Muddy Lake Research Articles

Integrated methodology for gas content assessment and prediction in shallow muddy lake sediments: acoustic mapping and correlation analysis

This paper provides a step-by-step description of integrated methodology for quantification and prediction of gas (methane, CH4) content dynamics in shallow aquatic sediments under changing spatial and temporal conditions. Presence of gas bubbles even in small concentrations significantly affects sediment compressibility, which in turn decreases sound speed in sediment. Our integrated methodology consists of two basic steps. In the first step, free gas content is evaluated by acoustic applications based on the sound speed inferred from the reflection coefficient from gassy bottom. The experimental bottom reflections are registered and compared to the simulated ones, using a geoacoustic inversion technique. The best match between the model and the experiment provides sediment sound speed estimate, which is converted into free gas content using a basic relation. In the second step, a multivariate linear regression is fitted for gas content and closed form expression of gas content dependence on the following predictors, which change spatially and temporally over the aquatic ecosystem, is obtained: 1) water depth, 2) short-leaving CH4 production rate peaks fueled by punctuated organic matter deposition; and 3) CH4 bubble dissolution rates.•Gas content and sound speed in the sediment are estimated via the geoacoustic inversion technique by matching the experimentally recorded and simulated bottom reflections•Only single source and receiver are required for the acoustic methodology•A multivariate linear regression is fitted for gas content to indicate its dependence on various predictors that change spatially and temporally over the lake

Crocodylian and dinosaur trace fossil assemblages from crevasse splay/levee and floodplain lake environments: middle Cenomanian Dunvegan Formation, northeast British Columbia, Canada

ABSTRACT Delta-plain strata of the Cenomanian Dunvegan Formation, in north-east British Columbia, represent anastomosed rivers that were flanked by vegetated crevasse splays, wetlands and shallow lakes. These rocks preserve a rich record of ankylosaur and ornithopod walking and wading tracks, as well as crocodylian swim traces and tracks. Analysis of depositional, erosional, and trace-making events reveals alternating phases of flooding and emergence that controlled both animal activity (walking, wading, swimming) and mud substrate consistency (firm vs. soupy), and hence fidelity of track preservation. Some tracks resemble those produced by both ankylosaurs and crocodylians, and confident attribution is difficult. The absence of tail drag marks suggests an ankylosaur trackmaker, whereas prominent claw marks might favour a crocodylian origin. Unusually large crocodylian swim traces suggest animals about 9 m, and possibly up to 12 m in length. These large traces may represent a precursor to the giant crocodylian Deinosuchus. Ankylosaur tracks are well-preserved in lake and channel-margin deposits, whereas crocodylian traces are most commonly preserved in firm muddy lake sediments that had experienced previous subaerial exposure and dewatering. Close integration of trace fossils with sedimentary and stratigraphic features makes it possible to reveal a detailed chronology of biotic and sedimentary events on palaeo-surfaces.

IMPACTS OF URBAN DEVELOPMENT ON AN OXBOW LAKE IN THE AMAZON REGION: LANDSCAPE AND GEOCHEMICAL EVIDENCE

Amapá Lake is an abandoned meander of the Acre River, the margins of which are occupied by agricultural settlements, smallholdings, fish farms and dirt roads. This paper discusses the impact of this occupation on the lake area based on field data and physical-chemical and elementary chemical analyses of the lake and river waters. The strong physical changes in the lake landscape have impacted the quality of its water. During the dry season (low water), when the temperature of the water reaches 30-34ºC, there is a proliferation of green algae and in the area affected by fish farms and smallholdings there is an increase in total dissolved solids, electrical conductivity, and transparency, while turbidity and total suspended solids become more concentrated during the flood season. The anthropogenic impact is also indicated by the higher levels of ammonia, chlorides, bromides, and phosphates in the water. However, the concentrations of dissolved metal ions are all consistent with those found in the Acre River, and other muddy water lakes and rivers in the Amazon basin, reflecting the mineralogy of the suspended material. The concentrations of most heavy metals (Pb, Cu, (Zn), Cd and Hg) are below 0.01 mg L-1, while that of Hg (0.006 mg L-1) is above the recommended limits for human consumption. Overall, the results of the study indicate that Amapá Lake is suffering ongoing impacts (physical and chemical) from urban development, which are mitigated by the periodic flooding of the river. Key words: Urban expansion, Environmental impacts, Waters, Heavy metals.

