Large-scale Erosion Research Articles

Organic carbon transport and C/N ratio variations in a large tropical river: Godavari as a case study, India

This study gives an insight into the source of organic carbon and nitrogen in the Godavari river and its tributaries, the yield of organic carbon from the catchment, seasonal variability in their concentration and the ultimate flux of organic and inorganic carbon into the Bay of Bengal. Particulate organic carbon/particulate organic nitrogen (POC/PON or C/N) ratios revealed that the dominant source of organic matter in the high season is from the soil (C/N = 8–14), while in the rest of the seasons, the river-derived (in situ) phytoplankton is the major source (C/N = l–8). Amount of organic materials carried from the lower catchment and flood plains to the oceans during the high season are 3 to 91 times higher than in the moderate and low seasons. Large-scale erosion and deforestation in the catchment has led to higher net yield of organic carbon in the Godavari catchment when compared to other major world rivers. The total flux of POC, and dissolved inorganic carbon (DIC) from the Godavari river to the Bay of Bengal is estimated as 756 × 109 and 2520 × 109 g yr−1, respectively. About 22% of POC is lost in the main channel because of oxidation of labile organic matter, entrapment of organic material behind dams/sedimentation along flood plains and river channel; the DIC fluxes as a function of alkalinity are conservative throughout the river channel. Finally, the C/N ratios (∼12) of the ultimate fluxes of particulate organic carbon suggest the dominance of refractory/stable soil organic matter that could eventually get buried in the coastal sediments on a geological time scale.

Erosion Control Using Wood Waste Materials

The field experiments presented here investigated the required thickness of wood waste materials used as a mulch to protect a soil slope against erosion. Each of the materials was subjected to large-scale erosion control testing at a field site with a slope of 1 vertical to 2 horizontal. Fourteen test cells (5′ W × 30′ L each) were prepared with different wood waste treatments; nine contained erosion control mulch applications at thicknesses of 3/4 to 3 inch. Two cells were left untreated as reference cells and three other cells were untreated but contained erosion control structures including wood waste filter berm, geosynthetic silt fence and hay bale silt barrier. The erosion control performance of each cell treatment was evaluated for 11 storm events of varying rainfall magnitude and intensity. Calibrated tipper buckets were used to measure the runoff from each cell and collection buckets were used to sample runoff and determine the mass of sediment eroded from each cell. Total rainfall and intensity of each storm was measured with an electronic rain gauge. The results showed that 0.75 inches of wood waste material is sufficient to reduce the erosion to levels below commonly used treatments. Using thicker applications of wood waste did not reduce the erosion much below that obtained with 0.75 inches. This effect was demonstrated with three different wood waste materials. In addition, one of the wood wastes (paper mill wood waste) was used as a filter berm to control eroded soil particles. The behavior of this berm was compared to standard methods of controlling these particles such as hay bales and geosynthetic silt fence, and found superior. Laboratory tests are planned to quantify the properties of wood waste that produce good filtering action in berms.

Modeling large-scale fluvial erosion in geographic information systems

Variants of the stream power model have become standard for large-scale erosion modeling in geographic information systems (GIS) because they can be applied over broad areas without the need for detailed knowledge of stream characteristics. GIS-based implementations of the shear stress, stream power per unit length and stream power per unit area models are closely related to one another and related also to empirical sediment yield models derived from continental-scale factor analyses. Based on a detailed examination of the implementation of stream power analyses at the scale of continental mountain ranges, we demonstrate that: (1) the careful selection of a digital elevation model (DEM) projection can minimize length and area distortion when analyzing large portions of the earth (such as the Himalaya or Andes) in the two-dimensional plane of a DEM. (2) The area-discharge proxy frequently employed in GIS-based stream power studies may not be appropriate for rivers that flow through significant rain shadows or climatic zones. (3) Decreasing the resolution of a DEM from 30- to the 900-m typical for studies of large extent decreased the mean slopes of 15 rivers in the Olympic mountains by 65%, increased the mean drainage basin size by 14%, and caused a 17% reduction in median main-stem channel length. (4) The coefficients k, m and n common to different versions of the Stream Power Law are themselves sensitive to grid resolution when determined from an analysis of area–slope plots. (5) Stream power per unit area decreased in the Olympics mountains as grid resolution decreased.

