RATES OF RECENT SEDIMENTATION IN LAKE PEPIN1

ABSTRACT: Sedimentation rates since 1954 in Lake Pepin, as determined from the content of fallout cesium-137 in the sediment profile, have exceeded 2.5 cm/yr in the upper part of the lake. These rates, although somewhat less than those of the previous half century (1895–1954), are sufficiently large that the upstream portion of Lake Pepin is threatened with conversion to a marsh within a century. The density of the sediments measured increased with depth in the sampled profile from 1.1 to 1.2 g/ml at the sediment surface to 1.4 to 1.5 g/ml at 2- or 3-m depth. There was little or no change in the patterns of textural composition or density of the sediment profile with depth and age over the past 80 to 150 years.     

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.     

PERSISTING SEDIMENT YIELDS AND SEDIMENT DELWERY CHANGES1

ABSTRACT: The Buffalo River is a tributary to the Mississippi River in west-central Wisconsin that drains a watershed dominated by agricultural land uses. Since 1935, backwater from Lock and Dam 4 on the Mississippi River has inundated the mouth of the Buffalo's valley. Resurveys of a transect first surveyed across the lake in 1935 and cesium-137 dating of backwater sediments reveal that sedimentation rates at the Buffalo's mouth have remained unchanged since the mid-1940s. Study results indicate that sediment yields from the watershed have persisted at relatively high levels over a period of several decades despite pronounced trends toward less cultivated land and major efforts to control soil erosion from agricultural land. The maintenance of sediment yields is probably due to increased channel conveyance capacities resulting from incision along some tributary streams since the early 1950s. Post-1950 incision extended the network of historical incised tributary channels, enhancing the efficient delivery of sediment from upland sources to downstream sites.     

HISTORICAL TRENDS IN SEDIMENTATION RATES AND SEDIMENT PROVENANCE,FAIRFIELD LAKE,WESTERN NORTH CAROLINA1

Sedimentation rates and sediment provenance were examined for lacustrine sediments deposited in Fairfield Lake, western North Carolina, during the past 111 years. Stratigraphic, radionuclide, and cartographic data indicate that sedimentation rates have increased several fold during the past three decades in response to localized development. The magnitude of increased sedimentation was surprising given limited development within the basin: 0.12 to 0.68 buildings/ha in 2000 in those parts directly delivering sediment to the dated cores. Thus, the analysis illustrates the potential sensitivity of watersheds in the southern Appalachians to changes in land cover. An approach that combined geochemical fingerprinting with sediment mixing models was subsequently evaluated to determine its ability to accurately estimate the contribution of sediment from (1) major bedrock formations that underlie the watershed and (2) potential sources associated with four land cover categories. Sediment sources in both analyses proved difficult to geochemically fingerprint to greater than 90 percent accuracy using data on acid‐soluble metals and selected isotopes of lead (Pb). The relative contributions of sediment from delineated sources, estimated by the mixing models, generally corresponded with known temporal and spatial patterns of land cover. However, the models were plagued by two significant problems — the chemical alteration of sediments as they were transported through upland streams to depositional sites within the lake and the loss of elemental mass. Thus, future investigations using the fingerprinting approach in this area of intense weathering, and presumably others, will need to modify the existing methods to more accurately elucidate changes in sediment provenance related to development.     

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.     

DELIVERY OF SUSPENDED SEDIMENT AND POLLUTANTS FROM NONPOINT SOURCES DURING OVERLAND FLOW1

ABSTRACT: Delivery of sediment and particulate pollutants from diffuse sources is shown to be related to the loss of sediment carrying energy of runoff during the overland flow phase. The loss is caused by the termination of rainfall and by reduction of flow energy during the recession phase of the overland flow hydrograph. It has been demonstrated both by theoretical analyses and experimental measurements that the saturated sediment concentration in overland flow is a function of rainfall erosivity and the runoff flow rate. The hypotheses were verified by field measurements from a small homogeneous watershed.     