Murrili meteorite’s fall and recovery from Kati Thanda

AbstractOn November 27, 2015, at 10:43:45.526 UTC, a fireball was observed across South Australia by 10 Desert Fireball Network observatories lasting 6.1 s. An ~37 kg meteoroid entered the atmosphere with a speed of 13.68 ± 0.09 km s−1and was observed ablating from a height of 85 km down to 18 km, having slowed to 3.28 ± 0.21 km s−1. Despite the relatively steep 68.5° trajectory, strong atmospheric winds significantly influenced the darkflight phase and the predicted fall line, but the analysis put the fall site in the center of Kati Thanda–Lake Eyre South. Kati Thanda has meters‐deep mud under its salt‐encrusted surface. Reconnaissance of the area where the meteorite landed from a low‐flying aircraft revealed a 60 cm circular feature in the muddy lake, less than 50 m from the predicted fall line. After a short search, which again employed light aircraft, the meteorite was recovered on December 31, 2015 from a depth of 42 cm. Murrili is the first recovered observed fall by the digital Desert Fireball Network (DFN). In addition to its scientific value, connecting composition to solar system context via orbital data, the recovery demonstrates and validates the capabilities of the DFN, with its next generation remote observatories and automated data reduction pipeline.

Persistence of remnant patches and genetic loss at the distribution periphery in island and mainland populations of the quokka

Understanding the spatial structure of populations is important in developing effective management options for threatened species, and for managing habitat connectivity for metapopulation function, and for demographic and genetic heterogeneity. We used genetic information to investigate the structure of populations of the quokka, Setonix brachyurus, in south-west Western Australia. We hypothesised that movement between known populations would be relatively rare and result in significant genetic structuring. Genetic analyses from 412 adult individuals at 14 nuclear markers (microsatellite) from 33 sampling locations identified structure, diversity and spatial separation of quokkas across their mainland distribution and on two islands. We identified nine inferred (K = 9) populations of quokka that would be otherwise difficult to define with standard ecological techniques. The highest genetic diversity was evident in a large central population of quokka in the southern forest area and genetic diversity was lower at the peripheries of the distribution. The Rottnest Island population contained 70% of the genetic diversity of the mainland populations but the genetic diversity of animals on Bald Island was markedly lower. Populations of quokka in the northern jarrah forest were the only ones to show evidence of recent or long-term population bottlenecking. Of particular interest was the recently identified population at the Muddy Lakes area (the only remaining locality on the Swan Coastal Plain), which was identified as being genetically associated with the southern forest population. Overall, spatial and population cluster analysis showed small insular populations in the northern jarrah forest area, but in the southern forests there appears to be a large panmictic population.

The significance and timing of sheetflood vs braided channel deposition on lacustrine fan deltas, Junggar Basin, NW China

ABSTRACTThe fan-delta reservoir play has become an important exploration target within the Junggar Basin, especially in the Mabei area within the Mahu Sag, where a fan-delta oil and gas pool has been recently discovered. The sedimentary characteristics, distribution patterns and formation mechanisms of the fan-delta clastic bodies in Lower Triassic Baikouquan Formation (T1b) were studied using seismic, well log and core data accompanied by a flume tank experiment and a modern analogue depositional study. The T1b in the Mabei area is composed of a fan delta consisting of fan-delta plain (including debris flow deposits, sheetflood deposits, braided channel deposits, and floodplain deposits), fan-delta front (including subaqueous reworked sheetflood deposits and distal sheetflood deposits) and muddy lake deposits. The sheetflood deposits, characterised by moderately sorted conglomerates with brown matrix, form during the peak flooding. They are widespread and sheet-like, occupying the major portion of the fan-delta plain. The braided channel deposits are characterised by well-sorted and clast-supported conglomerates and pebbly sandstones, formed later during the falling flow stage. Owing to the decline in volume and velocity, and the formation of continuously braided fluid flow, the sediments of the flood period are reworked, generating the braided channels. Laterally, the braided channels occur as belts of clastic bodies surrounded by continuous sheetflood deposits. Vertically, the braided channels are interbedded between the sheetflood deposits. The subaqueous reworked sheetflood deposits are characterised by greyish-green, well-sorted and clast-supported conglomerates, whereas the distal sheetflood deposits are characterised by well-sorted sandstones, intercalated with mudstone. The subaqueous reworked sheetflood and distal sheetflood deposits are a distal partly subaqueous extension of the main sheetflood deposits, albeit reworked by basinal currents and waves. The distal sheetflood deposits form on distal fringes of the sheetfloods and are more thoroughly reworked by longshore and wave currents. The braided channel, subaqueous reworked sheetflood and distal sheetflood deposits can form high-porosity reservoirs. These findings challenge the common view and suggest that the channelised facies on the fans are not the main flood events; rather, the more extensive sheetfloods are the major flood events.

