Surficial Drainage Research Articles

Comprehensive database of land subsidence in 143 major coastal cities around the world: overview of issues, causes, and future challenges

Subsidence refers to the gradual lowering or sudden sinking of the ground surface and is known to impact human lives in terms of damages to the infrastructures, utility lines, and buildings as well as changes in the surficial drainage systems and groundwater conditions. The impacts of land subsidence will be greater in the future, considering the sea level rise, population growth, intensification of coastal erosion and extreme events, as well as increase in flood risk or freshwater salinization, mostly in coastal cities. The main aim of this work is to provide an open-source, peer-reviewed, and comprehensive database identifying the main and secondary causes of land subsidence in 143 coastal cities. We highlight the potential impacts of subsidence that are still unknown in some at-risk cities and non-existence of mitigation measures. The database additionally shows that mitigation measures, specifically those addressing subsidence due to groundwater extraction, have proven successful in the past. The proposed database aims to increase the knowledge on the subsidence phenomenon and also global awareness of land subsidence issues among researchers, the scientific community, stakeholders, and policymakers in terms of urban planning and development.

Assessing landscape features and geomorphic processes influencing sediment dynamics in a geomorphologically highly active Mediterranean agroecosystem: The upper Val d'Arda case study (Northern Apennines, Italy)

The management of Mediterranean Agroecosystems is crucial to prevent or mitigate sediment-related threats like soil erosion, water quality issues, and reservoir sedimentation. Hence, the identification of type, extent, and location of sediment sources and sinks, in relation to geological and landscape features, is a key prerequisite for identifying erosion hot spot areas and assessing sediment dynamics at catchment scale. This study was carried out in the upper Val d'Arda catchment (Northern Apennines, Italy). We applied geomorphological field mapping together with multi-temporal photointerpretation, GIS-based hydrological modelling, terrain analysis and photogrammetry. Our approach was aimed at exploring the main landscape features, as well as identifying the variety of processes influencing sediment dynamics. In this poorly studied area, we identified a high heterogeneity in terms of type and extent of sediment sources and related processes. Landslides are the most common processes contributing to the sediment yield. Their morphogenesis, extent and location are controlled mainly by lithology and topography. Large ancient earthflows have been the most important geomorphic factor in shaping the landscape, strongly influencing the surficial drainage pattern and the morphological evolution of the main fluvial systems. Upland rill-interrill erosion is another main sediment source, particularly on arable lands. Piping and gullying locally contribute to the development of badlands, and have a high potential for soil loss and sediment production. Finally, fluvial processes significantly contribute to the sediment yield with several active fluvial erosion scarps and bank erosion source areas. Alluvial sediments are locally stored in transient landforms such as bars or more permanent sinks such as floodplains or river terraces. Our study shows that an integrated methodology based on fieldwork, geomorphological mapping, GIS applications and proximal sensing methods is an effective approach to characterize complex geomorphic systems in the Northern Apennines.

Effectiveness of a Geocomposite-PVD system in preventing subgrade instability and fluidisation under cyclic loading

Instability of subgrade soil sometimes associated with soil fluidisation can lead to uncontrollable deformation and failure at a critical number of loading cycles for a given cyclic deviator stress and frequency. Although numerous laboratory experiments on the performance of Prefabricated Vertical Drains (PVDs) and geocomposites have already been carried out in the past, how effectively this combination can mitigate the potential for subgrade fluidisation under repeated (cyclic) loading is still not properly understood. The primary objective of this paper is to evaluate the integrated role of PVDs and geocomposite in preventing subgrade fluidisation using Dynamic Filtration Apparatus (DFA). Laboratory experiments show that the continuous dissipation of EPWP and the substantial reduction in drainage path lengths by PVDs can prevent subgrade fluidisation at shallow depths, while geocomposite can provide adequate surficial drainage and effective confinement at the ballast/subgrade interface. By measuring the Excess Pore Pressure Gradients (EPPGs) during cyclic loading, the test results convincingly reveal the promising performance of PVD-geocomposite combination under different loading conditions.

