1987. Potential sediment production decreased from an average of 29,892 to 11,122 and then to 3,589 Mg/yr for the same time periods as above, respectively. Reductions in deposition and sediment production corresponded to reductions in cultivated and abandoned cropland which became perennial pasture. Together, cultivated and abandoned cropland accounted for 59 percent of the watershed in 1937, 24 percent in 1954, and 10 percent in 1962. Roadway erosion, stream bank erosion, stored stream channel sediment, and long-term precipitation were considered, but none seemed to play a significant role in changing sediment deposition rates. Instead, the dominant factor was the conversion of fields to perennial pastures. The effect of conservation measures on reservoir sedimentation can now be quantified for many reservoirs where historical data is not available.     

Recent Changes of Sediment Yield in the Upper Yangtze, China

/ Reservoir sedimentation is one of the many environmental problems associated with the Three Gorges Project in China. The rate and characteristics of sedimentation that directly affect the operating life of the reservoir are closely related to soil erosion and sediment transport dynamics in the upstream catchment and to the ability to manage the throughput of sediment-laden waters. The recent changes in sediment yield were examined using gauging data from 187 stations of varying sizes from less than 100 km2 to larger than 1,000,000 km2 in the Upper Yangtze basin between 1956 and 1987. Whereas many previous studies have concentrated on the trends in the main channel of the Yangtze, the distributed pattern of changes across the whole catchment is complex. Results from time series analysis indicate ten stations, mainly located in the Dadu and Wu tributaries (with a total incremental catchment area of 78,963 km2) have shown increasing trajectories of sediment yield, and six stations, located in the upper Jialing and Tuo tributaries (with a total incremental area of 27,816 km2) have experienced decreasing trajectories. By dividing the time series into three components, it is possible to map significant decadal changes in sediment yields that can be related to phases of deforestation and the construction of water conservancy projects. Most of the observed decreases in sediment yield are associated with large reservoir schemes on tributary rivers. The lack of evidence for increasing sediment input to the Three Gorges area masks a considerable variation in sediment conveyance and storage within the Upper Yangtze catchment.KEY WORDS: Sediment yield; Reservoir sedimentation; Three Gorges Project; Time series analysis; China     

Assessing Riparian Conservation Land Management Practice Impacts on Gully Erosion in Iowa

Well-established perennial vegetation in riparian areas of agricultural lands can stabilize the end points of gullies and reduce their overall erosion. The objective of this study was to investigate the impacts of riparian land management on gully erosion. A field survey documented the number of gullies and cattle access points in riparian forest buffers, grass filters, annual row-cropped fields, pastures in which the cattle were fenced out of the stream, and continuously, rotationally and intensive rotationally grazed pastures in three regions of Iowa. Gully lengths, depths and severely eroding bank areas were measured. Gullies exhibited few significant differences among riparian management practices. The most significant differences were exhibited between conservation and agricultural management practices, an indication that conservation practices could reduce gully erosion. Changes in pasture management from continuous to rotational or intensive rotational grazing showed no reductions in gully erosion. It is important to recognize that more significant differences among riparian management practices were not exhibited because the conservation and alternative grazing practices had recently been established. As gully formation is more impacted by upland than riparian management, gully stabilization might require additional upland conservation practices. The existence of numerous cattle access points in pastures where cattle have full access to the stream also indicates that these could be substantial sources of sediment for streams. Finally, the gully banks were less important sediment contributors to streams than the streambanks. The severely eroding bank areas in streams were six times greater than those in the gullies in the monitored reaches.     

