Major River Channels Research Articles

A national-scale high-resolution runoff risk and channel network mapping workflow for diffuse pollution management

Managing diffuse pollution from agricultural land requires a spatially explicit risk assessment that can be applied over large areas. Major components of such assessments are the precise definition of both channel networks that often originate as small channels and streams, and Hydrologically Sensitive Areas (HSAs) of storm runoff that occur on land surfaces. Challenges relate to regions of complex topography and land use patterns, particularly those which have been heavily modified by arterial drainage. In this study, a national scale, transferrable workflow and analysis were developed using a specifically commissioned LiDAR survey. Research on the first half of Northern Ireland (6927 km2) is reported where field-edge drain to major river channels were mapped from 1 m (16 points per metre) digital terrain models, and in-field HSAs were defined across over 400,000 fields with a median field size of 0.86 ha. Manual drainage mapping supplemented with a novel automated drainage channel correction process resulted in an unparalleled high-resolution national drainage network with 37,320 km of channels, increasing mapped channel density from 0.9 km km−2 to 5.5 km km−2. The HSAs were based on a Soil Topographic Index (STI) system using hillslope and contributing area models combined with soil hydraulic characteristics. In all, 249 km2 of runoff risk HSAs were identified by extracting the top 95th percentile of the modelled STI as the areas with the highest propensity to generate in-field runoff. At field and individual farm scale these targeted risk maps of diffuse pollution were delivered to over 13,000 farmers and form part of the nationwide Soil Nutrient Health Scheme programme.

Groundwater dominates terrestrial hydrological processes in the Amazon at the basin and subbasin scales

The hydrology of the Amazon River basin (ARB) has been extensively studied; however, critical scientific gaps remain regarding key processes that govern hydrologic dynamics and the resilience of the rainforest. This inhibits the understanding of hydrological considerations needed for sustainable forest management under climate change and growing human stressors. Here, using high resolution (∼2km), long-term simulations from a process-based hydrological model (LEAF-Hydro-Flood), we investigate the dominant hydrological processes across the ARB, their key roles in shaping basin functions, and the decadal evolution therein. Results indicate that shallow groundwater (<5m deep) strongly modulates the seasonality of the surface fluxes across the ARB and at least 34% of the Amazonian Forest is supported by groundwater during the dry season. A two-month lag between the seasonal peak of evapotranspiration (ET) and river discharge is a key mechanism that potentially prevents the rainforest from tipping into savanna. The dry season in the ARB is getting drier and the wet season is getting wetter, pointing to an accelerating hydrologic cycle. The ARB is dominantly energy limited, however, our results imply that in the absence of groundwater support, and with less than ∼125 mm/month of precipitation, the basin could have become water-limited over some regions. The long-term basin-averaged ET—dominated by transpiration—changed with a split pattern of ±9% in the past three decades. Similarly, water table depth (±19%) and runoff (±29%) changed with heterogenous patterns across the ARB. Further, the contribution of canopy interception loss and ground evaporation changed heterogeneously across the ARB in response to deforestation. River discharge did not change substantially due to the crucial buffering role of groundwater, but terrestrial water storage (TWS) decreased (increased) in the 2000s (2010s) compared to that in the 1990s. Although groundwater is the dominant contributor to total TWS, the dynamics of TWS over the major river channels are controlled by flood water, given relatively shallow groundwater. This study provides crucial insights on the dominant hydrological processes in the ARB to inform forest management practices.