Remote sensing of water tracks

AbstractWater tracks are an intrinsic part of the surficial drainage network in the foothills of the Brooks Range, Alaska. They preferentially transport water off hillslopes and represent the interplay between hydrology, vegetation, geomorphology, and permafrost characteristics. This research on mapping the location of water tracks builds on previous work which demonstrated that different types of water tracks exist due to difference primarily driven by geomorphology. We used a combination method where spectral classifications, texture, and topography were fed into random forests to identify the water track classes. The most accurate distributions were obtained for the organic‐rich and wide water track classes. The distinct linear shapes of the water tracks could also be visualized for many of the classes, especially in areas where the water tracks were particularly discrete. The biggest challenges to mapping the water tracks were due to class imbalances and high variability within and overlapping between classes. This research presents a significant step forward in understanding periglacial landscape dynamics.

Holocene flood frequency reconstruction from speleothems in northern Spain

Extreme precipitation events may cause flooding in the subsurface as well as surficial drainage networks, and these flood events may be preserved in the speleothem archive. We describe here a study of stalagmites from the Cueva Rosa, a system with a perennial cave stream in a lower active level and abundant speleothems in a fossil gallery 6–8 m above the active level. Several constrictions in the lower level act as bottlenecks at discharges of 8–11 m3/s during high discharge events, flooding both lower and upper galleries. Because the cave stream is the only efflux for the small surface watershed (1 km2), it is possible to estimate the critical rainfall intensity rates and runoff required to flood the upper gallery. In the upper gallery, historical flooding is constrained by 14C dates of wood fragments which register both a both post-bomb event and event at 420 yr BP. The latter event appears to coincide with deposition of thick mud deposit postdated by speleothem growth since 324 yr BP. A mid-Holocene (8.1–5.3 ka BP) speleothem from the upper gallery contains 26 detrital layers composed of clays and quartz grains evident in sectioned stalagmites and in Al content in LA-ICPMS analyses. The 9 most pronounced layers reach a thickness of 0.1–0.3 mm in the central growth axis. Petrography confirms that calcite crystal growth is continuous through these detrital layers and that they represent decantation events rather than hiatus in calcite deposition. In the mid-Holocene, large events have average recurrence of around 300 years, although large events are absent from the period from 7.3 to 6.3 ka and more frequent in the older and younger portions of the stalagmite. In the last four centuries, two major events have partially buried an actively growing stalagmite, showing that extreme precipitation events capable of flooding the upper gallery remain a persistent feature of the climate.

Connectivity of overland flow by drainage network expansion in a rain forest catchment

Soils in various places of the Panama Canal Watershed feature a low saturated hydraulic conductivity (Ks) at shallow depth, which promotes overland-flow generation and associated flashy catchment responses. In undisturbed forests of these areas, overland flow is concentrated in flow lines that extend the channel network and provide hydrological connectivity between hillslopes and streams. To understand the dynamics of overland-flow connectivity, as well as the impact of connectivity on catchment response, we studied an undisturbed headwater catchment by monitoring overland-flow occurrence in all flow lines and discharge, suspended sediment, and total phosphorus at the catchment outlet. We find that connectivity is strongly influenced by seasonal variation in antecedent wetness and can develop even under light rainfall conditions. Connectivity increased rapidly as rainfall frequency increased, eventually leading to full connectivity and surficial drainage of entire hillslopes. Connectivity was nonlinearly related to catchment response. However, additional information on factors such as overland-flow volume would be required to constrain relationships between connectivity, stormflow, and the export of suspended sediment and phosphorus. The effort to monitor those factors would be substantial, so we advocate applying the established links between rain event characteristics, drainage network expansion by flow lines, and catchment response for predictive modeling and catchment classification in forests of the Panama Canal Watershed and in similar regions elsewhere.

Seabed gallery intake technical feasibility for SWRO facilities at Shuqaiq, Saudi Arabia

Surficial drainage features that allow periodic flash-flood discharges to the shallow marine environment cause operational problems for seawater reverse osmosis water treatment plants at Shuqaiq, Saudi Arabia. The facility at Shuqaiq was designed to produce 212,000,m3/day of fresh water and it has experienced operational difficulties. An investigation was conducted to assess the technical feasibility of developing a subsurface intake system that would improve the feedwater quality to this and other facilities. A seabed gallery system was found to be technically feasible to supply the required 530,000 m3/day inflow. The design contains nine cells, each yielding 66,250 m3/day with an infiltration rate of 10 m/day. The filter would have a hydraulic retention time of 7 h. Each cell contains five layers with the total filter bed thickness of 5 m. Design of the filter under-drain system was optimized to maintain an equal flow rate at the top of the filter.