Management of Sedimentation in Tropical Watersheds

/ The sedimentation of reservoirs is a serious problem throughout the tropics, yet most attempts to control sedimentation in large river basins have not been very successful. Reliable information on erosion rates and sources of sediments has been lacking. In regions where geologically unstable terrain combines with high rainfall, natural erosion rates might be so high that the effects of human activity are limited. Estimates of natural erosion in these situations often have been poor because of the episodic nature of most erosion during large storms and because mass-wasting may supply much of the sediment. The predominance of mass-wasting in some watersheds can result in an unexpectedly high ratio of bedload to suspended load, shifting sedimentation to "live" rather than "dead" storage within reservoirs. Furthermore, the inappropriate use of the Universal Soil Loss Equation to assess the effectiveness of erosion control measures has led to inaccurate estimates of the sediment reduction benefits that could accrue to watershed treatment efforts. Although reducing erosion from cultivated areas is desirable for other reasons, efforts aimed at reducing reservoir sedimentation by controlling agricultural sources of erosion may have limited benefits if the principal sources are of natural origin or are associated with construction of the dams and reservoirs and with rural roads and trails. Finally, the most appropriate locations for watershed rehabilitation depend on the magnitude of temporary storage of colluvium and alluvium within the river basin: Where storage volume is large and residence time of sediment very long, reducing agricultural erosion may have limited impacts on sedimentation within the expected life of a reservoir. Systematic development and analysis of sediment budgets for representative watersheds is needed to address these limitations and thereby improve both the planning of river basin development schemes and the allocation of resources towards reducing sedimentation. When sedimentation of reservoirs is the key issue, sediment budgets must focus especially on channel transport rates and sediment delivery from hillsides. Sediment budgets are especially critical for tropical areas where project funds and technical help are limited. Once sediment budgets are available, watershed managers will be able to direct erosion control programs towards locations where they will be most effective. KEY WORDS: Tropical watersheds; Sedimentation; Reservoirs; Erosion control     

SEDIMENT DEPOSITION ESTIMATION FROM CESIUM-137 MEASUREMENTS1

ABSTRACT The movement of fallout ¹³⁷Cs carried by soil particles was studied as an indicator of erosion and sedimentation in the Allerton watersheds and 4-H Memorial Lake located near Monticello, Illinois. Sediment deposition was greater in the waterway draining from watershed IB than in the waterway from watershed IA. At the average rate of 2.3 cm/yr of sediment deposition in the lake (from 1954 to 1979), there will be a loss of over 2 meters of water depth in the next century. However, there appears to be a decreasing rate of sediment deposition in the 4-H Memorial Lake as a result of improved conservation practices on the watersheds and the increased effectiveness of vegetated waterways and buffers for retaining sediment.     

Soils and vegetation of Santa Barbara Island,Channel Islands National Park,California, USA

The multifaceted development of an erosion surface on Santa Barbara Island, Channel Islands National Park, California, has led to this study of the relationship between soils and vegetation. A dry Mediterranean climate and past attempts at farming and introductions of alien species have led to vegetative degradation accompanied by both gully and surface erosion. Soil and vegetation analyses show this erosion to be in a location of transition. The soils are Typic Chromoxererts (Vertisol Order) with high clay, salinity, and sodium contents. The vegetation is ecotonal in nature, grading from a principally alien annual grassland withAvena fatua andAtriplex semibaccata to a shrub community dominated by the nativeSuaeda californica. Management toward revegetation and stabilization of this island ecosystem will be difficult with high clay, saline-sodic soils and disturbed vegetation.     