Erosion and exhumation in the Himalaya from cosmogenic isotope inventories of river sediments

The outward erosional flux is a key factor in the tectonic evolution of mountain belts and there is much debate about the feedbacks between tectonics, erosion and climate. Here we use cosmogenic nuclides ( 10Be and 26Al) analysed in quartz from river sediments from the Upper Ganges catchment to make the first direct measurements of large-scale erosion rates in a rapidly uplifting mountain belt. The erosion rates are highest in the High Himalaya at 2.7±0.3 mm/yr (1σ errors), fall to 1.2±0.1 mm/yr on the southern edge of the Tibetan Plateau and are 0.8±0.3 to <0.6 mm/yr in the foothills to the south of the high mountains. These relative estimates are corroborated by the Nd isotopic mass balance of the river sediment. Analysis of sediment from an abandoned terrace suggests that similar erosion rates have been maintained for at least the last few thousand years. The data presented here, along with data recently published for European river catchments, demonstrate that a log–linear relationship between relief and erosion rate holds over three orders of magnitude variation in erosion rate and between very different climatic and tectonic regimes. The erosion rate estimates from cosmogenic nuclides correlate well with exhumation rates calculated from previously published apatite fission track ages in the Indian Himalaya. This confirms that much of the exhumation in the Himalayan mountain chain is now balanced by erosion. However, exhumation rates calculated from high blocking temperature systems, such as 40Ar/ 39Ar in muscovite, imply lower exhumation rates. Rocks presently at the surface must have undergone a three- to six-fold increase in exhumation rate within the last few million years. We show how this could be explained either by climatic forcing of erosion rate changes or by tectonics. Published evidence for equally rapid changes of exhumation rate in the past and the probable diachroneity in the time at which the present exhumation rates accelerated imply that tectonics has moderated at least some of the change in exhumation rates.

Material transport from the near shore to the basinal environment in the southern Baltic Sea: II: Synthesis of data on origin and properties of material

The Pomeranian Bight (southern Baltic Sea) is a mixing zone between waters of the Baltic Proper and the river Oder, which drains a densely populated and highly industrialised catchment of central Europe. The bight is a nondepositional area, and all material produced in its water column, from erosion of strata at the seafloor and cliffs, and delivered by rivers, is transported near the seafloor to the depositional areas of the Arkona, Bornholm and Gdansk basins. In this contribution, we assess the origin, transformation and mass fluxes of material through the bight based on an integrated field study conducted in the period 1996–1998. The transport mechanism is by wave- and current-induced resuspension and settling cycles, which effectively enrich organic-rich material and associated substances (organic pollutants, heavy metals) in deeper water; the estimated transport time is less than 6 months. The phases in which the material is transported are suspended matter in the water column, a particle- and aggregate-rich benthic boundary layer of <1 m above the seafloor and a layer of fluffy material fed from the two other sources that covers the sandy near-shore sediments as a discrete phase; it collects up to 130 g m −2 of particulate material after quiescent periods lasting several days. It is easily resuspended at shear velocities around 5 cm s −1 and is recycled into the suspended matter and benthic boundary layer pools of material. In deeper waters (>20 m water depth), the fluffy layer is not readily distinguished from the underlying soft, organic-rich sediment and the change in physical and chemical properties is gradual. The organic matter passing through the coastal zone in the southern Baltic is unaffected by biological or chemical modifications in composition. We find no evidence for a preferential removal of nitrogen or phosphorus, even if the speciation of phosphorus changes from biological compounds to minerals. The compositional changes which we see, i.e., in the nitrogen isotopic composition and in trace metal concentrations, are mainly caused by dilution of the river signal. In the case of polyaromatic hydrocarbons (PAH), different solubilities and compound stabilities affect the concentrations as well and result in the enhanced accumulation of stable compounds in Arkona Basin. Seasonal changes are pronounced in the amount of freshly produced biomass, as is seen in phytoplankton pigments and their degradation products, but significant amounts of fresh biomass are swept out of the bay and supply the Arkona Basin benthic community with additional nutrition. An imbalance in carbon import and export emerges from mass balance calculations: 50,000 t of organic carbon per year may be exported, which cannot be accounted for by known sources in the river or the bight. We may exclude erosion of early Holocene peat exposures at the seafloor as a possible source, but cannot exclude either errors in our export estimate, or large-scale erosion of other organic carbon pools, for example, the stock of seaweed and its substrate in the Greifswalder Bodden.

