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Erosion and water-quality investigations are often conducted on small plots or watersheds, and under natural or artificial precipitation. Research objectives require flow measurement and sampling when large sediment concentrations occur. [...]

The analysis of (super 10)Be and (super 26)Al in bedrock (n=10), colluvium (n=5) and alluvial sediments (n-59) is made, which suggested that erosion rates in the Great Smoky Mountains are controlled by the subsurface bedrock erosion and diffusive slope process. The comparison between cosmogenic nuclide data with other measured and calculated erosion rates are used to conclude this analysis.

A simple procedure is proposed to assess the vulnerability of bridge piers in rivers, taking into account the phenomena governing fluvial dynamics during flood events. The procedure requires an estimation of the maximum scour depth of the soil surrounding both the pier and the foundation as well as an analysis of the bearing capacity of the pier-foundation-soil geotechnical system. The scour depth is determined in terms of the physical and mechanical properties of the streambed soil, the shape of the pier foundation and the destabilizing effects induced by hydrodynamic forces. The coupling of both the hydraulic and geotechnical analyses enables to identify the most significant factors characterizing scour depth and affecting pier vulnerability. Two levels (low, medium) of allowable vulnerability, bounded by an extreme condition of high vulnerability, are defined and analytically determined in function of the maximum scour depth and the foundation depth. Specific diagrams corresponding to each category of foreseen actions allow a quick evaluation of the vulnerability of a bridge pier. CE Database subject headings: Stream erosion; Bridge maintenance; Algorithms; Bearing capacity; Shallow foundation; Scour; Bridges, piers.

Soil aggregates low in organic matter and clay contents are generally susceptible to disintegration at low rainfall energies. This study was conducted to evaluate the effectiveness of ferrihydrite ([Fe.su.5] H[O.sub.8] * 4[H.sub.2]O) at stabilizing such aggregates, using five soils with a wide range of physical and chemical properties. The soils were amended with ferrihydrite at rates equivalent to 0, 0.34, 3.36, 16.80, and 33.60 Mg [ha.sup.-1], packed to a depth of 7.6 cm in plexiglass cylinders, and then exposed to simulated rainfall at an intensity of 64 mm [h.sup.-1] for 1.5 h. The erodibility data indicated that as ferrihydrite increased from 0 to 16.80 Mg [ha.sup.-1] on acid soils, infiltration increased an average of 21.5% while runoff and soil loss decreased 20 and 40%, respectively. Conversely, infiltration decreased 37% while runoff and soil loss increased 21 and 34%, respectively for alkaline soils. Further, sediment size distributions measured at these same ferrihydrite rates indicated that the >250-, and 250- to 53-[micro]m fractions increased 24 and 22% for acid soils and decreased 15 and 14%, respectively in alkaline soils. The

Temporal estimates of surface soil dry aggregate-size distributions (DASD) are needed to evaluate soil management systems and estimate wind erosion. This study was conducted to determine the most accurate and precise DASD to estimate aggregate-mean diameter and amount of wind-erodible soil. Over 5400 surface samples of soil dry aggregates were collected at various times throughout the year for 2 to 4 yr at 24 locations in six states. The soils represented a wide range of management systems and intrinsic soil properties, including mineral and organic soils. We evaluated four DASDs: the log-normal determined by two methods, fractal and Weibull distributions, and compared estimates of the aggregate-mean diameter and amount of wind-erodible soil derived from the distributions. We evaluated log-normal distributions expressed as amount oversize (LNO) and undersize (LNU) and tested the effect of using different smallest and largest sieve openings sizes. The Weibull distribution is the most accurate because the ranges of error of the Weibull were generally smaller than all other distributions over the full range of sieve sizes tested, rarely exceeding 0.15. The Weibull distribution is the most precise because only the Weibull had an error mode of [+ or -] 0.05 in all sieve-size classes tested. Using substantially different sizes for the smallest sieve-size openings had a great effect on estimates of aggregate-mean diameter and wind-erodible soil, but only when using LNO. Using substantially different sizes for the largest sieve size openings had a great effect on estimates of aggregate-mean diameter but little effect on estimates of wind-erodible soil.

In a headwater catchment in the Oregon Coast Range, we find that solid-phase mass losses due to chemical weathering are equivalent in the bedrock and the soil. However, the long-term rate of mass loss per unit volume of parent rock is greater in the soil than in the rock. We attribute this finding to the effects of biotic processes in the soil and to hydrologic conditions that maximize contact time and water flux through the mineral matrix in the soil. This result stems both from earlier work in which we demonstrated that rock and soil contribute equally to the solute flux and from arguments presented here that the basin is in dynamic equilibrium with respect to erosion and uplift. The silica flux of 10.7 [+ or -] 7.1 t.[km.sup.-2]x[yr.sup.-1] from the basin is several times larger than the flux from older soils elsewhere, but comparable to the flux from sites with similar physical erosion rates. This result argues that physical denudation or uplift rates play an important role in setting the chemical denudation rate. Physical processes appear to influence chemical-weathering rates in several ways. First, they limit chemical evolution by removing material, thus setting the residence time within the weathered rock and the soil. Second, bioturbation mixes rock fragments into the more reactive soil and maintains high soil porosity, allowing free circulation of water. Because the weathering in the soil is more intense than in the rock, we argue that the chemical denudation rate will diminish where uplift rates--and, hence, physical-denudation rates--are great enough to lead to a bedrock-dominated landscape. Chemical denudation rates will increase with physical-denudation rates, but only as long as the landscape remains mantled by soil. Keywords: chemical erosion, denudation, physical weathering, soil dynamics, uplift, weathering.

