Drainage Basin Size Research Articles

A data set of monthly freshwater fluxes from the Greenland ice sheet’s marine-terminating glaciers on a glacier–basin scale 2010–2020

The loss of mass from the Greenland ice sheet causes an increasing influx of freshwater to the Greenlandic fjords and the oceans. Freshwater fluxes from marine-terminating glaciers are important to understand fjord circulation and ecosystem dynamics. Here, we present a data set constructed by reformulating existing products into a shared temporal and spatial framework. We combine three publicly available data sets of solid-ice discharge (iceberg), liquid-surface runoff (runoff) and basal melt to present a cohesive overview of the flow of freshwater from marine-terminating glaciers to the Greenlandic fjords. We also calculate glacier drainage basins and compare our findings to previous studies showing that drainage-basin sizes may vary considerably depending on how they were reconstructed. The data set will be a valuable asset to oceanographic, glaciological and marine biological research activities.

Beach pollution from marine litter: Analysis with the DPSIR framework (driver, pressure, state, impact, response) in Tuscany, Italy

• DPSIR applied to beach contamination through data search and fieldwork. • Fieldwork allowed to gain insight into local pressures, state, and health impact of beach-stranded. • Main local responses: waste removal from drainage system and recycling of stranded material. • Local and regional efforts are required to build a systemic view of beach contamination. Beaches are affected by the accumulation of natural and anthropogenic material; however, this environmental issue has not yet been explored from a One Health perspective. In this paper, the conceptual framework of DPSIR (Drivers-Pressures-State-Impact-Response) was used to understand the beach-stranded material issue in a systemic way and a data-based classification for some environmental indicators was developed to support the DPSIR analysis. The model was applied to an Italian coastal municipality as a case study, through the collection of data from a variety of data sources: publicly accessible database, data from a stakeholders’ network (i.e., coastal authority, solid waste company, sewerage company, drainage consortium), and fieldwork consisting in microbiological analysis of stranded material and underlying sand, visual census of macrolitter along beach and waterways. In the study area, solid wastes production was a high pressure (768 kg/capita/year), but in situ visual observations of floating wastes at the outlet of the canals revealed that the contribution of local waterways to marine litter was negligible, thus suggesting the effectiveness of the measures adopted along local waterways by the drainage consortium (i.e., grids at the drainage pumping stations). Nevertheless, very high quantity of anthropogenic wastes was counted during the beach litter surveys (603 items/100 m), probably as a result of coastal current pathway that transported material from major watercourses (>100 km 2 drainage basin size; 23 items/h). On the contrary, local sewage production represented a very high pressure (>33,000 m 3 /km) that impacted on the microbiological quality of the stranded material with moderate to high level of fecal bacteria indicators detected in the beach cast. The underlying sand was affected by such contamination, with most of the sample within the provisional limit set by WHO for enterococci in beach sand (60 CFU/g) that was associated to a health risk of<5 % of gastroenteritis attributable to accidental ingestion of sand; nevertheless, some enterococci peak values (980 MPN/g) could be associated to a health risk for gastroenteritis>10 %. The beach-stranded material was collected without separating the sand, with annual quantity of 1,243 kg/m, that was processed in a dedicated facility allowing to recover up to 98 % of sand and biomass after the treatment, with moderate expenditure for the coastal municipality (22 €/m). Overall, this study allowed to better figure out the cause-effect relationships underlying the accumulation of stranded material along shoreline and the effectiveness of the management practices toward beach-stranded material. Therefore, the usage of the DPSIR framework as structuring model to understand the problem of stranded material could be useful for beach managers and administrators, and its adoption within beach management programs is worth for improving beach quality.

Long-term morpho-structural development of major normal fault zones, Gran Sasso area, Central Apennines (Italy)