Paleogeographic evolution and avulsion history of the Holocene Rhine-Meuse delta, The Netherlands

AbstractApproximately 200,000 lithological borehole descriptions, 120014C dates, 36,000 dated archaeological artifacts, and gradients of palaeochannels were used to reconstruct the Holocene evolution of the fluvial part of the Rhine-Meuse delta. Ages of all Holocene channel belts were stored in a Geographical Information System database that enables generation of palaeogeographic maps for any time during the Holocene. The time resolution of the palaeogeographic reconstruction is about 200 years.During the Holocene, avulsion was an important process, resulting in frequent shifts of areas of clastic sedimentation. Palaeogeographic evolution and avulsion history of the Rhine-Meuse delta are governed by complex interactions among several factors. These are: (1)Location and shape of the Late Weichselian palaeovalley.In the Early Holocene, rivers were confined to the LateWeichselian valley. When aggradation shifted upstream, the margins of the valley were crossed by newly formed channel belts. (2)Sealevel rise, which resulted in back-filling of the palaeovalley. (3)River channel pattern.In the central-western part of the delta, a straight anastomosed channel pattern with large-scale crevassing developed as a result of sealevel rise and the associated decrease of stream power. (4)Neotectonics.Differential tectonic movements of the Peel Horst and Roer Valley Graben seem to have influenced river behaviour (formation of an asymmetrical meander belt, location of avulsion nodes in fault zones), especially from 4500–280014C yr BP when the rate of sealevel rise had decreased. After 280014C yr BP sealevel rise further decreased, and tectonic influence still may have influenced avulsions, but from then on other factors became dominant. (5)Increased discharge, sediment load and/or within-channel sedimentation.After 280014C yr BP, meander wavelenghts increased, which is interpreted as a result of increased bankfull discharge and/or within channel sedimentation. After 200014C yr BP both discharge and sediment load increased as a result of human influence. (6)Coastal configuration.The limited number of tidal inlets and extensive peat formation restricted the number of avulsions in the western part of the delta, and enhanced channel reoccupation. (7)Composition of the substrate and river banks.Meandering river channels tended to adhere to the sandy margins of the LateWeichselian palaeovalley, and high channel sinuosity is found in areas where river banks consisted of sand. Peat formation, which was most extensive in the western part of the back-barrier area especially between 4000 and 300014C yr BP, more or less fixed the river pattern at that time, hampering avulsions. (8)Marine ingressions, e.g. the 1421 AD St. Elizabeth’s flood caused large-scale erosion in the southwestern part of the fluvial deltaic plain, resulting in a shift of the main drainage to the SW. (9)Human influence.Since about 1100 AD human influence dominated the palaeogeographic evolution. Rivers were embanked and natural avulsions did no longer occur.

Runoff control of soil degassing at an active volcano. The case of Piton de la Fournaise, Réunion Island

Airborne atmospheric and in-situ soil gas analyses demonstrate that distal gas leaks are non-existent at Piton de la Fournaise (PF) despite frequent eruptions. Airborne traverses made at a constant altitude indicated that no detectable large-scale CO 2 degassing occurs from the volcano. In-situ soil gas traverses (CO 2, 4He, 222Rn) across the active rift-zones, the summit crater and the caldera rims, show both a lack of positive anomalies and evidence of strong depleted concentrations for He and CO 2 with respect to air. Depletions in soil He is explained by intense rain water infiltration. A more unexpected result lies in the very low CO 2 concentrations (down to 92 ppm) in bare soils in the summit region. Such a depletion was not previously reported and is attributed to considerable basalt weathering operating as the result of huge runoff. A model has enabled both the runoff and the CO 2 consumption occurring within the basaltic layers to be calculated. Data obtained are consistent with values expected from large-scale erosion models and with the local calculated diffusive flux of CO 2 in soils. The huge runoff, together with the basaltic lithology of the volcano, are found to be the main factor controlling the soil gas signatures at PF.