Occurrence of Unusual Unarmored, Unlithified Fossil Mud Balls in Plio-Pleistocene Lacustrine Sediments, Kashmir, India

AbstractDespite documentation of fossil armored mud balls in the geological record, no fossil unarmored mud balls have been previously reported. Here, we report for the first time the occurrence of fossil unarmored mud balls at two locations from 2.6 Ma lacustrine sediments in the Kashmir basin, India. The two locations are stratigraphically equivalent. At location A, brown mud balls of various shapes and sizes occur with large rounded and angular blocks of brown and gray mud in a random manner, indicating that they were formed quickly by short transport and by the slumping of lake beach cliff, triggered by an earthquake. This conclusion is supported by the presence of slump folds within this mud ball unit. The external morphology and internal structure of these mud balls suggest their growth by rolling over the muddy lake beach in a unidirectional transport by strong tidal waves. The mud balls at location B are gray in color and smaller in size than those at location A. Location B mud balls show high sph...

Density-Flow Deposition In A Fresh-Water Lacustrine Rift Basin, Paleogene Bach Long Vi Graben, Vietnam

Abstract The aim of this study is to describe density-flow facies variability and interpret their flow evolution in a fresh-water rift-lake system. The data were collected from the 500-m-thick Paleogene lacustrine oil-prone source-rock succession penetrated by the Enreca-3 core-hole at the intersection of the Song Hong and the Beibuwan basins, Vietnam. The sedimentological data collected are supplemented with source-rock screening data to get an insight into the origin of mud in the density flows. A wide range of density-flow facies are recognized and can be assigned to turbulent, transitional, and laminar flow processes. The beds range from centimeter-scale, mudstone-rich beds to meter-scale cohesive debrites and hybrid beds. Similarly to the “classic” hybrid beds, the centimeter-scale beds are interpreted to record flow transformation and concentration. The hybrid bed motifs include essentially similar bed divisions (H1–H5; Haughton et al. 2009) to those described from marine basins. Moreover, transitional-flow facies are particularly common and include transitional current ripples and variously developed cyclic banding and lamination. These facies occupy a fixed position below and/or at plug-flow units in the bed motifs, suggesting that flow dynamics related to plug-flow development governed their development. Source-rock screening data show that most of the mudstone in hybrid beds represents lake-bottom and lake-margin mud, which was likely assimilated into the density flows along their path. Similarly, sedimentological data show common evidence for interaction between flow and the muddy lake floor such as sheared flame structures, deformed mud intrusions, and interbeds in turbidite facies. The assimilation of the lake-bottom mud into the density flows probably played a key role in modulating flow turbulence and explains the common occurrence of transitional-flow facies and indications of late-stage flow concentration in these strata.

An Artifact Assemblage from Area B at the Grace Creek Site (41GG33), Gregg County, Texas

The Grace Creek #1 site (41GG33, GC–1) was situated on a natural alluvial rise on the east side of Grace Creek, about 0.4 km north of its confluence with the Sabine River. On the north side of the site was an abandoned Sabine River lake bed, while to the south was an old channel, as well as a channel lake (Muddy Lake), of the Sabine River. Jones divided the site into three areas (A, B, and C); a midden deposit was apparently located in Area B on the central part of the rise. Buddy Calvin Jones identified and worked at the Grace Creek #1 site between 1954 and 1956, while the site was being destroyed for the construction of an earthen dike along Grace Creek and the Sabine River. In addition to the extensive surface collection of projectile points, lithic tools, and ceramic sherds he found there, in areas A–C, Jones also conducted limited excavations in areas where apparently organically–stained soil and possible feature stains were noted on the scraped surface of the site. In these excavations, he documented midden deposits, a flexed burial in the midden deposits in Area B, two pit features in this area, and several small (ca. 10 cm in diameter) post holes in Area C. Jones' map of the site did not indicate the location of the excavations in Area C, but Jones suggested that aboriginal houses were likely present here. The ceramic artifacts discussed in this article are from a fire pit in Area B that was excavated by Buddy Jones in October 1956. There are also a number of arrow points in the collections from the site, as well as a large ceramic elbow pipe. These materials are in the collections of the Gregg County Historical Museum in Longview, Texas.