Formation of phreatic caves in an eogenetic karst aquifer by CO2 enrichment at lower water tables and subsequent flooding by sea level rise

ABSTRACTFormation of extensive phreatic caves in eogenetic karst aquifers is widely believed to require mixing of fresh and saltwater. Extensive phreatic caves also occur, however, in eogenetic karst aquifers where fresh and saltwater do not mix, for example in the upper Floridan aquifer. These caves are thought to have formed in their modern settings by dissolution from sinking streams or by convergence of groundwater flow paths on springs. Alternatively, these caves have been hypothesized to have formed at lower water tables during sea level low‐stands. These hypotheses have not previously been tested against one another. Analyzing morphological data and water chemistry from caves in the Suwannee River Basin in north‐central Florida and water chemistry from wells in the central Florida carbonate platform indicates that phreatic caves within the Suwannee River Basin most likely formed at lower water tables during lower sea levels. Consideration of the hydrological and geochemical constraints posed by the upper Floridan aquifer leads to the conclusion that cave formation was most likely driven by dissolution of vadose CO2 gas into the groundwater. Sea level rise and a wetter climate during the mid‐Holocene lifted the water table above the elevation of the caves and placed the caves tens of meters below the modern water table. When rising water tables reached the land surface, surface streams formed. Incision of surface streams breached the pre‐existing caves to form modern springs, which provide access to the phreatic caves. Phreatic caves in the Suwannee River Basin are thus relict and have no causal relationship with modern surficial drainage systems. Neither mixing dissolution nor sinking streams are necessary to form laterally extensive phreatic caves in eogenetic karst aquifers. Dissolution at water tables, potentially driven by vadose CO2 gas, offers an underappreciated mechanism to form cavernous porosity in eogenetic carbonate rocks. Copyright © 2012 John Wiley & Sons, Ltd.

Assessment of spatial properties of karst areas on a regional scale using GIS and statistics – the case of Slovenia

In Slovenia, 43% of the territory is karst, including 42% of all protected water sources and 53% of all water-protection areas in the country. Over 95% of drinking water is obtained from groundwater, so assessment of karst areas and their spatial distribution is essential to better understand the water in the lithosphere and for the assessment of the hydrogeochemical properties of the groundwater in a large part of Slovenia. These groundwater resources are susceptible to degradation or pollution, and a regional karstification-intensity map was developed to assist in the management of these water resources and to interpret the chemical composition of the groundwater. For the purpose of classifying stratigraphic units into karstification-level classes, three parameters were analyzed in the outcrops of units with carbonate content using GIS and simple spatial statistics: the presences of sinks and cave entrances and the absence of a surficial drainage network. Where at least two of the three parameters showed a positive relation with karstification, the unit was regarded as intensely karstified, while the rest were regarded as less karstified. The former areas cover 24% and the latter 21% of Slovenian territory.

Evolution of salt diapir and karst morphology during the last glacial cycle: Effects of sea-level oscillation, diapir and regional uplift, and erosion (Persian Gulf, Iran)