Pyrosequencing reveals bacteria carried in different wind-eroded sediments

Little is known about the microbial communities carried in wind-eroded sediments from various soil types and land management systems. The novel technique of pyrosequencing promises to expand our understanding of the microbial diversity of soils and eroded sediments because it can sequence 10 to 100 times more DNA fragments than previous techniques, providing enhanced exploration into what microbes are being lost from soil due to wind erosion. Our study evaluated the bacterial diversity of two types of wind-eroded sediments collected from three different organic-rich soils in Michigan using a portable field wind tunnel. The wind-eroded sediments evaluated were a coarse sized fraction with 66% of particles >106 μm (coarse eroded sediment) and a finer eroded sediment with 72% of particles <106 μm. Our findings suggested that (i) bacteria carried in the coarser sediment and fine dust were effective fingerprints of the source soil, although their distribution may vary depending on the soil characteristics because certain bacteria may be more protected in soil surfaces than others; (ii) coarser wind-eroded sediment showed higher bacterial diversity than fine dust in two of the three soils evaluated; and (iii) certain bacteria were more predominant in fine dust (, , and ) than coarse sediment ( and ), revealing different locations and niches of bacteria in soil, which, depending on wind erosion processes, can have important implications on the soil sustainability and functioning. Infrared spectroscopy showed that wind erosion preferentially removes particular kinds of C from the soil that are lost via fine dust. Our study shows that eroded sediments remove the active labile organic soil particulates containing key microorganisms involved in soil biogeochemical processes, which can have a negative impact on the quality and functioning of the source soil.     

DECISION FRAMEWORK FOR SEDIMENT CONTROL IN MUDDY CREEK WATERSHED1

ABSTRACT: The tailwater of Bridgewater Dam, below Lake James, North Carolina, is a designated trout stream. It has environmental attributes for a good cold water fishery with the exception of high suspended sediments. Muddy Creek, a tributary about 1.5 km downstream of the dam, is a major source of sediments. The Muddy Creek Watershed Restoration Initiative was established to develop and implement a sediment control plan. The Watershed Analysis Risk Management Framework was applied to simulate soil erosion and sedimentation and to help determine appropriate action. The simulated sediment concentrations of the river were comparable to observed data from November 1994 to November 2001. For the base condition, the sediment load was 135,000 kg/d from surface erosion and 1,300,000 kg/d from bank erosion. Increasing the buffer strip from existing 50 to 80 percent to 100 percent of stream segments would only reduce surface erosion to 70,400 kg/d with little change in sediment concentrations. Eliminating riverbank erosion would reduce the sediment load from 920,000 to 87,700 kg/d. The bank stabilization project would not only lower suspended sediment concentrations for Muddy Creek, but also reduce the lake sediment accumulation in the downstream Lake Rhodhiss by approximately 13 percent.     

Protecting the Green Behind the Gold: Catchment-Wide Restoration Efforts Necessary to Achieve Nutrient and Sediment Load Reduction Targets in Gold Coast City,Australia

The Gold Coast City is the tourist center of Australia and has undergone rapid and massive urban expansion over the past few decades. The Broadwater estuary, in the heart of the City, not only offers an array of ecosystems services for many important aquatic wildlife species, but also supports the livelihood and lifestyles of residents. Not surprisingly, there have been signs of imbalance between these two major services. This study combined a waterway hydraulic and pollutant transport model to simulate diffuse nutrient and sediment loads under past and future proposed land-use changes. A series of catchment restoration initiatives were modeled in an attempt to define optimal catchment scale restoration efforts necessary to protect and enhance the City’s waterways. The modeling revealed that for future proposed development, a business as usual approach to catchment management will not reduce nutrient and sediment loading sufficiently to protect the community values. Considerable restoration of upper catchment tributaries is imperative, combined with treatment of stormwater flow from intensively developed sub-catchment areas. Collectively, initiatives undertaken by regulatory authorities to date have successfully reduced nutrient and sediment loading reaching adjoining waterways, although these programs have been ad hoc without strategic systematic planning and vision. Future conservation requires integration of multidisciplinary science and proactive management driven by the high ecological, economical, and community values placed on the City’s waterways. Long-term catchment restoration and conservation planning requires an extensive budget (including political and societal support) to handle ongoing maintenance issues associated with scale of restoration determined here.     