Although the Central Apennines of Italy are frequently struck by earthquakes related to normal faulting, so far, no detailed regional study into long-term development and mode of normal faulting exists. In this study, we focus on the Assergi fault zone (AFZ) and the Campo Imperatore fault zone (CIFZ) in the Gran Sasso area.Both fault zones are S-dipping and ~ 20 km in length. Range front faults with ~1 km of footwall topography are associated with basins in the hanging wall. To create footwall relief, long-term throw rates of >0.6 mm/a are required. We compared geologic throw with morphologic markers, such as range and triangular facet height, drainage basin size, spacing between drainage basin outlets, and surface offsets. The morphologic analysis allows to distinguish fault sectors that share morphological characteristics but typically include several mapped fault segments. The history of faulting of the CIFZ indicates that normal fault segments first acquire offset at a constant length until reaching a threshold, and then link with neighbouring fault segments into longer fault zones.Our study of the fault zones suggests the following general concept for long-term development of range-front fault morphology and associated slopes: (1) Surface offset by active faulting is followed, or accompanied, by incision of numerous small alluvial-fan catchments with narrow outlet spacing into the fault footwall. (2) Upon further fault growth, large triangular facets separated by gullies develop. By headward and lateral erosion the small fan catchments merge into fewer and larger catchments with larger outlet spacing. (3) During fault dormance, fault scarps degrade to lower-dipping slopes typically veneered by scree. On these slopes, scarps excavated by differential erosion expose Riedel shears of the main faults.

New geomorphological and archaeological evidence for drainage evolution in the Luangwa Valley (Zambia) during the Late Pleistocene

This is the first systematic investigation of two distinctive geomorphological features recorded in the central Luangwa River valley, Zambia. A series of low hills was found to be capped by thin (~1 m) gravel deposits containing stratified Stone Age artefacts. More widespread gravels occur on the margins of the Luangwa River floodplain lacking stratified artefacts. The previously unreported hilltop deposits are interpreted as remnants of a dissected land-surface, and the valley floor gravels as redeposited clasts from c. 20 m of down-cutting. Clast analysis and drainage basin size analysis support a hypothesis of gravel deposition by unconstrained debris flows from the distant Muchinga escarpment, or from an intermediate zone. Excavation of a perched deposit revealed a coarsely stratified Stone Age record indicating periodic emplacement of artefact-bearing gravels over an extended period. Deposition of these perched gravels continued into the Late Pleistocene (~77 ka), based on OSL dating, after which the current dissected landscape formed. We hypothesize further, based on a regional record of landscape instability and core data from Lake Malawi, that fan formation in the valley was linked to periods of extended aridity and reduced vegetation cover followed by episodic erosional events on the return to wetter conditions. We argue that the subsequent dissection of the land-surface is the end state of a sequence of responses to base-level changes and climate change.

Luminescence dating of Quaternary alluvial successions, Sellicks Creek, South Mount Lofty Ranges, southern Australia

Quaternary alluvial and colluvial sediments infill major river valleys and form alluvial fans and colluvium-filled bedrock depressions on the range fronts and within the Mount Lofty Ranges of southern Australia. A complex association of alluvial successions occurs in the Sellicks Creek drainage basin, as revealed from lithostratigraphy, physical landscape setting and optically stimulated luminescence (OSL) ages. Correlation of OSL ages with the Marine Oxygen Isotope record reveals that the alluvial successions represent multiple episodes of alluvial sedimentation since the penultimate glaciation (Marine Isotope Stage 6; MIS 6). The successions include a penultimate glacial maximum alluvium (Taringa Formation; 160 ± 15 ka; MIS 6), an unnamed alluvial succession (42 ± 3.2 ka; MIS 3), a late last glacial colluvial succession within bedrock depressions (ca 15 ka; MIS 2) and a late last glacial alluvium (ca 15 ka; MIS 2) in the lowest, distal portion of Sellicks Creek. In addition, the Waldeila Formation, a Holocene alluvium (3.5 ± 0.3 ka; MIS 1), and sediments deposited during a phase of Post-European Settlement Aggradation (PESA) are also identified. The age and spatial distribution of the red/brown successions, mapped as the Upper Pleistocene Pooraka Formation, directly relate to different topographic and tectonic settings. Neotectonic uplift locally enhanced erosion and sedimentation, while differences in drainage basin sizes along the margin of the ranges have influenced the timing and delivery of sediment in downstream locations. Close to the Willunga Fault Scarp at Sellicks Creek, sediments resembling the Pooraka Formation have yielded a pooled mean OSL age of 83.9 ± 7 ka (MIS 5a) corroborating the previously identified extended time range for deposition of the formation. Elsewhere, within major river valleys, the Pooraka Formation was deposited during the last interglacial maximum (128–118 ka; MIS 5e). In general, alluviation occurred during interglacial and interstadial pluvial events, while erosion predominated during drier glacial episodes. In both cases, contemporaneous erosion and sedimentation continued to affect the landscape. For example, in the Sellicks Creek drainage basin, which lies across an actively uplifting fault zone, late glacial age sediments (MIS 2) occur within the ranges and near the distal margin of the alluvial fan complex. OSL dating of the alluvial successions reported in this paper highlights linkages between the terrestrial and marine environments in association with sea-level (base-level) and climatic perturbations. While the alluvial successions relate largely to climatically driven changes, especially in major river valleys, tectonics, eustasy, geomorphic setting and topography have influenced erosion and sedimentation, especially on steep-sloped alluvial fan environments.KEY POINTSLuminescence dating of the Sellicks Creek alluvial fan complex reveals that sedimentation occurred predominantly during the later stages of glacial cycles accompanying lower sea-levels than present.Luminescence dating confirms that the stratigraphically lower portions of the Pooraka Formation are beyond the range of radiocarbon dating.Upper Pleistocene alluvial fan sedimentation at Sellicks Creek correlates with pluvial events in southeastern Australia.