Marine, fluvial and cave sediments, and karst phenomena were studied and dated by 14C, U-series, and OSL methods to determine the evolution of the Namakdan diapir and the world's longest salt cave (3N Cave) during the Holocene and the Last Glacial. Sea-level oscillations, the uplift rate of the diapir and its surroundings, and erosion are the main factors influencing the diapir morphology. Although the diapir uplift rate has been constant for the last 50 kyr (∼ 4 mm/yr at a distance 600 m from the diapir edge), the uplift rate decreases with the distance from the diapir center. Drag-induced host rock deformation extends for ∼ 300 m from the outside edge of the diapir, and host rocks in this zone have an uplift rate of 0.4–0.6 mm/yr, which is 2–3 times greater than the regional uplift rate. Based on known sea-level oscillations, radiometric dating, and geological evidence, the Namakdan diapir was repeatedly flooded by sea water between 130 and 80 kyr BP. Submarine residuum composed mainly of gypsum and dolomite formed cap rock on the diapir. After ∼ 80 kyr BP, surficial drainage network and karst development started. Blind valleys and their corresponding cave systems evolved continuously for ∼ 20–30 kyr. Between 9 and 6 cal kyr BP the rate of sea-level rise exceeded the Namakdan diapir uplift rate by the factor of 3. As a consequence upward incision of cave streams (paragenetic trend) occurred, and blind valleys near the seashore were filled with gravels. Cave passages now accessible on the Namakdan and Hormoz diapirs started to form 3–6 cal kyr BP when sea level stabilized and downward stream incision began. Older cave levels are still preserved but are filled with sediments and salt precipitates. A comparison of the Namakdan diapir evolution with data from the Hormoz and Larak diapirs shows that the evolution of diapir morphology is strongly affected by the differences in uplift rates and geological settings. The general scheme of the evolution of the Namakdan diapir is believed to be partly applicable to many other diapirs in coastal settings.

THE NATURE OF THE ALLUVIAL DIAMOND DEPOSITS OF THE VENTERSDORP DISTRICT, NORTHWEST PROVINCE, SOUTH AFRICA

Research Article| September 01, 2009 THE NATURE OF THE ALLUVIAL DIAMOND DEPOSITS OF THE VENTERSDORP DISTRICT, NORTHWEST PROVINCE, SOUTH AFRICA T.R. MARSHALL; T.R. MARSHALL Explorations Unlimited, PO Box 6578, Homestead, 1412, South Africa e-mail: marshall.tania@gmail.com Search for other works by this author on: GSW Google Scholar G.A. NORTON G.A. NORTON Rockwell Diamonds Inc, South Africa e-mail: glenn@rockwelldiamonds.com Search for other works by this author on: GSW Google Scholar South African Journal of Geology (2009) 112 (2): 109–124. https://doi.org/10.2113/gssajg.112.2.109 Article history first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation T.R. MARSHALL, G.A. NORTON; THE NATURE OF THE ALLUVIAL DIAMOND DEPOSITS OF THE VENTERSDORP DISTRICT, NORTHWEST PROVINCE, SOUTH AFRICA. South African Journal of Geology 2009;; 112 (2): 109–124. doi: https://doi.org/10.2113/gssajg.112.2.109 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySouth African Journal of Geology Search Advanced Search Abstract Numerous models have been presented over the years to explain the nature and distribution of the diamondiferous alluvial deposits on the dolomite plain in the Ventersdorp district. These have, typically, highlighted the importance of karst processes both prior to, and coincidental with, deposition of the gravels. Karst has a particularly distinctive method of landscape sculpturing arising from greater rock solubility in natural waters than elsewhere. Solution may not always be the prevalent process, or even the dominant one, but it does play a greater role than in other kinds of landscape. It’s most critical effect lies in the enlargement of underground voids, causing increased permeability of the rock. As more water passes through the rock, the voids grow to become caves. Greater solubility of a substance is accompanied by a greater liability for the dissolved load to come out of solution, so chemical deposition can contribute in an important way to landform building in karst terranes, both on the surface and underground. In karst landscapes, the generally harmonious pattern of surficial drainage is broken up and closed depressions take over the landscape to varying degrees. Superficially, it lacks organisation and the drainage system has to be sought underground.The general stratigraphic sequence of the alluvial gravel deposits of the Ventersdorp district is shown to comprise an economic lower gravel (Lower Gravel Package and the Transition Zone Gravels), a sub-economic clay-rich unit and an economic upper gravel unit (Upper Gravel Package), all of which have been overlain by Kalahari clays/sands and the Hutton soil profile (including colluvial diamondiferous gravel units). It is fundamental to the model to appreciate that the gravels were deposited in a karst system where:The dolomitic bedrock contacts with the gravels may be steeply dipping to verticalThe mode of gravel deposition is not typical fluvial alluvialPeriodic subsidence has taken place during depositionDeposition has taken place over a long time (at least since the Mesozoic), resulting in the build-up of a very thick gravel sequence.Furthermore, the runs are often controlled by cave formation along linear, fracture or fault zones which allowed the increased flow of ground water that led to higher than normal levels of dissolution of the dolomites and resulted in the formation of sinkholes and channels. Some of the sinkholes are more than 100 m deep. The sedimentary infill of the sinkholes is variable and depends on the depth and subsidence history of each individual sinkhole.Numerous issues have been raised with respect to the potential sources of both the diamonds and the gravels. Among the major problems is the limited variation in grade and quality of the diamonds over a significant area and the relatively high grade of the lower gravels that show a distinct lack of reworking and re-concentration throughout their vertical extent. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Stratigraphic and Ecophysical Characterizations of Salt Pools: Dynamic Landforms of the Webhannet Salt Marsh, Wells, ME, USA