Interferometric synthetic aperture radar (InSAR) is a powerful geodetic tool used to construct digital elevation models of the earth's topography and to image centimeter-scale displacements associated with crustal deformation and the flow of ice sheets. The past decade has seen significant improvements in our understanding of earthquakes, volcanoes, and glaciers as a direct result of this technology. Geomorphology and hydrology can also benefit from InSAR. A small but growing body of work shows that it is possible to interferometrically measure centimeter-scale motions associated with unstable slopes, land subsidence, fluctuating soil moisture and water levels, and deforming river-ice cover. Maps of interferometric correlation provide information about the structure of snow and reveal areas disturbed by erosion, sedimentation, flooding, snowfall, and aufeis growth. At present, such techniques are underdeveloped and largely overlooked by the geographic and radar communities. This article reviews these emerging geomorphic and hydrologic applications of InSAR and presents a first demonstration of motion detection for river ice. Key Words: erosion, floods, hillslopes, interferometry, land subsidence, river ice, SAR, snow, soil moisture.

Late Cenozoic warping of the Colorado piedmont involved interplay of tectonic forcing, river erosion, and isostatic response to erosion. Modeled erosional isostasy closely replicates the observed pattern of deformation, but accounts for only about half Its magnitude. The remainder reflects tectonic rock uplift that increases southward across the piedmont, likely reflecting proximity to the northward-propagating Rio Grande Rift. This differential uplift triggered differential erosion, concentrated on southern piedmont river systems, particularly the Arkansas River, which led in turn to differential isostatic rock uplift focused on the Arkansas drainage. Covariation of tectonic uplift, erosion, and isostatic compensation across the piedmont reflects a positive feedback between uplift-induced erosion and erosion-induced isostasy, which has progressed to the point that isostatic uplift is approximately equal to the initial tectonic forcing. Keywords: Colorado, erosion, flexure, isostasy, Ogallala, tectonics.

The subaqueous portion of a levee bank in the Sacramento-San Joaquin River Delta of central California was instrumented to quantify the impact of boat-generated waves. Typical erosion rates associated with recreational craft are too small for direct measurement of bank retreat on a per-boat-passage basis; therefore, two independent analytical methods of estimating linear erosion were developed based on colocated suspended sediment concentration and velocity time series. The algorithms were tested using data measured during a field experiment in which a 7.5 m boat was driven past the site over a range of speeds to generate waves of varying size. A cross-shore array of electromagnetic current meters and optical back-scatterance sensors measured the character of boat-generated waves and the resultant sediment suspension. In near-bank, shallow-water (d DOI: 10.1061/(ASCE)0733-950X(2002) 128:4(152) CE Database keywords: Sediment suspension; Levees; Erosion; Ship motion; California; Measurement.

Landsliding is the dominant mass-wasting process in humid-temperate uplands and an important regulator of sediment yield from steep-land drainage basins. Information about the magnitude and frequency distribution of landslides has been derived from aerial photography, but it has proved difficult to set limits on the long-term scaling behavior of landsliding because the requirements of spatial and temporal coherence and the large number of observations necessary to undertake magnitude versus frequency analyses are not easy to fulfill. We use a 2250-yr-long record of hillslope erosion associated with extreme hydrologic events preserved in sediments from Lake Tutira, New Zealand, to investigate scaling in landslide deposits. Both the magnitude versus frequency distribution of sediment layers attributed to landsliding and the distribution of time intervals between landsliding events take the form of power laws, the former with an exponent b = 2.06 and the latter with an exponent b = 1.4. These results suggest that the erosional events originate from a self-organized critical process, and are in agreement with observations of scaling in turbidite deposits and grain flows in controlled laboratory experiments. The implications are that the aggregate behavior of landsliding at the catchment scale is orderly and that the stratigraphic record preserves a unique, long-term perspective on a fundamental geomorphic process and the extreme hydrologic events that trigger it. Keywords: erosion, self-organized criticality, sediment production, landsliding.