Tropical Asian mega‐delta ponds: Important and threatened socio‐ecological systems

AbstractThis paper uses multimedia to showcase the narratives and lived experiences of those who live and work in tropical Asian mega‐deltas, and as such is the first journal article of its kind in the field of Regional Geography. Using videos, photography and audio this paper describes the characteristics of ponds and their place in the intrinsically connected human‐environmental fabric of these delta regions. The aim is to bring to life descriptive inventories and provide greater weight in support of our conclusion that tropical Asian mega‐delta ponds are important and threatened systems. River deltas comprise just 1% of land cover worldwide but support the livelihoods of more than 500 million people. Delta research has historically focused on the major river channels and the socio‐ecological role of ponds has been overlooked despite their large number and surface area. Ponds are intrinsically linked to daily life (potable water, sanitation, bathing, washing), industry (aquaculture, agriculture) and the natural‐cultural heritage (religion, folklore) of deltas. In contrast to the larger river channels, ponds are likely to be significant stores and processors of nutrients, including carbon, and pollutants at annual to decadal scales, on account of their heavy anthropogenic use and smaller individual sizes. Consequently, they are severely polluted water sources and pose significant public health risks. In this review, we use case studies from three Asian mega‐deltas (the Red River Delta and the Mekong River Delta, Vietnam, and the Ganges‐Brahmaputra‐Meghna Delta, India and Bangladesh) to highlight the importance of Asian mega‐delta ponds as important socio‐ecological systems in their own right. We discuss future environmental challenges, knowledge gaps on the ecological function and biodiversity of these habitats, management and policy practices, and the capacity of ponds to achieve the UN Sustainable Development Goals.

Rapid incision events since 3.7 ka in the Anjihaihe Gorge along the northern Tianshan range, China

Young river terrace levels distributed along active tectonic regions and major river channels record the histories of the most recent tectonic activities and geomorphological evolution. There are 18 terraces in the upper reach of the Anjihaihe River, along active zones of the northern Tianshan in China. The surface of these young terrace insets is covered by Aeolian loess and fluvial deposits. Consequently, optically stimulated luminescence (OSL) dating was used to reveal the burial ages of those sediments, and to calculate the tectonic deformation and fluvial incision rate. However, the ages of young quartz sediments are difficult to determine due to the weak and dim OSL signal characteristics, and therefore geological and geomorphological information was not forthcoming. Recently, however, a “simplified procedures” methodology was developed to overcome these dating problems. Loess deposits can constrain the minimum abandonment age and fluvial sands were used to provide the age limits of formation, which correspond to the maximum and minimum incision rates. The elevation of each terrace above the modern river bed was obtained using a Digital Elevation Map with 12-m resolution, Google Earth images, and portable Global Positioning System measurements. Together with the results of the OSL dating and the elevation measurements, we conclude that the Anjihaihe River has incised the gorge to a depth of ~260 m since 3.7±0.2 ka, with a maximum average incision rate of 7.2±0.1 cm/a. The incision record can be divided into four major stages: ~3.7−3.0, ~3.0−2.4, ~2.4−1.3 ka, and from 1.3 ka to the present; the respective average incisive rates are 3.1±0.3, 15±0.3, 8.5±0.2, and 4.6±0.4 cm/a. The results indicate that the incision occurred in a series of steps which coincided with environmental changes in the region. As a result, this set of 18 strath terrace levels has recorded the aggradational-degradational surface processes of the Anjihaihe River gorge since ~3.7 ka. Before ~3.7 ka, a thickness of ~260 m of fluvial sands was aggraded above the pre-Quaternary bedrock. Subsequently, the river incised these sand layers until T11, and then, before the formation of T11, younger fluvial sands were deposited on the top of older deposits after 2.4 ka. The AJH River then continued to vertically erode these younger sands until 1.3 ka, when the incision reached the pre-Quaternary bedrock. From 1.3 ka to the present, at least five strath terraces were formed in the gorge. These surface processes and rapid incisive rates are ascribed to the specific lithological and geomorphological characteristics of the area, under the influence of climate change. First, the bedrock of the terrace levels is composed of pre-Quaternary sandstones, and more homogeneous, less consolidated siltstones and sandy mudstones. In addition, the upper terraces are covered by mixed cobbles and sands with a thickness of ~100 m above the bedrock, and the lower terraces by a thin layer of fluvial sands above the pre-Quaternary bedrock. Second, the variations in the flux of river water and sediments during episodes of climate change influenced the slope of the terrace surface, resulting in changes in aggradational-degradational processes and in the energy of the incision. Climate change was also a driving force, because the abrupt changes in incision rate correspond well to anomalous climatic events. Overall, the specific geomorphological characteristics and location of the Anjihaihe River have resulted in its recording an unusually rapid incision rate within the Anjihaihe Gorge.