Salt pools are water-filled depressions common to north-temperate salt marshes. In Wells, ME, USA, cores reveal a unique salt pool signature consisting of water-saturated dark-gray mud often containing fragments of Ruppia maritima. Cores through pool sediment reenter salt marsh peat, not tidal flat sediment, demonstrating that most pools are of secondary origin. A principal component analysis of attribute data collected from 119 pools defines three distinct pool types: those with (1) surrounding high-marsh vegetation and thick heavily undercut banks (40% of the variance), (2) surrounding low-marsh vegetation and thicker slightly undercut banks (18% of the variance), and (3) surrounding low-marsh vegetation and less thick moderately undercut banks, containing R. maritima and a surficial drainage (15% of the variance). Cores and spatiotemporal analyses of aerial photographs between 1962 and 2003 reveal dramatic salt marsh surface dynamism suggesting that salt pools influence the geomorphological evolution of coastal marshes.

Johnstuart Webb (1920–2007)

In 1978, publication of The Wolfson Geochemical Atlas of England and Wales , which showed the spatial distribution of concentrations of nineteen elements in the fine-grained (<0.177 mm) sieved fraction of samples of ~50,000 active stream sediments, collected at a spatial density of ~1 sample per 2.5 km2 over the whole of England and Wales where surficial drainage was present, established proof-of-concept of the multi-element regional geochemical atlas as a tool capable of providing geochemical data reflecting changes in bedrock geology and overburden, as well as both natural and anthropogenic sources of pollution. It represented the culmination of a programme of systematic research begun in 1954, when Webb established the Geochemical Prospecting Research Centre at the Imperial College of Science and Technology (ICST), London. His farsighted vision of geochemical atlases as a strategic national requirement, capable of providing information of relevance to fundamental geology, mineral exploration, agriculture and animal/human health, eventually became fully realised in the UNESCO Division of Earth Sciences International Geochemical Mapping Project, inaugurated in 1988. Webb was born in Balham, London, on 28 August 1920, the eldest child of George Stewart Webb and his wife Caroline Rabjohns ( nee Pengelly). Educated at St. Mary’s School, Balham (1925–30) and Westminster City School, London (1930–38), he graduated BSc (First Class Honours) in Mining Geology from ICST in 1941, having been awarded the Murchison Medal (1939), the Brough Medal (1940), the Clement le Neve Foster Prize (1941) and the Cullis Testimonial Fund (1941). After a brief period as assistant mining geologist to the U.K. Government’s Non-Ferrous Metallic Ores Committee, he served as a sapper in the Royal Engineers (1941–43) and was commissioned Second Lieutenant before being transferred to the Geological Survey of Nigeria (1943–44) as an economic mineralogist to look for tantalite, as at the time tantalum …

Chemical evolution of thermal springs at Arenal Volcano, Costa Rica: Effect of volcanic activity, precipitation, seismic activity, and Earth tides