Is tectonic uplift or climate change the primary driving force behind continent-scale erosion? Here we examine these two hypotheses for tropical South America by comparing a high-resolution record of terrigenous sediment accumulation rates from the western tropical Atlantic Ocean with continental tectonic reconstructions and both low-latitude and high-latitude climate indices that span the past 13 m.y. Over the full record, terrigenous sedimentation rates most closely follow lowland aridity (as recorded by abundance ratios of terrigenous oxide minerals), an indication that basin-integrated climate is an important control of erosion. Step-like transitions in the flux and composition of terrigenous material near 8 and 4.5 Ma, however, follow dated tectonic events, which suggests progressive expansion of the drainage basin into less humid or less vegetated regions that were more prone to erosion. On long time scales, high-latitude climate variability and sea level (as recorded by [[delta].sup.18]O) do not closely track terrigenous sedimentation. We conclude that regional climate change provides a mechanism for long-term erosion of tropical South America; tectonics plays a significant, but secondary, role in this area, perhaps by shaping the drainage basin to include regions in which the climate is more favorable to erosion. Keywords: Amazon, South America, climate, Ocean Drilling Program, tectonics, sedimentation.

********** Soil loss was modeled to assess erosion potential at Camp Williams Army National Guard Training Facility in northern Utah (40.5° N, 112° E). Erosion models are available to estimate [...]

1 Large, infrequent disturbances can exert a dominant influence on the structure and dynamics of forested landscapes and in Westland, New Zealand there is evidence of at least three massive earthquakes within the last 650 years. We reconstructed the history of forest disturbance in two study areas, totalling 1412 ha, to quantify the role of such disturbance in structuring the conifer/hardwood forests. 2 We divided the study area into different land-surface types, aged trees on each land surface and identified cohorts of trees established in response to past disturbance. The type of disturbance (tree fall or erosion/sedimentation event) that initiated cohort establishment was determined from the type of land surface and other physical evidence. We also dated abrupt growth releases or suppressions in tree rings to aid disturbance history reconstruction. 3 Erosion and sedimentation events dominated the disturbance regime, affecting 86% of the study area in the last 650 years. Catchment-wide episodes of forest cohort-initiating disturbances were centred around 1820-30, 1710-20, 1610-20 and 1460 AD. Of the 51 cohorts identified in the study area, 47 were initiated during one of these episodes, when disturbance by erosion or sedimentation affected from 10-50% of the study area. Consequently, over 80% of the forested area currently comprises simple, first generation cohorts of trees established after catastrophic disturbance. Only 14% of the study area is more complex, all-aged forest. 4 Three disturbance episodes coincide with the three most recent Alpine fault earthquakes (c. 1717, 1630 and 1460 AD), while one coincides with earthquakes recorded to the south of the study catchment in 1826 AD. Age structures from throughout Westland show that extensive, similar-aged, post-earthquake cohorts of trees are a feature of the region, suggesting that infrequent, massive earthquakes are the dominant coarse-scale disturbance agent, triggering episodes of major erosion and sedimentation and leaving a strong imprint on the forest structure. Key-words: age/forest cohort, disturbance history reconstruction, erosion and sedimentation, landslides, terraces, tree rings

We document the neglected phenomenon of lateral erosion (`toe-cutting') of alluvial fans by non-incising axial river channels. Field examples from the Holocene of the Big Lost River basin, Idaho and the Plio-Pleistocene of the Rio Grande Rift, New Mexico help to establish architectural models with more general application to basin analysis. The process of toe-cutting can lead to complete fan destruction and may be a response to climate change, tectonic tilting, fault propagation or a combination of these variables. It gives rise to: near horizontal erosion surfaces cut in fan sediment; steep fan-margin scarps; progressive up-fan incision from the scarp by a network of channels; soil formation up-fan away from the incised channel network; a deposit of axial alluvium that overlies the erosion surface and onlaps the scarp. Once avulsion occurs to take the axial channel away from the bajada margin, distinctive `healing-wedges' of fan alluvium prograde across abandoned axial river channel and floodplain deposits, gradually onlapping the eroded scarp and its upstream network of incised channels. Toe-cutting has important stratigraphic basin analysis and economic consequences: bajada deposits subject to the process exhibit appreciable extra groundwater and petroleum reservoir potential in the intercalations of more porous and permeable axial fluvial sediments. Keywords: Idaho, Rio Grande, alluvial fans, erosion, basin analysis.

This paper looks at the use of laser technology, which has been used to provide high resolution images of underwater objects, to examine the morphology and size of sand particles eroded during benthic sediment transport under turbulent shear flow. Implications of sediment and carbon cycling within estuaries are discussed.

Erosion rates in the Namib Desert have reached a steady state with little change over time. The Namibian escarpment is not retreating and the northern edge of the Namib Sand Sea has been unshifting.