Spatial Variability of Groundwater Arsenic Concentration as Controlled by Hydrogeology: Conceptual Analysis Using 2-D Reactive Transport Modeling.

Combined geological, hydrogeological, and geochemical controls on the arsenic concentration of contaminated aquifers in SE Asia were explored by two‐dimensional (2‐D) reactive transport modeling of data sets from Bangladesh, Cambodia, and Vietnam. For each site, the field data are summarized and used to create a conceptual 2‐D reactive transport model that elucidates characteristic features influencing the groundwater arsenic concentration. Comparison of models for Bangladesh and Vietnam indicates that fine‐grained layers overlying young sandy aquifers generate shallow high arsenic groundwater because low vertical groundwater velocities allow sufficient time for kinetic As release from the sediment. The low vertical groundwater velocity below major river channels, predicted by the model, also creates long groundwater residence times, leading to high arsenic groundwater. Young aquifer sediments release more arsenic than older sediments, and alternating young and older sediments create complex patterns of high and low arsenic groundwater. Over time, floodplain basins will subside, and river channels migrate, causing sedimentation and erosion on the floodplain while creating local environments with evolving hydrogeology and groundwater geochemistry. We have developed a three‐step model for the evolution of the Red River floodplain with sedimentation and shifting channels over the last 6000 years. The results show comparable timescales between the dynamics of arsenic release and of river migration, causing complex groundwater As distributions, comprising geochemical palinopsia of long vanished rivers.

Groundwater Occurrence from Hydrogeomorphological Study of Hard Rock Terrain of Part of Southwestern Nigeria

Abstract Studies of structural and hydrogeomorphological units (HGU) that are indicators of groundwater occurrence were carried out across an area extent of more than 700 km2 within the hard rock terrain of southwestern Nigeria. These studies integrated geological remote sensing techniques (RST) and geographical information system (GIS) methods to generate thematic maps that included elevation, drainage, lineaments and vegetation index for characterising the attributes of groundwater occurrence across the area. The results revealed that the lineament system is mainly rectilinear with major trends of NNW-SSE and NE-SW on the gneiss, NW-SE and NE-SW on porphyritic granite and NNE-SSW, NW-SE and E-W on migmatite. The discharge zones in the area are the lowland terrains underlain by gneiss and amphibolite. Similarly, variably directional discontinuities that are related to rock contacts are equally laden with groundwater. Conversely, the recharge areas are the high-lying terrains characterised by higher fracture density and underlain by porphyritic granite and migmatite. Additionally, there are evidences of groundwater seepage along the major river channels. Therefore, besides the rock structures, landform is another crucial factor that guides groundwater distribution in the study area.

Relocation of River Storage From Global Hydrological Models to Georeferenced River Channels for Improved Load‐Induced Surface Displacements

AbstractVariations in terrestrial surface water storage cause elastic crustal displacements of several millimeters in the vertical direction on daily to seasonal time scales. Locally, strong signals with exceptionally high amplitudes can be observed along the major river channels. As the horizontal resolution of global hydrological models is typically limited to 0.5° × 0.5°, the generalized model drainage network leads to simulated river mass distributions that are not necessarily collocated with the real courses of the rivers. Moreover, the spreading of locally concentrated surface water on the coarse model grids leads to substantially underestimated amplitudes of simulated hydrologically induced surface displacements. We therefore develop a relocation procedure to improve such hydrological loading estimates, especially at locations near the river banks. We separate the water masses stored in the modeled river network from the total water storage and relocate them on a georeferenced river map with higher resolution. Applying the relocated river masses, simulated hydrological loading amplitudes increase by 0.2–5 mm for the vertical load‐induced surface displacement along the major rivers, at the Amazon even up to additional 15 mm. The amplitudes for the horizontal load‐induced surface displacements are slightly increased, up to maximum 3 mm. Besides changes in the distance to the mass loads, the horizontal displacement fields is also heavily affected by significant changes in the directions to the relocated river mass load. The comparison of modeled hydrologically induced surface displacement time series with Global Positioning System observations shows a significantly improved fit for stations in close proximity to larger rivers when applying the relocation procedure.