Coastal erosion due to long-term human impact on mangrove forests

A coast in southern Vietnam, which is located in a wide and flat alluvial fan and neighbors tidal rivers fringed by wide mangrove swamps, has been eroded continuously by approximately 50 m/year since the early 20th century. Based on field observations and numerical experiments, it is inferred that this large scale erosion is caused by the transition of mangrove vegetation resulting from the long-term impact of humans since the late 19th century. This eroded coast is not in direct contact with mangrove swamps, but is strongly affected by the existence of mangrove forests through the intermediation of neighboring tidal rivers. Thus, with a view to coastal protection, it is argued that the mangrove vegetation in adjacent areas should be managed more sensitively.

Spatial coincidence of rapid inferred erosion with young metamorphic massifs in the Himalayas

A spatially distributed rate-of-erosion index (EI) based on models of bedrock river incision documents a strong spatial correspondence between areas of high erosion potential and young metamorphic massifs as well as structural highs throughout the Himalayas. The EI is derived from slopes and drainage areas calculated from a hydrologically cor- rected digital elevation model (GTOPO30) combined with precipitation data (IIASA) to generate synthetic annual stream discharges. These variables drive three generalized pro- cess models to produce EI maps that, while differing in detail, provide an internally con- sistent, spatially continuous index of large-scale erosion rates. The large spatial variation in potential erosion rates in the Himalayas suggested by the EI patterns contrasts with the uniform convergence of the Indian subcontinent. If these EI gradients persist through time, they support the emerging view of a positive feedback between localized, rapid ero- sion and upward advection of lower crust.

Large-scale erosion rates from in situ-produced cosmogenic nuclides in European river sediments

We have calculated long-term erosion rates of 20–100 mm/kyr from quartz-contained 10Be in the bedload of middle European rivers for catchments ranging from 10 2 to 10 5 km 2. These rates average over 10–40 kyr and agree broadly with rock uplift, incision and exhumation rates, historic soil erosion rates, and erosion rates calculated from the measured sediment loads of the same rivers. Moreover, our new erosion rate estimates correlate well with lithology and relief. However, in the Regen, Neckar, Loire, and Meuse catchments, cosmogenic nuclide-derived erosion rates are consistently 1.5–4 times greater than the equivalent rates derived from measured river loads. This may be due to the systematic under-representation of high-magnitude, low-frequency transport events in the gauging records which cover less than a century. Alternatively the discrepancy may derive from spatially non-uniform erosion and preferential tapping of deeper sections of the irradiation profile. A third explanation relates the high cosmogenic nuclide-derived erosion rates to inheritance of an elevated Pleistocene erosion signal. Uncertainties associated with the cosmogenic nuclide-derived erosion rate estimates are not greater than the potential errors in conventional estimates. Therefore, the cosmogenic nuclide approach is an effective tool for rapid, catchment-wide assessment of time-integrated rates of bedrock weathering and erosion, and we anticipate its fruitful application to the Quaternary sedimentary record.

Sediment Loss from a Waste Rock Dump, ERA Ranger Mine, Northern Australia

During the 1994/95 wet season, runoff, suspended load and bedload loss from large scale erosion plots under natural rainfall events were measured at three sites with different treatments on the Energy Resources of Australia Ranger waste rock dump (cap, soil, fire). The fire site has well established trees, the soil site smaller shrubs and the cap site minimal vegetation. All three sites are located on the flat areas of the waste rock dump with similar slopes and cap, fire and soil are local names for the sites. The quantity of bedload eroded from the soil and fire sites decreased during the monitoring period, however, the same trend was not observed on the cap site. Bedload loss from the fire and soil sites is significantly log‐linear with time and reduces at a decreasing rate until becoming statistically constant toward the later part of the first half of the wet season. For corresponding storms at each site, bedload erosion was highest from the unvegetated and unripped cap site and lowest from the well vegetated fire site. Site specific linear relationships between bedload and total sediment load have been derived and can be used to derive total sediment load in the absence of measured suspended load.

Ice shelves in the Pleistocene Arctic Ocean inferred from glaciogenic deep-sea bedforms.