Checklist of cyanoprokaryotes and algae in the large tropical River Congo (Africa)

This study presents data on the floristic composition of cyanoprokaryotes/cyanobacteria and algae in the tropical River Congo, the second longest river in Africa after the Nile, which at global scale is the second most important river after Amazon in terms of drainage basin size and freshwater discharge. The results in this paper are based only on the materials collected from the mainstem of the middle part of the River Congo, in a stretch of 1,450 km, in two sampling campaigns conducted in contrasting hydrologic conditions: in the high water (HW) period, in December 2013 (33 sites), and in the low, falling water (FW) period in June 2014 (38 sites). Totally 520 taxa of 7 divisions (Cyanoprokaryota – 76, Cryptophyta – 1, Euglenophyta -17, Pyrrhophyta – 8, Chlorophyta – 108, Streptophyta – 38, Ochrophyta – 272: 242 Bacillariophyceae, 24 Chrysophyceae, 1 Synurophyceae, 3 Tribophyceae and 2 Eustigmatophyceae), were identified in the phytoplankton samples but few of them (16) were strictly tropical. Algal diversity was higher during the FW (431 taxa) compared with HW (314 taxa) and floristic similarity between both periods was only 57% based on 213 common taxa. According to the frequency of distribution, most of the phytoplankters occurred rarely and only two diatoms were found in all sites during both studied periods: Aulacoseira ambigua and Nitzschia lancettula. They were followed by the widespread Aulacoseira agassizii, A. granulata var. angustissima, Nitzschia cf. lacuum, Staurosirella leptostauron and Staurosirella pinnata. Cyanoprokaryotes and chlorophytes were found along the whole river flow, but with different species composition.

Growing Spatial Scales of Synchronous River Flooding in Europe

AbstractRiver flooding is a common hazard, causing billions of dollars in annual losses. Flood impacts are shaped by the spatial scale over which different rivers flood simultaneously, but this dimension of flood risk remains largely unknown. Using annual flood data from several thousand European rivers, we demonstrate that the flood synchrony scale—the distance over which multiple rivers flood near synchronously—far exceeds the size of individual drainage basins and varies regionally by more than an order of magnitude. These data also show that flood synchrony scales have grown by about 50% over the period 1960–2010. Detrended flood synchrony values are serially correlated, implying that years with spatially extensive floods tend to follow one another. These findings reveal that flood risks are correlated well beyond the individual drainage basins for which flood hazards are typically assessed and managed.