Using stable isotopes 18O and 2H of lake water and biogeochemical analysis to identify factors affecting water quality in four estuarine Amazonian shallow lakes

AbstractStable isotopes analyses of oxygen and hydrogen of lake water were used to estimate the effect of evaporation (E) on the water quality of four shallow lakes in the Amapá State coast—Amazon/Brazil. These lakes, with different size and hydrologic conditions, were sampled during the course of the 2015/2016 El‐Niño (record‐breaking warming/drought in the Amazon rainforest). Hydrometeorological and water quality parameters were simultaneously performed to the isotopic sampling. The results showed that the evaporation process and the water quality can be explained by climate season and distances from the Atlantic Ocean. Lake evaporation losses ranged from ≈0–22% during the wet season in April/2016 and ≈35.7% during the dry season in November/2015. As expected, the evaporation of lake water was greater during the dry season, but it was higher for lakes farther away from the Atlantic Ocean compared with more coastal lakes due to tidal preponderance and the influence of major river channels. The more inland estuarine lakes showed a lower level of salinity (0.00–0.03 ppt) compared with those closer to the Atlantic Ocean (0.01–0.08 ppt). The El Niño phenomenon, with a lower precipitation in the Amazon basin, may initiate salinization of lakes closer to the Atlantic Ocean. Furthermore, strong mean seasonal variations of evaporation (0.06 ≤ E ≤ 0.22) and other hydrologic parameters were observed (precipitation, water temperature, and water depth), with significant effects on the water quality such as salinity, dissolved oxygen, chlorophyll (p &lt; .05). We conclude that the occurrence of the extreme climatic events can disrupt the biogeochemical and hydrological balance of these aquatic ecosystems and salinization of lakes closer to the Atlantic Ocean.

Lithofacies characters and depositional processes of a Middle Miocene Lower Siwalik fluvial system of the Himalayan foreland basin, India

The Lower Siwalik sequence in south-eastern Uttarakhand state of India records fluvial sedimentation during middle Miocene in the Himalayan foreland basin. Three distinct lithofacies associations (‘A’, ‘B’ and ‘C’) have been identified, each one of which comprises one or more of the eight major lithofacies and points to specific sub-environments of fluvial depositional setting. The lithofacies association ‘A’ is sandstone dominated and suggests deposition mainly by 0.09–2.0m/s flowing traction currents in braided streams. The lithofacies association ‘B’ is sandstone–mudstone dominated and suggests deposition by 0.1–1.0m/s flowing traction currents as well as suspension fallout in meandering streams and adjoining overbank areas. The facies association ‘C’ is mudstone dominated, characteristically pedogenically altered and disassociated with any major channel sandstone body. It represents reworking of pre-deposited sediments by ephemeral sheet and shallow channelized flows in the upland interfluve regions. The palaeoflow pattern reveals regional radial outward sediment dispersal by a distributary fluvial system, indicating convex-up transverse surface profile. The dominance of fine- to medium-grained sand and general absence of coarser extrabasinal clasts suggests that this distributive fluvial system of meandering and braided streams was located in the middle-distal part of a megafan. The upland interfluve surface developed following the cessation of megafan surface aggradation due to incision of major river channels.

Numerical simulations of global‐scale high‐resolution hydrological crustal deformations

High‐resolution load‐induced crustal deformations calculated from numerical models are tested for their ability to predict hydrologically‐induced station height variability, as they are known to be large enough to affect epoch‐wise parameters obtained from the analysis of global geodetic networks. Loading contributions due to terrestrial water storage as given by global hydrological models are calculated on a 0.5° global regular grid with daily temporal resolution. Apart from the dominant seasonal variations, the hydrological loading signal discloses also rapid changes exceeding 1 mm in several regions that can be associated with major precipitation events and river floods. Locally strong loading signals with exceptionally high amplitudes, in many cases even with nonseasonal nature, occur along the major river channels. Only high‐resolution loading calculations considering also the water mass anomalies stored in the model river flow can resolve the correct amplitudes in the surrounded area up to 100 km distance. The comparison of the modeled hydrological surface deformation with GPS station time series shows that high‐resolution hydrological loading estimates based on global‐scale models are able to explain a considerable fraction (up to 54%) of the observed vertical station movements caused by continental water storage variations.