Ross River virus risk associated with dispersal of Aedes (Ochlerotatus) camptorhynchus (Thomson) from breeding habitat into surrounding residential areas: muddy lakes, Western Australia.

Rapid population growth in Western Australia has resulted in increased development of land for residential housing, and new developments are often proposed close to water because of intrinsic aesthetic values. However, this placement may place future residents at risk of mosquito-borne disease, of which Ross River virus (RRV) disease is the most common in Australia. Mosquito dispersal data were combined with a spatial analysis of human RRV cases to show that mosquitoes dispersed readily from larval habitat into surrounding low- and high-density residential areas and that residents living within 2 km of mosquito breeding habitat had a significantly higher rate of RRV disease. This finding highlights the importance of planning authorities in state and local governments to consider the implications of mosquito-borne disease risks when assessing residential development applications.

Authigenic carbonates related to active seepage of methane-rich hot brines at the Cheops mud volcano, Menes caldera (Nile deep-sea fan, eastern Mediterranean Sea)

On the passive margin of the Nile deep-sea fan, the active Cheops mud volcano (MV; ca. 1,500 m diameter, ~20–30 m above seafloor, 3,010–3,020 m water depth) comprises a crater lake with hot (up to ca. 42 °C) methane-rich muddy brines in places overflowing down the MV flanks. During the Medeco2 cruise in fall 2007, ROV dives enabled detailed sampling of the brine fluid, bottom lake sediments at ca. 450 m lake depth, sub-surface sediments from the MV flanks, and carbonate crusts at the MV foot. Based on mineralogical, elemental and stable isotope analyses, this study aims at exploring the origin of the brine fluid and the key biogeochemical processes controlling the formation of these deep-sea authigenic carbonates. In addition to their patchy occurrence in crusts outcropping at the seafloor, authigenic carbonates occur as small concretions disseminated within sub-seafloor sediments, as well as in the bottom sediments and muddy brine of the crater lake. Aragonite and Mg-calcite dominate in the carbonate crusts and in sub-seafloor concretions at the MV foot, whereas Mg-calcite, dolomite and ankerite dominate in the muddy brine lake and in sub-seafloor concretions near the crater rim. The carbonate crusts and sub-seafloor concretions at the MV foot precipitated in isotopic equilibrium with bottom seawater temperature; their low δ13C values (–42.6 to –24.5‰) indicate that anaerobic oxidation of methane was the main driver of carbonate precipitation. By contrast, carbonates from the muddy lake brine, bottom lake concretions and crater rim concretions display much higher δ13C (up to –5.2‰) and low δ18O values (down to –2.8‰); this is consistent with their formation in warm fluids of deep origin characterized by 13C-rich CO2 and, as confirmed by independent evidence, slightly higher heavy rare earth element signatures, the main driver of carbonate precipitation being methanogenesis. Moreover, the benthic activity within the seafloor sediment enhances aerobic oxidation of methane and of sulphide that promotes carbonate dissolution and gypsum precipitation. These findings imply that the coupling of carbon and sulphur microbial reactions represents the major link for the transfer of elements and for carbon isotope fractionation between fluids and authigenic minerals. A new challenge awaiting future studies in cold seep environments is to expand this work to oxidized and reduced sulphur authigenic minerals.

Early Pleistocene freshwater communities and rodents from the Pasinler Basin (Erzurum Province, north-eastern Turkey)