Методи оцінювання антропогенних загроз біорізноманіттю прісноводних екосистем

The industrial and agricultural activities of mankind, which includes chemical pollution, building of dams and hydroelectric power stations and their use of the biomass of freshwater ecosystems, have had considerable impacts on freshwater ecosystems, the abundance of aquatic species, and the vital capacity of those species’ populations. A variety of aquatic organisms are feasible indicators and they can be used for evaluation water quality through analysis of their population diversity, composition and abundance. The most significant threats for freshwater biodiversity and the integrity of aquatic ecosystems are activities in the watershed’s terrestrial realm, disturbances occurring in aquatic and riparian environments and threats that affect freshwater organisms. There are changes of functional structure of biological community due to reducing biodiversity and the dominance of opportunistic species. A lot of changes in characteristics of biological communities can be assessed by using different kinds of evaluations, including the index of biological integrity, saprobity system and geographic information system technology. An effective predictor of great number of disturbances to freshwater ecosystems is the density of human population. Approaches that directly evaluate the level of threat to freshwater biodiversity typically use species composition and abundance data. Methods that indirectly assess the biotic community evaluate the ecological integrity of aquatic ecosystems. For large-scale, resource-limited, time-constrained biodiversity conservation planning, the most appropriate method for assessing the impairment of freshwater systems is relativistic evaluation of biological integrity using geospatial data. For aquatic threat assessments, the most meaningful level of analysis is the drainage basin and its subbasins. The concept of ecoregions covers all freshwater ecosystems. This approach provides biologically meaningful units for further large-scale studies of existing and potential threats to biodiversity. The size of drainage basins can be defined from digital elevation models, which exist as global network in high quality resolution. At the level of drainage basin, using high-resolution remote sensing imagery is often feasible from a financial and workload perspective.

Geomorphologic proxies for bedrock rivers: A case study from the Rwenzori Mountains, East African Rift system

Geomorphic proxies yield useful insights into understanding long-term endogenic and exogenic response to erosion and/or rock uplift rates. By evaluating areal proxies (including asymmetry factor (AF), mountain front sinuosity (Smf), hypsometric integral (HI), geophysical relief, and shape factor (Shp), and linear proxies (including normalized steepness index (ksn), length-gradient index (SLk) and Chi gradient (Mχ), the erosion and/or rock uplift rates can be quantified. We carried out morphotectonic analysis in the Rwenzori Mountains, which represents an anomalously uplifted Precambrian horst within the western branch of the East African Rift system (EARS). This study aims to: (1) evaluate the relationship between geomorphic proxies and drainage basin's maturity; (2) evaluate the usefulness of geomorphic proxies as recorders of erosion and/or rock uplift rates; (3) evaluate the sensitivity of each geomorphic proxy to the drainage basin size and geometry, stream order, glaciers extent, and local structures; (4) explore internal correlation within the geomorphic proxies; and (5) contribute to the understanding of morphotectonic evolution of the Rwenzori Mountains. For this, we computed the stream's ‘Good of Fitness’ (R2, an indicator of the drainage basin's maturity) and geomorphic proxies for the drainage basins and their streams in the Rwenzori Mountains from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) digital elevation model (DEM). Subsequently, we correlated the areal geomorphic proxies with each other and with R2. Also, we correlated the linear geomorphic proxies with each other and with published erosion rates obtained from cosmogenic 10Be analysis. Our results show that the areal geomorphic proxies (AF, Smf, HI, relief, and Shp) — considering the drainage basin size and geometry, stream order, glacier extent, and local structures — can be applied to locally evaluate the maturity of the drainage basin. We also found that the linear geomorphic proxies (ksn, SLk, and Mχ) have strong positive correlation with the erosion rates, they are effective in detecting fault activities, and some of them (ksn and Mχ) are highly correlated with each other. Additionally, our geomorphic proxy results suggest that the north and south sectors of the Rwenzori Mountains are in relative tectonic quiescence but that the central sector is experiencing higher levels of erosion and rock uplift.

Landforms characteristic of inter-ice stream settings on the Norwegian and Svalbard continental margins

The suite of sedimentary landforms produced on continental shelves by fast-flowing ice streams is typified by features that are elongate and streamlined in the direction of past ice flow (e.g. Stokes & Clark 2001; Ottesen et al. 2005 a ; Livingstone et al. 2012). By contrast, the slower-flowing parts of an ice sheet, located between ice streams and fed by drainage basins that are often an order of magnitude or more smaller than the interior basins feeding the ice stream, have a very different set of characteristic submarine landforms; these are orientated predominantly transverse to the direction of past ice flow (Ottesen & Dowdeswell 2009; Klages et al. 2013, 2016). In addition, on many high-latitude shelf-banks glacial landforms produced beneath and at the margins of ice sheets are often reworked by the ploughing action of iceberg keels, sometimes to the point where these primary depositional landforms are almost obliterated (e.g. Dowdeswell et al. 1993, 2014). Submarine landforms from the western margin of the former Eurasian Ice Sheet, which extended across the shallow banks and deeper intervening troughs on the continental shelves of Norway and Svalbard at the Last Glacial Maximum (LGM) (Svendsen et al. 2004; Ottesen et al. 2005 a ), are used to illustrate this inter-ice stream palaeo-glacial setting (Fig. 1). In these locations, ice flux is limited by relatively small drainage-basin size and ice flow is likely to be slow relative to the 103–104 m a−1 velocities of adjacent ice streams (Dowdeswell & Siegert 1999). Fig. 1. ( a ) Location of study areas offshore of Norway and Svalbard (red boxes; map from IBCAO v. 3.0). Bathymetric data showing the pattern of shallow banks and intervening cross-shelf troughs offshore of ( b ) NW Norway and ( c ) NW Spitsbergen. WI, Wijdefjorden; KR, Krossfjorden. ### Description The …