This paper presents the dendrochronological markers used to date the erosive activity of ice and waves on the shores of two large lakes in Northern Quebec. Damages to trees are dated by detailed analysis of anomalous tree rings in response to the shore geomorphic activity associated with extreme water levels over the last 2 centuries. Spring floods are dated using ice scars on trees and shrubs. Lake ice breakup occurs 4-6 weeks after snowmelt. Marks on trees made by ice displaced by the wind indicate the flood levels. Once ice breakup is complete, waves erode forest edges. Tree tilting, dated by counting rings in compression wood, mark high lake levels at the beginning of summer. At submerged sites that are not subject to waves and ice, stress related to prolonged submersion results in narrow tree rings. Trees located in the immersion zone show narrow rings compared to wider rings in unsubmerged trees. However, tree response is not immediate; they can primarily respond to sequences of several consecutive years of stress, reducing the resolution of dating to 2-3 years. Rings of submerged trees are denser than average. These high water level markers, as well as population recruitment showed by the age structure of shoreline trees and shrubs, which establish during intercalary years of low water levels, were used to date the extreme levels of two large lakes at the interface between the upper boreal and subarctic regions in northeastern Canada. During the past 200 years, the annual extreme levels have been increasing, becoming even more pronounced in the 1930s, and may be due to a century-long increase in snow precipitation. The summers seem to be drier. The 1930s, 1950s, and 1970s were marked by a high frequency of high water levels. Intercalary periods of low water levels allowed regeneration of shoreline forests and recovery of trees damaged during these events. It appears that episodes similar to the recent period of low waters experienced in the American northeast since 1982 may have occurred in the past during the 1920s, 1940s, and 1960s.

Forward modelling of maturity against vitrinite reflectance and chemical data for a number of onshore boreholes was used to test a range of subsidence and thermal histories for the Pennine Carboniferous Basin. The preferred models suggest maximum palaeotemperatures were reached during late Cretaceous or possibly early Tertiary burial, consistent with the outcome of previous apatite fission-track studies. Models in which maximum palaeotemperature was attained in the late Carboniferous are considered less likely: rapid burial under a normal heat flow requires an unrealistic thickness of missing late Westphalian strata, whilst high regional heat flow is incompatible with the observed vitrinite reflectance gradients. The lithologies assumed for missing strata have a significant influence on the range of estimates of eroded thicknesses. Preferred estimates of Cenozoic erosion range from around 1.6 km in Mesozoic basinal areas to around 2.8 km within areas of present Carboniferous outcrop. Keywords: Carboniferous, vitrinite reflectance, maturity, subsidence, erosion.

To obtain in situ measurements of sediment erodibility in defined bottom shear stress environments, a portable, straight flume was built, tested, and deployed in the field for six experiments at three locations in Quincy Bay of Boston Harbor, Mass. The flume had a 1.0-m-long inlet section, which included a boundary-layer trip and a roughened, plexiglass bottom; this design prevented erosion of the sediment bed in the boundary-layer-development region. Downstream of the inlet section was a 1.2-m-long sediment test section, which had a laboratory-verified, uniform bottom stress. In the absence of algal mats, our flume experiments on sites exhibiting a range of bed properties indicated quite uniform erodibility, with a critical shear stress tau(sub c) of 0.10 +/- 0.04 Pa and an erosion rate constant M of 3.2 +/- 0.2 X 10(super -3) kg m(super -2) s(super -1) Pa(super -1) (R(super -2) = 0.92, N = 17, where N is the total number of erosion rate measurements made in the absence of algal mats). The measured rates were consistent with those of many other in situ studies. We observed markedly reduced erodibility in early October 1995 when the sediment was covered by a benthic diatom mat, and measured erosion rates were lessened by 50-80%. The possibility of depth-dependent sediment erodibility in near surface (top 3 mm) was investigated by calculating a set of depth-dependent erosion parameters. The parameters obtained suggested that both the critical shear stress and the erosion rate constant were depth-sensitive (both doubling by 1 mm into the sediment).

There are four main mechanisms through which glacial erosion may produce topographic relief, according to research giving quantitative information about the processes of erosion and relief production. These mechanisms are ice buttressing of rock slopes, valley widening and formation of hanging valleys and overdeepenings. There are three mechanisms by which glacial erosion may lead to a reduction in some elements of relief. These are reduction of fluvial erosion downstream of glaciers, concentration of erosion at higher elevations and possible acceleration of summit-lowering rates in the near-glacial environment.

The disturbance of bed sediments by the wash of ship's propellers, and the resulting structural and environmental impact, is an increasing problem. Consequently, there is a need for an accurate method of scour prediction. The scouring action of the wash generated by a number of propellers in the confines of quay structures was investigated in the laboratory. Empirical equations have been developed for the prediction of the maximum depth of scour, for any given exposure period, for both flee expanding jets and those in close proximity to harbor structures.

Experiments have been done in order to determine the effects of particle size and bulk density on the erosion of quartz particles. Average particle sizes ranged from 5 to 1,350 p,m, while bulk densities ranged from approximately 1.65 to 1.95 g/[cm.sup.3]. For the larger particles, the sediments behaved in a noncohesive manner, i.e., they consolidated rapidly and the surface eroded particle by particle. For the smaller particles, the sediments behaved in a cohesive manner, i.e., they consolidated relatively slowly and the surface eroded in chunks. Erosion rates were a very strong decreasing function of density for the finer particles and were essentially independent of density for the larger particles. For a particular bulk density and shear stress and as the particle size increased, the erosion rate increased rapidly for the smaller particles, reached a maximum, and then decreased rapidly for the larger particles. Critical stresses for erosion were strongly dependent on particle size and, for the smaller particles, were also strongly dependent on bulk density.