An Early Pleistocene lacustrine faunal assemblage from Pasinler (Erzurum Province, north-eastern Turkey) is described, which encompasses 13 mollusc, 5 fish, 2 amphibian, 1 reptile, and 1 mammal species. The autochthonous freshwater mollusc fauna indicates shallow stagnant waters and a fine-sandy to muddy lake bottom, grown with submersed plants. The fish community (Leuciscus sp., Rutilus sp., Chondrostoma sp., Leuciscinae sp. 1 and 2, and Capoeta sp.) and autochthonous herpetofauna (Latonia sp. and Natrix sp.) provide evidence of a well-oxygenised palaeolake with rich periphyton and partially rocky to gravelly bottom. Moreover, the presence of the terrestrial snail Caspicyclotus cf. akramowsii, the peri- to semiaquatic Latonia, Natrix, and Allophaiomys cf. pliocaenicus reflects wet-shore habitats surrounding the lake. The specimens of Latonia sp. from Pasinler represent the youngest fossils of Latonia recorded from western Asia. Apparently, Latonia survived well into the Early Pleistocene in western Asia and southern Europe. Due to progressive cooling during the Late Pleistocene, however, the genus finally became extinct to the north of its present distribution area. The presence of the arvicolid Allophaiomys cf. pliocaenicus allows for dating the locality at ca. 1.0 to 1.55 Ma. The overlying alluvial sediments of the Pasinler Beds mark the onset of sedimentation of the Palaeo-Araks River into the basin and are Middle to Late Pleistocene in age.

Study of Water Aquaculture Coleus Blumei to Improve the Landscape Water Body Quality in Eutrophic Waters

Choose to use different size a coleus blumei，with natural palm tree hair fiber for suspend to carry a body and regulate to go together with heavy to constitute to certainly plant system, observe a coleus blumei the growth condition in water body and improvement fluid matter effect in eutrophic waters. The results showed that coleus blumei completely can give birth very well in water suspend growth；that every coleus blumei could bring about many fibrous roots which reached 0.10cm to 0.15cm in the first day and that the longest fibrous roots reached about 16.5cm to 24.0cm during 15 days. The biomass measurement discovers that the plant more big its biomass increase more big，the plant is more small it the clean increment biomass be more big.The water body clarity raises to from the 41cm to 97cm and compare to change into almost transparent clear water for the muddy lake water；and the concentrations of CODCr,TP and TN were decreases to 80.0%，88.6% and 93.8% respectively.The DO concentrations also kept in certain satisfied level for using the coleus blumei method.

The Wealden (non-marine Lower Cretaceous) of the Wessex Sub-basin, southern England

The Wealden Beds (non-marine Lower Cretaceous) of the Wessex Sub-basin, southern England, are exposed principally in coastal sections on the Isle of Wight and in Dorset. Geological Conservation Review sites within these strata have been extensively documented since the earliest days of geological enquiry in Great Britain. The succession is dominated by the alluvial Wessex Formation which demonstrates a broad east–west transition from meanderplain lithofacies to coarser-grained alluvial sediments, in relative proximity to the Cornubian source massif. The meanderplain sediments on the Isle of Wight are of international importance for their plant and animal fossils, the latter including many dinosaurs and their trackways. Upper Barremian transgression resulted in the spread of muddy lakes and coastal lagoons from the Weald Sub-basin into the eastern part of the Wessex Sub-basin, around or through the Purbeck–Isle of Wight structure. The resulting richly fossiliferous mudrock-dominated strata are now represented by the Vectis Formation on the Isle of Wight and in Swanage Bay, Dorset. The Geological Conservation Review sites in the Wessex Sub-basin are documented and interpreted, with particular reference to research history, chronostratigraphy, structural context, palaeoenvironments, palaeobiology and palaeoclimatology. New directions for research are proposed, as applicable.

Surviving quokka (Setonix brachyurus) population on the Swan Coastal Plain, Western Australia

A survey of the Muddy Lakes area on the Swan Coastal Plain was carried out to determine the presence of quokkas. Extensive on-ground searches found a carcass, juvenile skull, and fresh scats, which were identified as belonging to quokka, using DNA analyses. This is currently the only known population remaining on the coastal plain.

The sedimentary record of drifting ice (early Wisconsin Sunnybrook deposit) in an ancestral ice-dammed Lake Ontario, Canada