It has been proposed that during Pleistocene glaciations, an ice cap of 1 kilometre or greater thickness covered the Arctic Ocean. This notion contrasts with the prevailing view that the Arctic Ocean was covered only by perennial sea ice with scattered icebergs. Detailed mapping of the ocean floor is the best means to resolve this issue. Although sea-floor imagery has been used to reconstruct the glacial history of the Antarctic shelf, little data have been collected in the Arctic Ocean because of operational constraints. The use of a geophysical mapping system during the submarine SCICEX expedition in 1999 provided the opportunity to perform such an investigation over a large portion of the Arctic Ocean. Here we analyse backscatter images and sub-bottom profiler records obtained during this expedition from depths as great as 1 kilometre. These records show multiple bedforms indicative of glacial scouring and moulding of sea floor, combined with large-scale erosion of submarine ridge crests. These distinct glaciogenic features demonstrate that immense, Antarctic-type ice shelves up to 1 kilometre thick and hundreds of kilometres long existed in the Arctic Ocean during Pleistocene glaciations.

Late Weichselian and Holocene palaeogeography of the Rhine–Meuse delta, The Netherlands

A large number of lithological borehole descriptions, 14C dates, archaeological artefacts, and gradients of palaeochannels were used to reconstruct the Holocene development of the Rhine–Meuse delta. Data were stored in a Geographical Information System database that enables generation of palaeogeographical maps for any time during the Holocene. The time resolution of the palaeogeographic reconstruction is about 200 years. During the Holocene, avulsion was an important process, resulting in frequent shifts of areas of clastic sedimentation. Palaeogeographic evolution of the Rhine–Meuse delta is mainly governed by complex interactions among several factors. These are: (1) Location and shape of the Late Weichselian palaeovalley. (2) Sea-level rise, which resulted in back-filling of the palaeovalley. (3) Peat formation, which was most extensive in the western part of the back-barrier area especially between 4000 and 3000 14C yr BP. This more or less fixed the river pattern at that time and resulted in few avulsions. (4) Neotectonics. Differential tectonic movements of the Peel Horst and Roer Valley Graben seem to have influenced river behaviour (formation of an asymmetrical meander belt, location of avulsion nodes), especially from 4500 to 2800 14C yr BP when the rate of sea-level rise had decreased. After 2800 14C yr BP, the sea-level rise further decreased, and the tectonic influence still may have influenced avulsions, but from then on, other factors became dominant. (5) Increased discharge, sediment load and/or within-channel sedimentation. After 2800 14C yr BP, river meanders of Rhine distributaries as well as the single channel of the Meuse show remarkable increases in wavelength, interpreted as a result of increased bankfull discharge and sediment load. Increased discharge may initially have been caused by a higher precipitation. After 2000 14C yr BP, both discharge and sediment load seem to have increased as a result of human influence. Alternatively, decreasing gradients (as a result of sea-level rise) may have caused an increased within-channel sedimentation and channel widening, which would also lead to increased meander wavelengths. (6) Composition of the river banks. Meandering river channels tend to adhere to the sandy margins of the Late Weichselian palaeovalley, and a high channel sinuosity is found in areas where river banks consist of sand. (7) Marine ingressions, e.g., the AD 1421 St. Elizabeth's flood, caused large-scale erosion in the southwestern part of the fluvial deltaic plain. (8) Human influence. Since about AD 1100 human influence dominated the palaeogeographic evolution. Rivers were embanked and dredged, meanders were cut off, and for the Meuse, a new distributary canal (the Bergsche Maas) was dug in AD 1904. Discharge distribution over the various channels is strictly controlled nowadays. An unusual feature of the delta is the prolonged existence of the Oude Rijn channel belt.