Erosion tests were run on soft cohesive sediment beds freshly deposited from a concentrated slurry, in a large rotating annular flume. A total of seven tests were carried out with the bed age varying between 1.8 and 18.9 days. A surface type erosion was observed for the applied shear stress range (0.1 to 1 Pa). Following the increase in the water flow, the erosion rate increases quickly to a peak value and then drops off to about zero. The eroded depth was less than 8 mm for all beds, but the resistance to erosion at a given depth and for a given bed structure increased with increasing bed age. This phenomenon was attributed to thixotropy. The erosional strength of a given sediment layer is found to be one order of magnitude lower than its undrained and drained mechanical shear strengths measured with innovative testing methods. Hypotheses are given to explain that difference. However, at constant density and soil-water chemistry, the erosional and mechanical strengths seem to be affected by the same factors, i.e., bed age, structure, and temperature.

This paper presents the results of a laboratory study on the erosion of noncohesive sediment beds made of three different mixtures by deeply submerged plane turbulent wall jets. The sediment mixtures had median sizes of 6.75, 1.32, and 1.62 mm, and geometric standard deviations of 1.32, 2.02, and 3.13. The main objectives were to study the effect of sediment gradation on the asymptotic scour size and determine an effective size for the sediment mixtures for predicting erosion. The experimental observations and analysis presented in this study established that the sediment nonuniformity has a significant effect on the size of the scour hole produced by the jet. The effective size of the sediment mixture for obtaining a good correlation for the depth of scour was found to be [d.sub.95] rather than [d.sub.50] for defining the densimetric particle Froude number, [F.sub.o]. The average reduction due to armoring in the maximum scour depth and dune height was found to be [approximately]60%, and it was found to be 50% for the scour hole length and the distance of the dune.

New apatite fission-track analyses from NW Britain indicate that a maximum of 2.5 km of erosion has occurred there during the Cenozoic, similar to values for SE Greenland and the east Greenland coast north of Scoresby Sund. The erosion may have been facilitated by magmatic underplating during break-up. However, at Kangerdlugssuaq, East Greenland, 4-6 km of erosion is measured since 45 Ma. Lower-mid-Eocene marine sedimentary rocks overlying the lavas on the Biosseville Coast indicate that magmatic underplating on the central Greenland coast substantially post-dated flood volcanism and break-up, behaviour not predicted by simple plume-rift models. Subsidence reconstructions of the Hebrides Shelf, and the east and west Greenland coasts, show that rapid, dynamic uplift was effectively synchronous at 63 Ma and preceded volcanism by [less than]1.6 million years. The magnitude of uplift on the Hebrides Shelf (c. 400 m) is compatible with a mantle temperature anomaly of c. 100 [degrees] C. These data suggest very rapid lateral flow of the impacting Iceland plume head. The predicted crossing of the plume by the east Greenland coast in the mid-late Eocene would account for post-rift magmatic underplating and dynamic support on the Greenland but not the European side of the North Atlantic basin. Keywords: Atlantic, uplift, erosion, underplating, fission-track analysis.

A study was conducted on the uplift and erosion of the San Bernardino Mountains (SBMs) in southern California. The low-temperature cooling history of the SBMs was analyzed using radiogenic helium thermochronometry. Results showed that the helium ages of the uplift predate the present uplift of the range and that exhumation was only several kilometers since the Late Cretaceous. These results suggest that the uplift and exhumation is associated with transpression along the San Andreas fault.

Erosion risk assessment should incorporate various data sources for increased accuracy in predictions. These include mapping geomorphologically sensitive regions, weather pattern research and continuous rain-gauge information. Deforestation and vegetation removal have contributed to extended periods of erosion in Spain's Pyrenees, resulting from timber logging, overgrazing, fires and slope cultivation.

Research on natural hazards and erosion in Scandinavia provides valuable environmental and safety data on mountainous regions. The data can help clarify links between climatic changes and hazardous events, and assist in the development of safety programs for tourism and other industries in mountain and downstream locations. Climatic warming has been found to contribute to both snowmelt- and rainstorm-triggered flooding.

Gently sloped summits and ridges (collectively referred to as summit flats) are abundant in many Laramide ranges in the western United States. The erosion rate of summit flats is [approximately]10 m/m.y., on the basis of the concentrations of cosmogenic radionuclides. Because erosion rates in valleys between summit flats are an order of magnitude faster, relief within these ranges is currently increasing by about 160 m/m.y. If summit-flat erosion is slower than rock uplift driven by the isostatic response to valley erosion, then this relief production could result in increased summit elevations. The mean depth of material eroded from a smooth surface fit to existing summit flats varies from 280 to 340 m in four Laramide ranges, based on geographic information system (GIS) analyses of digital elevation models. This erosion would result in a maximum of 250-300 m of rock uplift, assuming Airy isostasy. However, because the Latamide ranges examined here are narrow relative to the flexural wavelength of the lithosphere, erosionally driven rock uplift is limited to [approximately]50-100 m. Over the past several million years, summit erosion would approximately offset this rock uplift. Therefore, we conclude that summit elevations have remained essentially constant even though several hundred meters of relief has been produced. On the basis of valley and summit erosion rates and the average depth of erosion, we estimate that relief production in Laramide ranges began at ca. 3 Ma. We hypothesize that this relief production was climatically driven and was associated with the onset or enhancement of alpine glaciation in these ranges.