ENVIRONMENTAL AND ECONOMIC DAMAGE CAUSED BY SEDIMENT FROM AGRICULTURAL NONPOINT SOURCES1

Environmental and economic damages caused by agricultural nonpoint source inputs of sediment and associated pollutants are examined. Widespread water quality problems are identified in lakes, rivers, and estuaries in agricultural areas, and billions of dollars of on-site and offsite costs result from this eroded soil every year. Some water bodies have been irretrievably damaged, and expensive rehabilitation programs are needed to remedy in-place water pollution problems if Clean Water Act goals are to be achieved. Unless effective abatement and rehabilitation programs are established, billions of dollars of benefits to future generations will not be realized as more waters become irretrievably damaged, and billions more will continue to be spent by government to treat symptoms of these sediment-related problems.     

SOLIDS DISTRIBUTION IN LAKE CHAPALA,MEXICO1

ABSTRACT: Lake Chapala is the largest natural water body in Mexico and also one of the most important shallow lakes in Latin America. For the past several years it has suffered various environmental problems such as the upstream overuse of water, contamination, and sedimentation. For the past 10 years the lake has had less than 50 percent of its historical water level over the past century. No criteria are reported in the literature that establish a water storage volume that will guarantee water quality conditions necessary for the survival of the lake. After determining the behavior of total solids concentrations in relation to the variations in the lake's depth, we proposed a minimum water column height of 5.0 m, representing a storage volume of about 5,000 Mm³. This volume would result in the recommended water quality standards for total dissolved solids. Calculated distribution maps show that the total solids concentration in the lake has increased since the end of the 1970s. The solids are primarily concentrated in the eastern part of the lake, as a consequence of the high solids discharged from the Lerma River, its main tributary.     

RESERVOIR SEDIMENTATION AND ITS CAUSES IN ILLINOIS1

ABSTRACT: During the years 1930 to 1978 a research program has been carried out in Illinois dealing with reservoir sedimentation. Detailed surveys have been made on 107 lakes. A generalized graph allows annual reservoir capacity loss in percent to be estimated based on size of watershed, lake capacity, and watershed ratio. For the 258 square miles of land on the watershed of Lake Springfield, complete soil maps were measured using a graphic digitizer. A computer readable map was produced. The Universal Soil Loss Equation was solved to provide upland soil losses for each acre of the watershed. Average watershed soil loss was 3.96 tons per acre per year, and 24 percent of this was delivered to the lake.     

A COMPARISON OF THREE METHODS FOR MEASURING RECENT RATES OF SEDIMENT ACCUMULATION1

ABSTRACT: The determination of sediment accumulation rates is important in understanding how these materials are affecting lakes and reservoirs ecosystems. In this study three methods were used to estimate sediment accumulation rates in the impounded backwater lakes behind Lock and Dam Nos. 8 and 9 on the upper Mississippi River. The three methods were: 1) a “spud” survey, 2) a survey of bottom contours, and 3) the use of fallout cesium-137. The field use of these three methods of determining sediment accumulation and the potential errors and merits involved in each method are discussed. The results from the field study in backwater areas along the upper Mississippi River showed the survey of bottom contour method gave the lowest rate of sediment deposition and the ¹³⁷Cs method gave the highest rates. Sediment accumulation rates from 0 to 7.8 cm per year were measured in the study area. All three methods are useful and have unique characteristics for determining rates and patterns of sediment accumulation. Thus the choice of a method to be used in a sediment survey is dependent on the type of information needed and the time available.     

RESERVOIR SEDIMENTATION1

ABSTRACT: Reservoirs, as well as lakes and estuaries, are subject to sediment accumulation. The rate at which sedimentation occurs is accelerated or diminished by man's activities. Acceptable rates of sedimentation are considered on the basis of costs of sediment storage and removal and a review of reservoirs constructed with sediment removal capabilities is given. The rates of reservoir sedimentation in the United States are summarized based on reliable data obtained from all sizes of reservoirs. Problems confronting the engineer concerning reservoir construction and maintenance are discussed on the basis of these data.     