Identifying large-scale erosion and deposition processes from airborne gamma radiometrics and digital elevation models in a weathered landscape

Patterns of erosion and deposition are difficult to identify and measure at catchment and regional scale but it may be possible to infer their distribution from remote sensing using easily measured surrogate variables. Airborne geophysical surveys provide data on gamma ray emissions from surface and near-surface material and allow estimation of K, Th and U content. Gamma ray signatures are largely determined by lithology but also change with weathering and with erosion and deposition, and may be used as a partial surrogate for those processes. Comparison of gamma ray signatures with topographic characteristics closely related to sediment transport capacity, downstream sorting of sediments, and the extent of erosion and deposition shows strong linkages. Studies in four small catchments in southeastern Australia indicate that K consistently increases as slopes become steeper while Th and U may either increase or decrease. This suggests the presence of fresh rock rather than weathered material and implies removal of material by erosion, although some patterns may result from systematic changes in lithology across catchments. Analysis by lithology confirms the increase in K with slope in granites, metamorphosed sediments and basalt, and also shows a tendency for U to decrease with slope in the granites and basalt. Gamma ray emissions vary only slightly with catchment area (a surrogate measure of water discharge) suggesting that water erosion is limited or that discharge is not closely related to area. Gamma radiometric profiles down hillslopes, averaged across the full range of gradients, show that, in most cases, radioelements initially decrease, probably reflecting increased weathering, but then increase close to valley floors because of accumulation of fine sediments. Analysis by lithology confirms the increase in radioelements close to valley floors in granites and metamorphosed sediments but the trend is less clear in basalt. Gamma ray profiles down floodplains are variable and indicate the amount of deposition and accumulation of weathered material in valley openings and exposure of bedrock in valley constrictions. Simple erosion and deposition models, based on the conservation of mass equation, and applied to the four catchments, show that all radioelements increase as the potential for deposition increases. They reach a low point in zones of no net erosion or deposition and subsequently increase as erosion becomes more intense and weathered material is lost from slopes. Analysis by lithology largely confirms this pattern. The usefulness of airborne geophysical survey data is limited by flight line spacing with most data being flown at a 200–400 m spacing in Australia. However, general trends in erosion and deposition can still be distinguished and there is capacity for calibrating long-term erosion and deposition models once better approaches to interpretation of gamma ray data have been developed. Copyright © 2000 John Wiley & Sons, Ltd.

Influence of sea level and basin physiography on emplacement of the late Pleistocene Herodotus Basin Megaturbidite, SE Mediterranean Sea

Radiocarbon 14C dates from pelagic intervals above a megaturbidite in the Herodotus Basin give direct evidence of emplacement at the beginning of the last glacial period, approximately 27,125 calendar years before present, as sea level lowered rapidly and entered a low stand phase. The Herodotus Basin Megaturbidite is a basinwide deposit that forms a recognisable acoustically-transparent layer on 3.5 kHz high-resolution seismic profiles and covers an area of approximately 40,000 km2. It thins from about 20 m in thickness proximally to some 10 m distally over a basin length in excess of 400 km. The total volume is estimated at around 400 km3, which is significantly more than the volume of sediment that could have been displaced from its most likely source area, the funnel-shaped marginal embayment of the Gulf of Salûm to the west of the Nile Cone. The additional material may have been derived, in part, from synchronous failures on other parts of the Libyan/Egyptian shelf and slope, but most is believed to have come from large-scale erosion at the base of the very large-volume turbidity current that was generated from the original slide. Detailed sedimentary analyses of cores recovered from the megaturbidite show its distinctive characteristics: graded sand, silt, mud and bioturbated mud units; poorly developed structures proximally becoming more distinct and ordered distally; variation in grain size and structures that suggest either flow separation/reflection around topographic highs and/or an inherently unstable flow; and a mixed-source bioclastic-terrigenous composition. These features, together with its size and lateral extent, would make any similar megabed an excellent marker horizon in basin analysis. A combination of factors was responsible for triggering the initial slide-debris flow event that evolved downslope into this mega-turbidity current. These include lowered sea level that destabilised the outer shelf — upper slope sediments, tectonic oversteepening of the margin, relatively high rates of sedimentation, and seismic activity.