A study on the vertical motions of rocks during convergent deformation and erosion at the surface is presented. Using a kinetic model, the study revealed that the rocks' upward or downward motion in the crust is determined by the relative rates of erotion and thickening and the initial depth in the crust. Furthermore, exhumation during thickening is found to only take place when rapid denudation occurs with the thickening process.

Sedimentological and provenance data from the lower Miocene-Pliocene Dumri Formation and Siwalik Group in western Nepal provide new information about the timing of thrust faulting and the links between erosional unroofing of the Himalaya and the Cenozoic 87Sr/86Sr record of the ocean. In western Nepal, the Dumri Formation is an [approximately equal to]750-1300. m-thick fluvial sandstone and overbank mudstone unit. The Siwalik Group is >4200 m thick and consists of a lower member (>850 m) of 2-12-m-thick fluvial channel sandstones and oxidized calcareous paleosols, a middle member (>2400 m) of very thick (>20 m) channel sandstones and mainly organic-rich Histosols, and an upper member (>1000 m) composed of gravelly braided river deposits. Paleocurrent data indicate that middle Miocene-Pliocene rivers in western Nepal flowed southward, transverse to the thrust belt, throughout deposition of the Siwalik Group. No evidence was found for an axial fluvial trunk system (i.e., the paleo-Ganges River) in Siwalik Group sandstones. A major increase in fluvial channel size is recorded by the transition from lower to middle Siwalik members at [approximately equal to]10.8 Ma, probably in response to an increase in seasonal discharge. Modal petrographic data from sandstones in the Dumri Formation and the Siwalik Group manifest an upsection enrichment in potassium feldspar, carbonate lithic fragments, and high-grade metamorphic minerals. Modal petrographic analyses of modern river sands provide some control on potential source terranes for the Miocene - Pliocene sandstones. The Dumri Formation was most likely derived from erosion of sedimentary and low-grade metasedimentary rocks in the Tibetan (Tethyan) Himalayan zone during early Miocene emplacement of the Main Central thrust. The presence in Dumri sandstones of plagioclase grains suggests exposure of crystalline rocks of the Greater Himalayan zone, perhaps in response to tectonic unroofing by extensional detachment faults of the South Tibetan detachment system. During deposition of the lower Siwalik Group ([approximately equal to]15-11 Ma), emplacement of the Dadeldhura thrust sheet (one of the synformal crystalline thrust sheets of the southern Himalaya) on top of the Dumri Formation supplied abundant metasedimentary lithic fragments to the foreland basin. A steady supply of plagioclase grains and high-grade minerals was maintained by deeper erosion into the Main Central thrust sheet. From [approximately equal to]11 Ma to the present, K-feldspar sand increased steadily, suggesting that granitic source rocks became widely exposed during deposition of the upper part of the lower Siwalik Group. This provenance change was caused by erosion of passively uplifted granites and granitic orthogneisses in the Dadeldhura thrust sheet above a large duplex in the Lesser Himalayan rocks. Since the onset of deposition of the conglomeratic upper Siwalik Group ([approximately equal to]4-5 Ma), fault slip in this duplex has been fed updip and southward into the Main Boundary and Main Frontal thrust systems. We obtained 113 U-Pb ages on detrital zircons from modern rivers and Siwalik Group sandstones that cluster at 460-530 Ma, [approximately equal to]850-1200 Ma, [approximately equal to]1.8-2.0 Ga, and [approximately equal to]2.5 Ga. An abundance of Cambrian-Ordovician grains in the Siwalik Group suggests sources of Siwalik detritus in the granites of the Dadeldhura thrust sheet and possibly the Greater Himalayan orthogneisses. The older ages are consistent with sources in the Greater and Lesser Himalayan zones. An overall upsection increase in zircons older than 1.7 Ga suggests increasing aerial exposure of Lesser Himalayan rocks. None of the detrital zircons (even in the modern river samples) yielded a Cenozoic age that might suggest derivation from the Cenozoic Greater Himalayan leucogranites, but this may be attributable to the inheritance problems that characterize the U-Pb geochronology of the leucogranites. When compared with recent studies of the 87Sr/86Sr composition of paleosol carbonate nodules and detrital carbonate in paleosols from the Siwalik Group, the provenance data suggest that erosion and weathering of metamorphosed carbonate rocks in the Lesser Himalayan zone and Cambrian-Ordovician granitic rocks of the crystalline thrust sheets in central and eastern Nepal may have played a significant role in elevating the 87Sr/86Sr ratio of middle Miocene synorogenic sediments in the Indo-Gangetic foreland basin and the Bengal fan, as well as global seawater.