Arenal Volcano in NW Costa Rica, Central America has been active during the last 37 years. However, only relatively low temperature springs have been identified on its slopes with temperatures less than around 60 °C. The springs are clustered on the NE and NW slopes of the volcano, close to contacts between the recent and older volcanic products or at faults that intercept the volcano. This volcano is located in a rain forest region with annual rainfall averaging around 5 m. During the last 15 years, the temperature and chemical composition of 4 hot springs and 2 cold springs have been monitored approximately every 3 months. In addition, two more thermal sites were identified recently and sampled, as well as two boreholes located on a fault NE of the volcano. Scatter plots of chemical species such as Cl and B suggest that the waters in these discharges belong to the same aquifer with a saline end member similar to Río Tabacón at the beginning of the study period (1990) and the deeper borehole (B-2) in 2004. The waters of Quebrada Bambú and Quebrada Fría represent a more dilute end member. Both long-term (over the 15 years) and short-term or seasonal decreases in concentration and steady or decreasing temperature are noted in NW springs. Springs located at the NE show increasing temperatures and ion concentrations, except for bicarbonate that has decreased in concentration for all the springs. This behavior is likely associated with a shallow source for the solutes and heat for this aquifer. To the NW the early lavas and pyroclastic flows have been cooling down, decreasing the contribution of leaching products to the infiltrating waters. To the NE, pyroclastic flows to the N during the last decade are contributing increasing concentrations of solutes and heat throughout water infiltration and circulation within the faults and the surficial drainage that has a NE regional trend. For the short-term or seasonal variations, concentrations of chemical constituents decrease when precipitation increases. However, correlations between concentrations and the number of seismic events per month and the modeled vertical tidal acceleration are also observed. The intrinsic periodic behavior of all these variables influenced by the Earth's rotation can complicate the interpretation of the chemical changes at hot springs. For example, variations in atmospheric pressure can affect the degassing and seismic tremor of a volcano as well as the rate of precipitation. Frequent monitoring and understanding of these variations is essential at every volcano if we want to use the variations in chemical composition of hot springs in volcanic monitoring.

The spatial distribution of groundwater age for different geohydrological situations in the Netherlands: implications for groundwater quality monitoring at the regional scale

The spatial distribution of groundwater age is a key factor determining the distribution of dissolved contaminants in the subsurface when contamination loadings have increased in time. The effects of surficial drainage and aquifer heterogeneity on the spatial distribution of groundwater age in unconsolidated aquifers in flat areas were investigated, and consequences were presented for the monitoring of contaminants from diffuse sources. First, the effects were assessed using model simulations. Second, the groundwater age distribution was evaluated in the two regional monitoring networks of Noord-Brabant and Drenthe using tritium measurements. Theoretically, a simple spatial distribution of groundwater age is present in homogeneous aquifers with natural groundwater recharge, characterized by a horizontal pattern of residence time isochrones and a gradual increase of groundwater age with depth. The model simulations show that the isochrone pattern becomes distorted in areas with a surficial drainage network, resulting in relatively old groundwater at shallow depth and larger spatial variation in groundwater age at a specific depth. This drainage effect on the spatial distribution of groundwater age is relatively large compared with effects of regional scale aquifer heterogeneity or spatially varying groundwater recharge. The effects of surficial drainage on the spatial distribution of groundwater age were confirmed by tritium measurements made in the regional monitoring networks of two provinces in the Netherlands. At about 24 m depth, the proportion of post-1950 groundwater in drained areas was significantly less and the spatial variation of groundwater age was larger than in recharge areas that lack a drainage network. The age of the groundwater appeared to be related to the drainage network density and the water table regime. A preliminary survey showed that contamination patterns in the two networks agree well with the proportion of post-1950 groundwater.

Glacial loading and unloading: a possible cause of rock salt dissolution in the Western Canada Basin