The infill of a supradetachment(?) basin: the continental to shallow-marine Upper Permian succession in the Dolomites and Carnia (Italy)

In the Dolomites and Carnia (eastern Southern Alps), the Upper Permian succession is represented by red beds of Val Gardena Sandstone, grading upwards and eastwards into the evaporitic and carbonate deposits of the Bellerophon Formation. An overall transgressive pattern is shown by the general trend of the depositional setting, which evolved from flashy alluvial fans, through multiple-channel bedload rivers, mixed-load sinuous rivers, terminal fans, coastal sabkha and evaporitic lagoon, to a shallow, low-gradient marine ramp. The inferred fluvial regime was subject to rapid and erratic fluctuations in discharge. Palaeosols are represented by calcic soils, and suggest a warm to hot, semi-arid or dry subhumid climate with strongly seasonal rainfall. Vertic features are associated with more inland alluvial complexes: they are missing in the terminal-fan deposits, suggesting greater aridity in lowland (coastal) areas. The Bellerophon Fm. consists of two units: a lower evaportte-bearing unit, deposited in a barred basin, and an upper shallow-marine carbonate unit, laid down on a very low-energy, low-gradient ramp. Five third-order sequences and the lower part of a sixth sequence, collectively showing a backstepping pattern, have been identified in the succession. Due to the presence of a very low-gradient ramp margin, and the consequent ineffectiveness of relative sea-level falls in producing large-scale erosion in coastal areas, it is suggested that, rather than eustatic changes, degradational episodes marking sequence boundaries in the red-bed succession reflect changes in the climate-modulated intrinsic variables of discharge and sediment supply, and/or tectonic uplift. Base level rises in, red-bed sequences are recorded by upward change from thick channel-belt sandstone bodies with an often high connectedness ratio, to progressively thinner channel deposits, ranging from isolated ribbons to channel-belt sandstone bodies potentially reaching high width/thickness ratios, encased in a comparably greater volume of overbank fines. This trend is thought to reflect the change from a confined geomorphic setting, with a limited area of potential avulsion, to an unrestricted setting with rivers free to move extensively; in addition, it documents the transition from an inland fluvial system with laterally migrating perennial or semi-perennial channels, to an ephemeral network of randomly migrating and frequently avulsing small terminal-fan distributaries, through a drastic downstream decrease in channel depth and discharge. Identification of key sequence-stratigraphic relationships within the red-bed succession was mostly aided by the presence of easily recognizable and regionally correlated marine and marginal marine bed packages, traceable landwards into alluvial deposits showing faint traces of tidal activity, interpreted as the equivalent of marine maximum-flooding sediments. They may grade upwards into progradational fluvial packages showing basinward increase in thickness. The Upper Permian deposits of the Southern Alps are considered part of an Upper Permian-Lower Triassic, second-order, structurally controlled sequence. The location of the basin on a thickened, previously active crust, affected by thermal perturbance after the last stages of the Variscan orogeny, the relatively reduced thickness of the basin fill, and predominantly long, transverse drainage networks, mostly derived from the denuded Insubric footwall, all suggest that sedimentation took place in a supradetachment basin, with a major detachment fault located in the palaeo-Insubric belt.

The Status of Women in Subsistence Agriculture in the Garhwal Himalaya, India

The rural people of the Garhwal region are generally characterized as economically deprived. Their basic source of income is traditional agriculture on small terraced fields with low productivity. Animal husbandry is neither productive nor profitable yet excessive and uncontrolled grazing has led to large-scale soil erosion; the only economic benefit from livestock is the dung which provides fertilizer for agricultural fields. The daily requirements of the inhabitants for fuel, food, and fodder are a cause of great concern in any attempt to achieve or maintain sustainable development and a balanced ecosystem (Negi, 1992). In view of this, an integrated watershed management project has been developed in the Garhwal Himalaya to improve rural agriculture and, at the same time, to prevent overexploitation of the natural resources. A study was undertaken in the area south of the River Bhagirathi. Here, watersheds defined by natural boundaries in each drainage basin were designated as 'sub-watersheds' or 'mini-watersheds' and these units were used to classify information on the development of land, water, and plant resources to meet the subsistence needs of the hill farmers and farm women. The Henval, Hindolakhal, and Muni-Ki-Reti subwatersheds were selected for field study in the south Bhagirathi area in the Tehri Garhwal district. Within these, five mini-watersheds were selected: the Agar (about 280 ha) on the eastern banks of the Henval; the Lawa (336 ha) and Amani (358 ha) on the north and south banks of the Hindolakhal; and the Simswara (104 ha) and Byasi (575 ha) on the north and west banks of the Muni-KiReti.