A relatively original point of view is introduced in order to classify sedimentation phenomena according to the spatial scale. Three categories are identified: large-, medium-, and short-scale models. The distinction is made in terms of the relationships between the hydrodynamic adaptation length, the sedimentologic adaptation length, and the study scale. This allows determination of the right type of mathematical modeling for each particular case. The main features of the models involved and of the associated numerical methods are briefly described. Case studies for the different types of models are presented for illustration.

Analytical solutions provide a simple and economical means of quickly estimating qualitative and quantitative responses of the shoreline to a wide range of environmental and engineering conditions. This paper presents analytical solutions for shoreline evolution in the vicinity of coastal structures, including detached breakwaters, seawalls, and jetties with and without wave diffraction. The solution for a detached breakwater illustrates the growth of a salient with time behind the breakwater and the associated initial shoreline retreat at locations across from the breakwater tips. A simple solution describing flanking of a seawall is obtained by using two different solution areas where the longshore sand transport rate and breaking-wave angle vary. A similar technique is used to model diffraction downdrift of a groin or jetty by allowing the wave angle to vary with the distance alongshore according to a specified function or by employing a large number of solution areas. Cases are also presented for the accumulation updrift a groin and the shoreline response in a groin compartment with a breaking wave angle that varies sinusoidally in time. A solution for a single groin illustrates the impact of bypassing on the updrift accumulation.

Fossil shells and paleosol carbonate from ancestral Himalayan river deposits provide a [sup.87]Sr/[sup.86]Sr record of lowland Himalayan river water during the late Neogene. Reconstructed [sup.87]Sr/[sup.86]Sr river values increased sharply in [...]

The CRP was initiated to remove environmentally fragile areas from crop production. Approximately 14.8 million hectares (36.5 million acres) of cropland, primarily west of the Mississippi, were enrolled in this [...]

Colluvium on foot slopes in parts of North America and Europe has been attributed to a major mass-wasting episode during the last glacial period. Stratigraphic evidence and 14C ages support this hypothesis for the northern part of the Upper Mississippi Valley. Colluvium in this region grades laterally into, or interfingers with, fluvial sediment beneath the late Wisconsinan Savanna terrace. Colluvial foot slopes are truncated by fluvial surfaces postdating the incision that created the Savanna terrace between 13 and 11 ka. Samples from within the colluvium have Inc ages between 18.6 and 12 ka. Ages of 28.9 and 20.3 ka have been obtained from beneath colluvium, and fluvial sediment inset into colluvial foot slopes has yielded ages between 12.5 and 9.8 ka. Widespread permafrost during the late Wisconsinan glacial maximum may explain the onset of accelerated mass wasting that produced the colluvium, although mass wasting apparently continued for some time during subsequent climatic warming. The results described here imply major, long-term, climatically driven fluctuations in sediment supply from hillslopes to the fluvial system in this region.

An experimental program has been conducted to examine the feasibility of using seawater solar ponds in mariculture operations along the Texas gulf coast to protect fish crops from the potentially lethal, cold temperatures experienced in outdoor ponds. Seawater solar ponds in the form of floating thermal refuge areas are proposed as a method for reducing the loss of heat from small sections of a pond. Gradient zone erosion under various ambient and operating conditions is examined. Comparisons with previous laboratory studies show a much lower entrainment rate in the natural environment. For conditions which are typical of those encountered in mariculture pond operation, the entrainment rate was found to depend only weakly on the Richardson number. For these conditions, a simple (linear) correlation of entrainment rate with wind speed was developed.

Mantle-plume activity has been proposed to explain Neogene and mid-Cretaceous magmatic events, as well as associated tectonism, in West Antarctica; but the arrival time and dimensions of plume influence have been hard to define and are still a subject of debate. Two low-relief erosion surfaces, one in West Antarctica and the other in New Zealand (herein named the Waipounamu erosion surface), provide a way of assessing plume activity by measuring vertical displacements associated with these events. Both surfaces bevel mid-Cretaceous rocks, and both represent prolonged intervals of erosional leveling in a stable tectonic environment. Overlying strata in New Zealand indicate that leveling was near completion in coastal regions by ca. 75 Ma and therefore must have begun around 85 Ma, when New Zealand was beginning to break away from West Antarctica. Fluvial erosion followed by subsidence and marine planation are clearly recorded by these strata, and a similar history seems likely for West Antarctica, accounting for isostatically corrected ice-free bedrock elevations that are well below sea level over much of the region. The absence of uplift at the time of breakup seems incompatible with a plume mechanism for continental breakup. By contrast, the present elevation of the West Antarctic erosion surface records an estimated maximum of [approximately equal to]3 km of tectonic uplift, associated with alkalic volcanism, beginning at ca. 28-30 Ma. We suggest that this event marks the inception of plume activity in West Antarctica. The resulting structure, the Marie Byrd Land dome, defines an area of plume influence that is smaller than the area defined by geochemistry, but is similar in scale to the Yellowstone plume.