Outcrops of pebbly mud (diamict) at Scarborough in Southern Ontario, Canada (the so-called Sunnybrook ‘Till’) are associated with the earliest incursion of the Laurentide Ice Sheet (LIS) into mid-continent North America some 45,000 years ago. The Sunnybrook is a blanket-like deposit containing deepwater ostracodes and occurs conformably within a thick (100 m) succession of deltaic and glaciolacustrine facies that record water depth changes in a large proglacial lake. Contextual evidence (associated facies, sedimentary structures, deposit geometry and landforms) indicates a low energy depositional setting in an ice-dammed ancestral Lake Ontario in which scouring by floating ice masses was an important process. U-shaped, iceberg-cut scours (with lateral berms) up to 7 m deep, occur on the upper surface of the Sunnybrook and are underlain by ‘sub-scour’ structures that extend several meters below the scour base. Ice-rafted concentrations of clasts (‘clast layers’), grooved surfaces formed by floating ice glissading over a muddy lake floor (‘soft sediment striations’) and melanges of sand and mud mixed by grounding ice keels (‘ice keel turbates’) are present and are all well known from modern cold environments. The wider significance of this depositional model is that the LIS margin lay east of Scarborough and did not overrun Southern Ontario. This finding is in agreement with recent data from the Erie Basin of Canada, Ohio, and Indiana where deposits formerly correlated with the Sunnybrook (and thus implying an extensive early Wisconsin ice sheet) are now regarded as Illinoian. A speculative hypothesis is proposed that relates deposition of the Sunnybrook and two younger deposits of similar sedimentology, to surge-like instabilities of the southern LIS margin.

LA AMISTAD EN EL PELIGRO DE J. DE VALDIVIELSO

The play portrays the history of salvation by means of the allegory of life as a journey from Paradise back to God. Two incidents are dramatized, which share the same basic pattern of temptation, fall, repentance, and reconciliation, but are developed very differently, each becoming a short allegory. The first is rather unusual: man falls into a muddy lake, and as Christ pulls him out he falls (becomes muddy, i.e. assumes human nature) and saves man. Cleaned with the waters of baptism, he is ready to continue his pilgrimage. I explain the philological background of the allegory; its early development; and its moral content.

Wind-blown Rocks and Trails on a Dry Lake Bed: An Alternative Hypothesis

Rocks with maximum dimensions between 5 and 50 cm, which have moved distances of 35 cm to 400 m over the dry, mudcracked upper surface of the seasonal lake at Magdalenasmeer in South Africa, have left well defined traces on the argillaceous material. This phenomenon is postulated to reflect a slick muddy lake surface that developed during the passing of a cold front in the middle of the winter season, and which developed as a result of dew precipitation related to high humidity, and two subsequent freezing nights. The resultant slick surface is thought to have represented a combination of damp mud polygons and ice-filled cracks in between, and movement of the rocks was as a result of gusts of wind on the second freezing night.

The effects of glacial surging on sedimentation in a modern ice-contact lake, Alaska

In June 1987, Carroll Glacier (Glacier Bay, southeast Alaska) began to surge southeastward toward Wachusett Inlet. Between 9 June and 1 July 1987, the terminus advanced with a mean velocity of 29 m/day, slowing afterward but attaining a net displacement of >1 km by July 1988. A proglacial lake lay directly in the path of the surge and became partly overridden, with only 25% of its former basin remaining exposed after 1 yr. The effects of the ice advance on the lake were profound. Early in the surge, lake-water samples attained suspended-sediment concentrations exceeding 9 g/L well above bottom. Extremely rapid sedimentation, locally attaining 8 m in 36 days, produced nearly uniform water depths of 8-9.5 m by August 1987. Sediment deposited during the surge was extremely soft and underconsolidated, composed of massive to irregularly layered muddy silt produced by disruptions of the lake floor and by remobilization of lake-bottom sediment by sediment gravity flows. A prominent push moraine, developed from a low ridge initiated by glacial loading, formed where the glacier overrode sandy substrate, but no similar feature was observed in the muddy lake floor. By July 1988, the surge had greatly diminished, and the remaining lake basin was slowly uplifted by displacement from the weight of the ice. By the end of August 1988, most of the lake floor was completely exposed and undergoing erosion by meltwater channels, causing abnormally high suspended-sediment loads at the outlet compared to the previous year when the lake was an effective sediment sink.

Turbid Density Currents in Reservoirs

ABSTRACT Physical features of turbid density currents in reservoirs are touched on in the first part of this paper: the plunging of the muddy fluid, the unsteadiness of the density current, the characteristics of velocity and concentration of density currents, the density current over a submerged dam and the submerged muddy lake formed by the density current upstream from the dam site. Besides the conditions for formation of density currents, the condition for maintenance of a density current that may vent sediment from a reservoir is discussed. The outflow silt discharge by a vented density current, calculated by a suggested approximate method, agrees fairly well, on the average, with measured values in the Guanting Reservoir and Lake Mead.