Soil erosion due to settled upland farming in the Himalaya: a case study in Pranmati watershed

SUMMARY A common concept is that upland agriculture undertaken by local communities is a major factor causing large-scale soil erosion and other environmental problems in the Himalaya. Attempts to measure soil loss from farm fields are limited. This study was undertaken to measure the rate of soil loss from fields sown with crops during the rainy season and to examine the factors determining the erosion rates in the Pranmati watershed which is characterised by settled organic farming on terraced slopes. About 43% of the total agricultural land in the watershed was on low sloping terraces (< 2°), 32% on medium sloping terraces (2-6°) and 25% on highly sloping terraces (6-10°). Potato was the most dominant crop, occupying 50.2% of the total cropped area, followed by Amaranthus paniculatus (22.6%), Eleusine coracana (11.2%), Echinocloa frumentacea (10%) and Oryza sativa (6%). Soil loss from different crop covers was in the range of 0.300-0.658 t/ha/yr on low sloping terraces, 1–7 t/ha/yr on medium sloping terraces, and 6.037-64.39 t/ha/yr on highly sloping terraces. Comparison of different crops revealed the highest soil loss was from potato fields, a cash crop in the watershed. Potato cultivation on highly sloping terraces accounted for 72.6% of the total soil loss from agricultural fields. The area under this crop is rapidly increasing because of increasing emphasis on a monetary economy. Organic manure input in potato fields (28.5 t/ha) was much higher as compared to traditional crops (7.8-15.5 t/ha). Byproducts of potato do not have any fodder value, while the traditional crops do have useful fodder byproducts. The increase in the area under potato also implies more intensive pressure on the forests. The threat of soil erosion causing unsustainability of upland agriculture seems to be due more to the cultivation of potato than to traditional subsistence crops. Factors related to erosion and land-use policy aspects are discussed in the paper.

Environmental Degradation and the Classic Maya Collapse at Copan, Honduras (a.d. 600–1250): Evidence From Studies of Household Survival

AbstractThe Late Classic Maya abandonment of the Copan Valley, Honduras, began in the ninth centurya.d. and lasted approximately 250–300 years. The relationship between local ecological setting and residential group abandonment is examined by applying event-history analysis to the known occupation spans of 140 residential mound groups, dated by obsidian hydration. Late Classic households in ecologically vulnerable sections of the Copan Valley—as measured by slope, soil type, and natural vegetation—had significantly higher risk of abandonment than households in more ecologically stable settings. Abandonment risk rises sharply in all regions at the end of the seventh centurya.d. Both computerized agricultural simulations and settlement demographic reconstructions indicate that increased levels of agricultural intensification necessary to meet the subsistence needs of Copan's growing population would have led to large-scale erosion in upland areas and a significant reduction of soil fertility in all regions of the valley at that time. Mound-group abandonment patterns tend to support the hypothesis that environmental degradation played a dominant role in the collapse of the Copan polity.

Syn-rift sedimentation and structural development of the Gediz and Büyük Menderes graben, western Turkey

The Büyük Menderes and Gediz graben of western Turkey developed during Miocene to Recent extension in the Aegean region. New mapping of structures and sedimentary lithofacies in exhumed basin-fill strata is used to reconstruct the evolution of the graben. Field evidence shows that extension was primarily accommodated by tilted fault-blocks 0.2–0.8 km wide, bounded by planar faults, with some modification by antithetic faulting. The basins are faulted on both margins, although they are highly asymmetric, with dominant 'footwall' margins characterized by steeper topography and greater thicknesses of exhumed strata. Within exhumed graben-fill sequences, lacustrine, axial fluvial, and laterally-derived sedimentary facies can be identified. Palaeocurrent orientations, divergent wedge geometries and intra-basin unconformities all indicate that exposed sediments are syn-tectonic. The present day map patterns demonstrate that the position of graben-bounding faults has migrated basinward with time. As a result, large-scale erosion and recycling of the uplifted basin fill have created extensive footwall-derived alluvial fans. These displace the axial drainage towards the hanging-wall margins, away from the locus of greatest subsidence.