EFFECTS OF SOIL TYPE,PLOT CONDITION,AND SLOPE ON RUNOFF AND INTERRILL EROSION OF TWO SOILS IN THE LAKE TAHOE BASIN1

ABSTRACT: Few studies have addressed sediment discharge due to interrill erosion from natural and minimally disturbed alpine and subalpine forested watersheds. Infiltration, runoff, and surface erosion of two Tahoe Basin soils under several conditions were investigated using rainfall simulation. A significant three-way interaction among soil type, plot condition, and slope was identified. Although high erodibiity was commonly associated with disturbance and/or high slope, this was not always the case. Soil type, plot condition, slope, and duration of the event were all found to be important factors in determining the amount of erosion. Decreased water clarity in Lake Tahoe has been partly attributed to increased algal growth associated with surface runoff and erosion from adjacent watersheds. Interpretive evaluation for resource management planning should be event based and carefully delineated on a sitespecific basis.     

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.     

NATURAL EROSION RATES AND THEIR PREDICTION IN THE IDAHO BATHOLITH1

ABSTRACT: Natural rates of surface erosion on forested granitic soils in central Idaho were measured in 40 m² bordered erosion plots over a period of four years. In addition, we measured a variety of site variables, soil properties, and summer rainstorm intensities in order to relate erosion rates to site attributes. Median winter erosion rates are approximately twice summer period rates, however mean summer rates are nearly twice winter rates because of infrequent high erosion caused by summer rainstorms. Regression equation models and regression tree models were constructed to explore relationships between erosion and factors that control erosion rates. Ground cover is the single factor that has the greatest influence on erosion rates during both summer and winter periods. Rainstorm intensity (erosivity index) strongly influences summer erosion rates, even on soils with high ground cover percentages. Few summer storms were of sufficient duration and intensity to cause rilling on the plots, and the data set was too small to elucidate differences in rill vs. interrill erosion. The regression tree models are relatively less biased than the regression equations developed, and explained 70 and 84 percent of the variability in summer and winter erosion rates, respectively.     

SEDIMENTATION CHARACTERISTICS OF GORGE-TYPE RESERVOIRS1

ABSTRACT. An investigation of the hydraulics of gorge-type reservoirs was conducted with scale models. Reservoir shapes were moulded within a large basin. Water was circulated using a centrifugal pump-motor unit and uniform sediment (specific gravity 2.65) with mean diameters of 0.20 mm and 0.60 mm were utilized. Observations were made to study sedimentation patterns from the commencement of sediment inflow until the final stage of a fully silted reservoir. In particular, the mode of deposition of the sediment beds, the mechanics of transportation and sediment bed slopes were investigated. These aspects of reservoir siltation were examined in relation to the factors which influence it, which included sediment characteristics and flow parameters. Bed slopes and flow depths were analyzed by various methods; the Kalinske equation in conjunction with the Manning and Einstein-Barbarossa relations as proposed by Doland-Chow produced the best results.     

A PROCEDURE FOR ESTIMATING OFF-SITE SEDIMENT DAMAGE COSTS AND AN EMPIRICAL TEST1

ABSTRACT This paper reports on research to develop a methodology for estimating agricultural off-site sediment damage costs and includes an empirical estimate of such damages for a watershed. The economics of off-site sediment damage costs are discussed as a theoretical basis for the procedures developed. A detailed methodology is described for estimating five different types of off-site sediment damages commonly associated with rural watersheds. The methodology is then applied to a central Illinois watershed and estimates of individual types of damage are made. The estimates are combined into an off-site sediment damage function for the watershed, and the usefulness of the damage function for analyzing off-site sediment damages and achieving a reduction in those costs is discussed.     