Quantitative analysis of randomness exhibited by river channels using chaos game technique: Mississippi, Amazon, Sava and Danube case studies

Abstract. This paper presents a numerical evaluation of the randomness which can be observed in the geometry of major river channels. The method used is based upon that of generating a Sierpinski triangle via the chaos game technique, played with the sequence representing the river topography. The property of the Sierpinski triangle is that it can be constructed only by playing a chaos game with random values. Periodic or chaotic sequences always produce an incomplete triangle. The quantitative data about the scale of the random behaviour of the river channel pathway was evaluated by determination of the completeness of the triangle, generated on the basis of sequences representing the river channel, and measured by its fractal dimension. The results show that the most random behaviour is observed for the Danube River when sampled every 715 m. By comparing the maximum dimension of the obtained Sierpinski triangle with the gradient of the river we can see a strong correlation between a higher gradient corresponding to lower random behaviour. Another connection can be seen when comparing the length of the segment where the river shows the most random flow with the total length of the river. The shorter the river, the denser the sampling rate of observations has to be in order to obtain a maximum degree of randomness. From the comparison of natural rivers with the computer-generated pathways the most similar results have been produced by a complex superposition of different sine waves. By adding a small amount of noise to this function, the fractal dimensions of the generated complex curves are the most similar to the natural ones, but the general shape of the natural curve is more similar to the generated complex one without the noise.

Use of a regional, relict landscape to measure vertical deformation of the eastern Tibetan Plateau

Field work and topography analysis show that remnant, local areas of a low‐relief landscape or erosion surfaces are geographically continuous across the southeastern Tibetan Plateau margin. We correlate these remnant surfaces as a paleolandscape that formed at low elevation. Remnants of this paleolandscape are preserved because incision of the fluvial system has been largely limited to major rivers and principal tributaries and has not yet progressed throughout the entire fluvial network. The incomplete adjustment of the fluvial system signals initiation of rapid bedrock incision into a developing plateau margin, and erosional denudation is concentrated in the major river channels. This interpretation contradicts earlier notions that low‐gradient, regional topography is the product of regional elevation reduction by intense landscape dissection due to the presence of several large southeast flowing rivers. The modern altitude of the reconstructed paleolandscape (or “relict landscape”) constrains the vertical displacement of the plateau surface in response to crustal thickening and subsequent erosion during the lateral growth of the Tibetan Plateau. Regional preservation of the relict landscape is consistent with minor surface disruption by late Cenozoic thrust faults and folds and supports a model of distributed lower crustal thickening. Because significant erosion is limited to narrow river gorges, an increase in the plateau elevation due to isostatic rebound is minor. Therefore we propose that the modern elevation of the relict landscape reflects isostatically compensated thickening of the lower crust.

Facies characteristics and cyclicity of Lower Siwalik sediments, Jammu area: a new perspective

The Lower Siwalik succession of the Jammu area has been distinguished into three major lithofacies associations: a sand-dominant association, a sandy-mud-dominant association, and a silty-heterolithic association. The sand-dominant association is made up of three lithofacies: cross-bedded sandstone, rippled silty sandstone and bioturbated sandy siltstone, which are organized in multi-storeyed sandbodies representing deposition in major river channels. The sandy-mud-dominant association is made up of two lithofacies, mottled clayey siltstone and interbedded sandstone, siltstone and mudstone, representing deposition in overbank areas of flood-plain and natural levee-crevasse splays. The sand-dominant association and sandy-mud-dominant association are grouped together as a channel-related succession and are products of processes in the river channel. The silty-heterolithic association consists of four lithofacies: mottled siltstone, mottled silty sandstone, bedded calcrete and mottled mudstone. They are considered to be deposits of Doab (upland interfluve) areas operating independently of present-day major river channels. These deposits have been formed in minor channels, sloping surfaces, and lakes and ponds of the interfluve regions. The cyclicity of both successions (channel-related and Doab-related) has been determined using a partial-independence statistical model.