Assessment of spatial variation of cesium-137 in small catchments

Surface contamination by bomb-derived and Chernobyl-derived 137Cs has been subject to changes due to physical decay and lateral transport of contaminated soil particles, which have resulted in an on-going transfer of radionuclides from terrestrial ecosystems to surface water, river bed sediments, and flood plains. Knowledge of the different sources of spatial variation of 137Cs is particularly essential for estimating 137Cs transfer to fluvial systems and for successfully applying 137Cs as an environmental tracer in soil erosion studies. This study combined a straightforward sediment redistribution model and geostatistical interpolation of point samples of 137Cs activities in soil to distinguish the effects of sediment erosion and deposition from other sources of variation in 137Cs in the small Mochovce catchment in Slovakia. These other sources of variation could then be interpreted. Besides erosion and deposition processes, the initial pattern of 137Cs deposition, floodplain sedimentation, and short-range spatial variation were identified as the major sources of spatial variation of the 137Cs inventory.     

Erosion control at construction sites on red clay soils

Five single-treatment methods used to stabilize seeded areas at urban and highway construction sites (asphalt-tacked straw, jute netting, mulch blanket, wood chips, excelsior blanket) were tested for their ability to control erosion of red clay soils by comparisons with exposed sites and multiple treatments. Sediment in runoff from experimental plots was characterized during low and high intensity precipitation from natural rainfall episodes during April, May, and June. Reductions in the total sediment concentration of runoff ranged from 28 percent (asphalt-tacked straw, 50 percent slope) to 90 percent (multiple treatments, 40 percent slope), with 85 percent of the eroded material composed of particles <0.04 mm in diameter. Larger size fractions were effectively reduced by all treatments tested regardless of slope (70 percent decrease). Established grass cover exceeded 90 percent on all plots after 60 days, but sediment release remained similar, attributable to high intensity rainfall, poor establishment of root systems, and piping on plots treated with tacked straw or jute netting. Results indicate that current stabilization methods shift sediment compostion toward a smaller particle size, causing single treatments to be minimally effective for controlling erosion of the major component of red clay soils. Because small particles have the greatest direct effect on aquatic biota, certain impacts of sedimentation may not be measurably lessened by single treatments in regions where red clays predominate even though the total sediment load is reduced by as much as 75 percent. Clearly, a multiple-treatment approach offers significantly greater control of erosion on red clay soils, however, current economic and construction policy represents a substantial deterrent to implementation.     

Cost/Benefit Considerations for Recent Saltcedar Control,Middle Pecos River,New Mexico

Major benefits were weighed against major costs associated with recent saltcedar control efforts along the Middle Pecos River, New Mexico. The area of study was restricted to both sides of the channel and excluded tributaries along the 370 km between Sumner and Brantley dams. Direct costs (helicopter spraying, dead tree removal, and revegetation) within the study area were estimated to be $2.2 million but possibly rising to $6.4 million with the adoption of an aggressive revegetation program. Indirect costs associated with increased potential for erosion and reservoir sedimentation would raise the costs due to increased evaporation from more extensive shallows in the Pecos River as it enters Brantley Reservoir. Actions such as dredging are unlikely given the conservative amount of sediment calculated (about 1% of the reservoir pool). The potential for water salvage was identified as the only tangible benefit likely to be realized under the current control strategy. Estimates of evapotranspiration (ET) using Landsat TM data allowed estimation of potential water salvage as the difference in ET before and after treatment, an amount totaling 7.41 million m³ (6010 acre-ft) per year. Previous saltcedar control efforts of roughly the same magnitude found that salvaged ET recharged groundwater and no additional flows were realized within the river. Thus, the value of this recharge is probably less than the lowest value quoted for actual in-channel flow, and estimated to be < $63,000 per year. Though couched in terms of costs and benefits, this paper is focused on what can be considered the key trade-off under a complete eradication strategy: water salvage vs. erosion and sedimentation. It differs from previous efforts by focusing on evaluating the impacts of actual control efforts within a specific system. Total costs (direct plus potential indirect) far outweighed benefits in this simple comparison and are expected to be ongoing. Problems induced by saltcedar control may permanently reduce reservoir capacity and increase reservoir evaporation rates, which could further deplete supplies on this water short system. These potential negative consequences highlight that such costs and benefits need to be considered before initiating extensive saltcedar control programs on river systems of the western United States.     