The ability to predict the stability of eroding riverbanks is a prerequisite for modeling alluvial channel width adjustments and a requirement for predicting bank-erosion rates and sediment yield associated with bank erosion. However, there are a number of limitations of existing bank-stability analyses that limit their physical basis and predictive ability. Some of these limitations are addressed through the development of a new bank-stability analysis. The new approach is applicable to steep, cohesive, nonlayered riverbanks that fail along planar failure surfaces. Pore-water and hydrostatic confining pressure terms are included in the analysis. The failure plane is not constrained to pass through the toe of the bank. The predictive abilities of four bank-stability analyses (Lohnes and Handy 1968; Huang 1983; Osman and Thorne 1988; and the present analysis) were assessed using field data. The new analysis is the most successful of the tested analyses in terms of predicting the stability of riverbanks with respect to mass failure.

Soil crust formation can result from the beating action of falling drops (structural crust) or the deposition of entrained eroded suspension in shell flow (deposional crust). To determine the relative importance of the two processes, an experiment was carried out with rainfall simulation under laboratory conditions. Clay soil exposed to the beating action of rain drops produced sheet erosion, which was deposited on a layer of clean sand downslope of the same continuous slope. The thin crust that was formed on the two parts of the slope, the clay and the sand, reduced the infiltration rates in time to values of 1 to 5 mm [h.sup.-1]. In comparison with the uniform clay slope, the main difference was the longer time needed to reach the low infiltration rates. Preventing the sand part from any rain impact beating extended the time needed to obtain the low infiltration rates that were nearly the same as those of the clay slope. The results provoke the question is the common theory of compacted crust by rain impact such an important factor?

Mine soils developed from mine spoils commonly have a wide range of particle size. The slopes of old spoil piles usually are marked by gullies due to years of uncontrolled erosion. These characteristics raise questions about applicability of available theories and models for estimating runoff and erosion. An investigation was made to determine whether available erosion models can work for mine soils and can account for gully erosion. The investigation at an abandoned surface mine consisted of measurement of soil and sediment properties, measurement of runoff and erosion, observations of armor by rock fragments on gully floor, and calculations with available theories of sediment transport and slope stability. The results at this site suggest that (1) predictions with the ANSWERS model have about the same accuracy as those made for agricultural lands; (2) armor provided by rock fragments are temporary as they are periodically removed by debris flows; (3) detachment by rainfall impact is the primary cause of erosion on short steep slopes; and (4) a simplified method can be used for estimating erosion on such slopes.

An eclectic range of stratigraphical, sedimentological, geochemical, archaeological and historical evidence relating to a tidal palaeochannel exposed at a deep erosional level in the modern intertidal zone on the Avon-Gloucestershire border demonstrates that tidal wetlands reclaimed during the Roman period ranged much further seaward than the modern coastline. Vigorous late medieval-early modern erosion, linked to the cool, disturbed conditions of the Little Ice Age, forced the coastline by the mid-seventeenth century to a location inland of its present line. A possible medieval landing place was destroyed and it was necessary to set back the flood defences. As the result of renewed mudflat-marsh growth, the coast at Hills Flats has built outward during modern times, but in three distinct stages and by no means as far seaward as its likely Roman position. Keywords: Severn Estuary, Flandrian, channels, salt marshes, coastal erosion.

The oldest greenstone successions with angular unconformity indicating an ancient erosion surface were newly discovered to be under the rocks of 3.46-Gyr Warrawoona Group in the Pilbara craton of Australia. This discovery supports the tectonic evolutionary models that believe during the first billion years of Earth's history, the early Archaeon period, considerable volumes of continental crust were already present The unconformity dates to between 3.47 and 3.46 Gyr and those greenstones must have been on an already rigid, buoyant and cool crust.

New estimates of long-term consumption of Japanese and Peruvian continental fore-arc margins together with recent drilling data (Ocean Drilling Program) on some other erosional island arcs such as Izu-Bonin, Mariana, or Tonga allow the calculation of a mean rate of 7 [+ or -] 3 km/m.y. landward migration of the arc-trench system on both east and west sides of the Pacific basin during Cenozoic time. Such relative arcward retreat counterbalances seaward migration of the slab hinge line to accommodate opening of the Atlantic Ocean. Retreat can thus be interpreted as a delay in seaward or retrograde migration due to upper mantle resistance to contraction of the Pacific basin. Mass transfers in subduction zones must be reconsidered in the light of these new estimates, especially the balance between net growth and loss of continental crust in recent geologic time. A 50 m.y. period is long enough to erode the initial width of the volcanic-arc and fore-arc massif of any subduction zone, assuming a mean trench - volcanic-arc landward migration of 5 km/m.y. and a mean trench - volcanic-arc distance of 250 km. This circumstance thus raises the question of whether the Izu-Bonin-Mariana and Tonga arcs are older than their reported Eocene age.