Upland interfluve (Doab) deposition: Alternative model to muddy overbank deposits

Major alluvial plains contain large tracts of fine-grained muddy sediments, deposited away from the main river channels, which are mostly classed as overbank or floodplain deposits. Systematic study of the Ganga plain shows that such large tracts of deposition of muddy sediments are located several metres above the major channels, and are not flooded by overtopping of the major river channels. These surfaces are here designated as upland interfluve areas (Doab) where deposition of fine-grained sediments takes place independent of the processes operating in the main channels. The surfaces show distinct depositional domains with characteristic deposits. These include higher sloping surfaces (mottled silt), lower flat surfaces (variegated clayey silt), gulleys (sandy silt), small channels (mottled silty sand), ponds (shelly sandy clayey silt), lakes (shelly clayey silt). These deposits are prone to diagenetic changes, especially the development of calcrete horizons. Redistribution of these domains through time produces characteristic mud-dominant alluvial stratigraphy as observed in the Late Quaternary deposits of the Ganga plain. This succession shows similarity to mud-dominant deposits of the Siwalik succession. These Doab deposits are distinct from the overbank deposits formed close to the river channels affected by channel processes. It is argued that many of the thick mud-dominant fluvial deposits of the ancient fluvial record are products of deposition in upland interfluve areas.

Is magmatic underplating the cause of post-rift uplift and erosion within the Cabora Bassa Basin, Zambezi Rift, Zimbabwe?

The Cabora Bassa Basin in northern Zimbabwe is an Upper Paleozoic karoo basin trending almost east-west. It has clearly recognisable gravity and magnetic signatures from which its dimensions are estimated to be approximately 150 km long and at least 62 km wide. Its southern boundary is marked by a north dipping, listric, master fault of possibly a ramp-flat-ramp geometry. Within the basin there is erosional truncation of over 2 km of sediment at the top of the stratigraphy and an absence of a post-rift thermal subsidence phase. Modern and major river channels are characteristically narrow, deep and without considerable amounts of silt on the river beds, whilst their valley sides are marked by multiple terraces. These ongoing erosional processes are evidence for sustained and possibly episodic uplift of the basin since the end of rifting in the Late Cretaceous/Early Tertiary. An evaluation of possible uplift mechanisms for the basin and its surroundings lends support to lithospheric thickening as the most likely mechanism. Compression and magmatic underplating and/or intrusion are two common ways of thickening the lithosphere. The absence of major compressional structures within the basin suggests that magmatic underplating and intrusion may have played a major role in lithospheric thickening. Major element data for the mantle-derived Jerama basalts indicate substantial hidden cumulates, which possibly thickened the lithospheric column. Modelling of gravity data, constrained by both seismic reflection results, and the densities of the surface rocks, show that the crustal thickness beneath the basin is in the range 19–23 km. The stretching factors from seismic and gravity models range from 1.6 to 1.9. There is up to 5 km difference between the expected and modelled crustal thicknesses beneath the basin, which could be explained by magmatic underplating.