Quantifying the Effects of Conservation Practices on Soil,Water, and Nutrients in the Loess Mesa Ravine Region of the Loess Plateau,China

A large number of soil and water conservation programs have been implemented on the Loess Plateau of China since the 1950s. To comprehensively assess the merits and demerits of the conservation practices is of great importance in further supervising the conservation strategy for the Loess Plateau. This study calculates the impact factors of conservation practices on soil, water, and nutrients during the period 1954–2004 in the Nanxiaohegou Catchment, a representative catchment in the Loess Mesa Ravine Region of the Loess Plateau, China. Brief conclusions could be drawn as follows: (1) Soil erosion and nutrient loss had been greatly mitigated through various conservation practices. About half of the total transported water and 94.8 % of the total transported soil and nutrients, had been locally retained in the selected catchment. The soil retained from small watersheds do not only form large-scale fertile farmland but also safeguard the Yellow River against overflow. (2) Check dam was the most appropriate conservation practice on the Loess Plateau. In the selected catchment, more than 90 % of the retained soil and water were accomplished by the dam farmland, although the dam farmland occupied only 2.3 % of the total area of all conservation measures. Retention abilities of the characteristic conservation practices were in the following order: dam farmland > terrace farmland > forest land and grassland. (3) The conservation practices were more powerful in retaining sediment than in reducing runoff from the Loess Plateau, and the negative effects of the conservation practices on reducing water to the Yellow River were relatively slight.     

Monitoring and management of streambank erosion and natural revegetation on the lower Gordon River,Tasmanian Wilderness World Heritage Area,Australia

The wash from high-speed tourist cruise launches causes erosion of the formerly stable banks of the lower Gordon River within the Tasmanian Wilderness World Heritage Area. Speed and access restrictions on the operation of commercial cruise vessels have considerably slowed, but not halted erosion, which continues on the now destabilized banks. To assess the effectiveness of restrictions, bank erosion and natural revegetation are monitored at 48 sites using erosion pins, survey transects, and vegetation quadrats. The subjectively chosen sites are grouped on the basis of geomorphology and bank materials. The mean measured rate of erosion of estuarine banks slowed from 210 to 19 mm/year with the introduction of a 9 knot speed limit. In areas where cruise vessels continue to operate, alluvial banks were eroded at a mean rate of 11 mm/yr during the three-year period of the current management regime. Very similar alluvial banks no longer subject to commercial cruise boat traffic eroded at the slower mean rate of 3 mm/yr. Sandy levee banks have retreated an estimated maximum 10 m during the last 10–15 years. The mean rate of bank retreat slowed from 112 to 13 mm/yr with the exclusion of cruise vessels from the leveed section of the river. Revegetation of the eroded banks is proceeding slowly; however, since the major bank colonizers are very slow growing tree species, it is likely to be decades until revegetation can contribute substantially to bank stability.     

The Use of Carbon and Nitrogen Isotopes to Study Watershed Erosion Processes1

Abstract:  Tracer studies are needed to better understand watershed soil erosion and calibrate watershed erosion models. For the first time, stable nitrogen and carbon isotopes (δ¹⁵N and δ¹³C) and the carbon to nitrogen atomic ratio (C/N) natural tracers are used to investigate temporal and spatial variability of erosion processes within a sub‐watershed. Temporal variability was assessed by comparing δ¹⁵N, δ¹³C, and C/N of eroded‐soils from a non‐equilibrium erosion event immediately following freezing and thawing of surface soils with two erosion events characterized by equilibrium conditions with erosion downcutting. Spatial variability was assessed for the equilibrium events by using the δ¹⁵N and δ¹³C signatures of eroded‐soils to measure the fraction of eroded‐soil derived from rill/interrill erosion on upland hillslopes as compared to headcut erosion on floodplains. In order to perform this study, a number of tasks were carried out including: (1) sampling source‐soils from upland hillslopes and floodplains, (2) sampling eroded‐soils with an in situ trap in the stream of the sub‐watershed, (3) isotopic and elemental analysis of the samples using isotope ratio mass spectrometry, (4) fractioning eroded‐soil to its upland rill/interrill and floodplain headcut end‐members using an unmixing model within a Bayesian Markov Chain Monte Carlo framework, and (5) evaluating tracer unmixing model results by comparison with process‐based erosion prediction models for rill/interrill and headcut erosion processes. Results showed that finer soil particles eroded during the non‐equilibrium event were enriched in δ¹⁵N and δ¹³C tracers and depleted in C/N tracer relative to coarser soil particles eroded during the equilibrium events. Correlation of tracer signature with soil particle size was explainable based on known biogeochemical processes. δ¹⁵N and δ¹³C were also able to distinguish between upland rill/interrill erosion and floodplain headcut erosion, which was due to different plant cover at the erosion sources. Results from the tracer unmixing model highlighted future needs for coupling rill/interrill and headcut erosion prediction models.     