Each of the five main bedded Devonian rock salts in the Western Canada Basin has been leached in places more-or-less continuously since shortly after deposition. For the most part, salt dissolution has been self-sustaining and relatively slow. At various times however, leaching has been significantly accelerated by large-scale external processes such as regional uplift and erosion (during the pre-Cretaceous hiatus), and regional faulting (during the mid-Late Cretaceous). The most recent phase of accelerated leaching occurred during the Quaternary, probably as a result of glacial loading and unloading. Two principal lines of evidence support the thesis that accelerated rates of salt dissolution occurred during the Quaternary. First, there is an apparent correlation between surface drainage patterns and the active dissolution margins of the Devonian rock salts. A significant percentage of the lakes and rivers in proximity to active salt dissolution margins are situated along the near-zero edge of the respective salt bodies. Second, salt-related subsidence features at the pre-Quaternary subcrop level have been mapped on seismic data in the immediate proximity of active salt dissolution margins. The Quaternary subsidence features associated with the Prairie Formation rock salt for example, parallel the active salt margin and can be up to several kilometers wide. The areal extent of these shallow subsidence features and the high volume of rock salt dissovled at these sites suggest that accelerated rates of leaching occurred during the Quaternary (relative to the average rate of dissolution throughout the Cretaceous). The magnitude of these shallow structural features demonstrates clearly that subsidence associated with Recent salt dissolution can influence surficial drainage patterns to the extent suggested by the distribution of the lakes and rivers in proximity to the salt dissolution margins. The accelerated rates of salt dissolution during the Quaternary are attributed to glacial loading and unloading, and associated processes. Glacial loading could result in the compaction of subsurface strata, an increase in geothermal gradient, the thermal expansion of the deep aquifers, and the expulsion of interstitial water. These effects could increase both the rate of centrifugal fluid flow within a basin and the rate of salt dissolution. Glacial unloading could also affect the rate of salt dissolution. This rapid decrease in applied load is manifested as glacial rebound, a process which could increase the porosity and fracture permeability of the subsurface, and decrease the geothermal gradient. Potentially, the rate of centripetal fluid flow (at least around the basin margins) could be increased. Increased centripetal fluid flow, accompanied by an influx of understurated glacial melt waters would enhance the rate of salt dissolution.

Ravine formation on steep slopes: Forward versus regressive erosion. Some case studies from Rwanda

Summary Observations on steep slopes in Rwanda reveal that human interventions on surficial drainage can introduce gullying on places where runoff normally leads to diffuse or ephemeral rill erosion. In such cases, an initial runoff incision or slide scar, respectively due to an artificial concentration of runoff or underflow, grows in downslope direction, mainly by runoff incision. This type of progressive or forward gully development contrasts with the more classical regressive development, here only observed in places where the flow gradient is less than 7°–8°.

Succession of depositional environments in the neogene basin at Aliveri, Evia (Greece)

The Neogene lignite-bearing sequence in the southern part of the Aliveri-Kymi basin was studied with regard to sedimentology, palynology and coal petrology in order to establish the type and succession of environments involved and to compare them with modern analogues. Initial sediments of the Prinias Group were deposited in a wide shallow depression by means of a low competent drainage system which formed sandy alluvial fans, interfan areas and ephemeral lakes as well as sand flats and vegetated mudflats in lateral sequences from the basin margins to a central perennial lake. Succeeding sediments of the Prinias Group were formed under the influence of the rising lake level in algal-marsh, various peat-forming and open-lake environments. Sediments of the Koustoumalou Group were deposited by a highly competent surficial drainage system in alluvial fan to braided and meandering fluvial environments. This drainage system followed the south-north trending axis of an asymmetrical graben while debris-flows moved in from the steep flanks to the east.

Late Quaternary lacustrine paleoenvironments in the Cuenca de México

A late Quaternary paleolimnological history from the Mexican highlands has been obtained by diatom analysis of short cores and stratigraphic sections of lake and marsh sediments from the Cuenca de México, the large, endorheic, graben basin that holds Mexico City. The records, dated by radiocarbon and tephrochronologic methods, extend back to about 30 ka BP and document the presence of extensive, saline lakes in the basin until 25 ka BP. Thereafter, lake levels fell and marginal sites became shallow and fresh under the influence of surficial drainage and (especially) spring discharge. A shallow, saline lake existed 18 ka BP in Texcoco, the central and lowest basin in the system, reflecting moderately increased effective moisture at that time. By 16 ka BP, Lake Texcoco had become so shallow that diatoms were no longer preserved. However, marginal sites nourished by spring flow recorded changes in the local hydrologic balance resulting from increased infiltration at higher elevations. These changes appear to coincide with glacial advances between about 14 and 10 ka BP on the volcanic mountains surrounding the basin. Dry climates with reduced infiltration characterized the early Holocene, but by 5 ka BP a modest increase in precipitation established the modern climatic regime. These lacustrine records offer important insights for evaluating the paleoenvironmental history of the Cuenca de México based on other evidence. They confirm glaciological, stratigraphic and palynologic data that suggest dry climates and the absence of large pluvial lakes in the Cuenca de México during and after the full glacial, but document climates of significantly increased precipitation at least 10 ka prior to 18 ka BP.