Abstract: Depositional Environments and Sandstone Diagenesis in the Tyler Formation (Pennsylvanian), Western North Dakota&nbsp;

Thirty-three oil well cores were examined and approximately 3,100 oil well logs were used to determine the depositional environments of the Tyler Formation (Pennsylvanian) in northwestern North Dakota. Ninety sandstone thin-sections were described and twenty-nine sandstone samples were analyzed by scanning electron microscope/X-ray microanalysis techniques to characterize sandstone diagenesis. The Tyler Formation in northwestern North Dakota is correlative with the lower unit of the Tyler Formation in southwestern North Dakota. The upper unit of the Tyler Formation in southwestern North Dakota is not present in northwestern North Dakota, contrary to previous workers suggestions. The Tyler Formation in northwestern North Dakota consists of varicolored mudstone, medium-gray claystone, thin bituminous coal beds, dark-gray shale and limestone, and gray sandstone. The upper unit of the Tyler Formation in southwestern North Dakota consists of gray sandstone, dark-gray limestone and shale, and varicolored mudstone. Lenticular sandstones present in the middle and rarely at the base of the formation, which occur in northeast-southwest linear trends, indicate that the Tyler Formation in northwestern North Dakota was deposited as a river channel system on a low-lying, prograding coastal plain. Depositional environments associated with the coastal plain include river channels, flood plains, lakes, estuaries, caliche paleosols, and backswamps. Throughout deposition of the Tyler Formation, several major river channels flowed in a predominantly southwest direction across a low-lying coastal plain and transported quartz sand toward a delta or shoreline, not identified in this study, at the margin of a shallow epeiric sea. Initially, the presence of detrital clay and infiltration clay and early hematite and chlorite coatings on quartz grains was the most important factor influencing porosity reduction in Tyler sandstones. The first major diagenetic event responsible for porosity reduction was the development of authigenic quartz overgrowths. Other cements that have contributed to porosity reduction in Tyler sandstones include amorphous silica (opal), calcite, ankerite, siderite, anhydrite, barite, hematite, and pyrite, In addition, authigenic kaolinite, illite, and chlorite have reduced porosity in Tyler sandstones. Dissolution of carbonate cement resulted in the formation of secondary porosity. Sandstone diagenesis took place shortly after sand was deposited in a marine environment, while sediments were being flushed with meteoric ground water, and while sands were being flushed with chemically evolved pore water released after diagenesis and compaction of interbedded mud-rich sequences. River-channel sandstones in northwestern and southwestern North Dakota and barrier-island sandstones present only in southwestern North Dakota contain considerable amounts of intergranular porosity. Petroliferous sandstones in southwestern North Dakota are fine- to medium-grained, well-sorted, texturally mature…

Magpie Goose, Anseranas-Semipalmata, Nesting on the Mary River Floodplain, Northern-Territory, Australia - Extent and Frequency of Flooding Losses

Magpie geese, Anseranas semipalmata, nest synchronously in large colonies on the seasonally inundated floodplains of the Northern Territory. Erratically variable wet season rainfalls may cause rises in floodplain water levels that destroy many nests. We describe such an event on the Mary River foodplain in which all of the nests within a monitored study area were drowned. Vulnerability to flooding (as indicated by variation in the depth at which eggs first submerged) varied significantly among vegetation types. Mean rises in water levels causing inundation of eggs varied from 67 cm in nests constructed on a base of Oryza rufipogon to 118 cm on nests constructed on Eleocharis brassii. Nests close to major river channels are more vulnerable to flooding. Analysis of patterns of river level rises since 1959 indicate that such catastrophic nest losses are relatively common, perhaps occurring as often as one year in seven. Lesser flooding losses probably occur in most years. Nesting colonies on the Mary River are likely to suffer higher flooding mortalities than on many other river systems.

Some general and theoretical considerations on the fish yield of African rivers

The factors regulating fish production from river systems remain poorly studied and understood. Rivers conform to physical and chemical laws which determine their morphology. From these laws relationships are calculated which estimate the total length and number of streams of different order on the African continent. Edaphic factors very less than morphological ones and the chemical and physical conditions in the major river channels tend to resemble each other closely. The present catch from African rivers, evaluated from catch statistics, by country and by river system, resembles a theoretical figure derived from the basin area. However, these statistics are drawn only from major fisheries and there remain a very large number of smaller streams whose production does not enter into this calculation. A theoretical approach to this problem is proposed which gives an estimate of annual yield of 530 000 t of fish at present levels of catch. Deviations from the theoretical yield in individual river systems arise from differences in both edaphic and morphological characteristics.