RUNOFF AND SEDIMENT RESPONSES TO CONSERVATION PRACTICES: LOESS PLATEAU OF CHINA1

ABSTRACT: Soil erosion is the most significant threat to land productivity and environmental quality on the Loess Plateau of China. The annual total sediment load of the Yellow River is 1.6 billion tons, with about 90 percent coming from soil erosion from the Loess Plateau. To reduce soil erosion from the Loess Plateau, conservation practices, including tree planting, ridge construction between fields and around gullies, terrace and ditch construction perpendicular to the main slope, and dam construction are being implemented. An evaluation of these conservation practices is required before they are implemented at the large scale. The objective of this study is to evaluate the effectiveness of conservation practices to control runoff and sediment yield from paired watersheds in the hilly gully region of the Loess Plateau. The advantage of the paired watershed approach is its sensibility in detecting differences in runoff and sediment transport by monitoring both watersheds during two periods, an initial period with no conservation practices and a treatment period with only one watershed subjected to conservation practices. Implementation of the conservation practices resulted in (1) cumulative runoff and sediment yield that were, respectively, 25 and 38 percent less from the treatment watershed than from the control, (2) a decrease in the number of rainfall events producing runoff and sediment transport (94 in the control versus 63 in treatment), and (3) a reduction in the maximum discharge and maximum suspended sediment concentration.     

水体底泥对污染物的吸附-解吸机理的研究

水体底泥污染已经成为世界范围内的一个重要的环境问题,其污染物主要通过大气沉降、废水排放、雨水淋溶冲刷等进入水体,最后沉积到底泥中并逐渐富集,使底泥受到严重污染。吸附-解吸是底泥污染的重要过程,为了控制水体底泥污染,对底泥对各种污染物的吸附-解吸机理及其影响因素的研究显得极其重要。本文总结了底泥中氮、磷和重金属等的吸附-解吸的影响因素及机理,对今后水体底泥污染方面的研究具有重要意义。     

REELFOOT LAKE SEDIMENTATION RATES AND SOURCES1

ABSTRACT: The three basins of Reelfoot Lake, which is located in northwestern Tennessee, were investigated using the Cs-137 tracer technique to determine rates of sediment deposition and to estimate the time before the basins will fill with sediment. Blue Basin, the largest of the three basins with 2922 ha, had an average annual sedimentation rate of 0.9 cm/yr from 1984 to 1984. The basin will become too shallow for most boating and recreational activities in about 200 years. Buck Basin, the central basin with 774 ha, had an average annual sedimentation rate of 1.1 cm/yr and will become too shallow for most recreational uses in about 100 years. Upper Blue Basin, the most upstream and smallest basin with 439 ha, had an average annual sedimentation rate of 1.7 cm/yr and will become too shallow for most recreational uses in about 60 years. Two important sources of sediment to Reelfoot Lake are erosion from a large number of soybean fields and channelization of many of the streams that flow into the lake. Changes in land management that would reduce erosion could increase the time the lake would remain usable for recreational activities.