MODELING THE EFFECTS OF URBANIZATION ON BASIN WATER YIELD AND RESERVOIR SEDIMENTATION1

ABSTRACT: The dam impounding White Rock Lake was completed in 1910 to provide water for the City of Dallas. Since then, land use on the watershed has changed from entirely rural to over 77 percent urban. A model called SWRRB (Simulator for Water Resources in Rural Basins) was utilized to determine the effect of urbanization on water and sediment entering the lake. The simulation results show that, if urbanization had not occurred, then the annual surface runoff would be 135 mm rather than 151 mm and the annual sediment yield would be 4.4 t/ha rather than 4.1 t/ha. Also, the effect of urbanization on delivery ratios was shown and a positive linear correlation was found. Finally, the weather generator in SWRRB was utilized to estimate the loss of reservoir capacity until 2050 for three different land use management scenarios.     

PREDICTING EROSION AND DEPOSITION ON A STRIPMINED AND RECLAIMED AREA1

ABSTRACT A rill-interrill erosion model was applied to a mined and reclaimed area. Soil loss from the interrill areas was estimated by the Universal Soil Loss Equation (USLE). The model considers the fate and ultimate disposition of the sediment from interrill areas along with the fate and destination of soil materials detached by the rill flow. The net sediment loss was predicted by comparing, for a given flow, the amounts of eroded soil to rill transport capacity. When applied to a selected stripmined and reclaimed site the model displayed the location of contributing areas and the amount of erosion and deposition. The predicted areal distribution of erosion and deposition was compared to measured data. Agreement between the predicted and measured values was within 25 percent.     

SOURCES OF VARIABILITY IN PHOSPHORUS AND CHLOROPHYLL AND THEIR EFFECTS ON USE OF LAKE SURVEY DATA1

Summer lake survey measurements of total phosphorus (TP) and chlorophyll a (CHLa) from 188 reserviors and natural lakes in the midwest were analyzed to determine the magnitude of major sources of variability. Median variance among replicate samples collected at the same location and time was about 7-8 percent of the mean for both TP and CHLa. Median observed temporal variability within summers was 27 percent of the mean for TP and 45 percent of the mean for CHLa. Median values of year-to-year variance in average TP and CHLa were 22 percent and 31 percent of the mean, respectively. A range of approximately two orders of magnitude was observed among individual estimates of variance in each of these categories. The magnitude of observed temporal variability was affected only slightly by variance among replicate samples on individual days and was weakly correlated with the length of time during which samples were collected from individual lakes. Observed temporal variation was similar between reservoirs and natural lakes when variances were calculated with logtransformed data. The magnitude of temporal and year-to-year variance can severely limit the power of statistical comparisons of TP and CHLa means, but has less effect on establishing relative rankings of lake means, Sources and relative magnitude of variability are important in the use of TP and CHLa data in regression models and in the planning of lake surveys and subsequent data analysis.     

COMPARATIVE EVALUATION OF LAND COVER DATA SOURCES FOR EROSION PREDICTION1

ABSTRACT: A fundamental problem in protecting surface drinking water supplies is the identification of sites highly susceptible to soil erosion and other forms of nonpoint source (NPS) pollution. The New York City Department of Environmental Protection is trying to identify erodible sites as part of a program aimed at avoiding costly filtration. New York City's 2,000 square mile watershed system is well suited for analysis with geographic information systems (GIS); an increasingly important tool to determine the spatial distribution of sensitive NPS pollution areas. This study used a GIS to compare three land cover sources for input into the Modified Universal Soil Loss Equation (MUSLE), a model estimating soil loss from rangeland and forests, for a tributary watershed within New York City's water supply system. Sources included both conventional data (aerial photography) and Landsat data (MSS and TM images). Although land cover classifications varied significantly across these sources, location-specific and aggregate watershed predictions of the MUSLE were very similar. We conclude that using Landsat TM imagery with a hybrid classification algorithm provides a rapid, objective means of developing large area land cover databases for use in the MUSLE, thus presenting an attractive alternative to photo interpretation.