Extreme weather: is ‘abnormal’ becoming the new ‘normal’?

Extreme weather: is ‘abnormal’ becoming the new ‘normal’?

Scale-dependent groundwater contributions influence patterns of winter baseflow stream chemistry in boreal catchments

Understanding how the sources of surface water change along river networks is an important challenge, with implications for soil-stream interactions, and our ability to predict hydrological and biogeochemical responses to environmental change. Network-scale patterns of stream water reflect distinct hydrological processes among headwater units, as well as variable contributions from deeper groundwater stores, which may vary nonlinearly with drainage basin size. Here we explore the spatial variability of groundwater inputs to streams, and the corresponding implications for surface water chemistry, during winter baseflow in a boreal river network. The relative contribution of recent and older groundwater was determined using stable isotopes of water (δ18O) at 78 locations ranging from small headwaters (0.12 km2) to fourth-order streams (68 km2) in combination with 79 precipitation and 10 deep groundwater samples. Results from a two end-member mixing model indicate that deeper groundwater inputs increased nonlinearly with drainage area, ranging from ~20% in smaller headwater subcatchments to 70–80% for catchments with a 10.6 km2 area or larger. Increases in the groundwater contribution were positively correlated to network-scale patterns in surface stream pH and base cation concentrations and negatively correlated to dissolved organic carbon. These trends in chemical variables are consistent with the production of weathering products and the mineralization of organic matter along groundwater flow paths. Together, the use of stable isotopes and biogeochemical markers illustrate how variation in hydrologic routing and groundwater contributions shape network-scale patterns in stream chemistry as well as patchiness in the relative sensitivity of streams to environmental change and perturbation.

Is Ganga the Longest River in the Ganga Basin, India?

The length of the main trunk in a river basin is an important morphometric parameter and it depends on the size of the drainage basin. The Ganga River Basin is one of the largest basins in the world with the Ganga River considered to be the main stem. Variable lengths of this river in the literature motivated us to study its exact length and also to test whether geomorphically it is the longest river in the basin. The results show that the maximum river length of 2758 km is attained when source is considered at the headstream of the Tons River. This length is more than any of the traditional lengths of the Ganga River in the literature. We propose to call the longest segment of the river in Ganga Basin as the Himalayan Foreland River.

An advanced Index of Biotic Integrity for use in tropical shallow lowland streams in Costa Rica: Fish assemblages as indicators of stream ecosystem health

This paper presents an advanced version of the Index of Biotic Integrity (IBI), a multimetric index to indicate ecosystem health. The multimetric index has been adapted in such a way that it not only indicates overall condition but also specific causes of environmental disturbance. The newly developed index (a) uses data of tolerant as well as intolerant species in a single metric to indicate environmental disturbance, (b) does not require knowledge about species from the literature, and (c) can be applied to artificial landscapes.The metrics proposed here consist of indicator species assemblages that are selected directly for their relationship with an environmental component or specific type of environmental degradation. Thus, each metric indicates a type of environmental concern, which enables conservation practices to be targeted more effectively. Species assemblages for each single metric consist of a combination of species that can be negatively and positively related to environmental disturbances, providing a better indication of stream ecosystem health.The area studied was assumed to be too diverse for one single index. Canonical Indirect-Gradient Principal Component Analysis indicated that the optimal division of subindices based on stream typology was for streams with drainage basin sizes <10km2 and >10km2. Pearson Product-Moment Correlations were used to identify relationships between anthropogenic disturbances and the composition and abundance of fish species at impacted as well as undisturbed sites. This index proved to be useful for indicating overall stream ecosystem health as well as local onsite environmental disturbances or the environmental components of greatest concern. This index does require extensive information about measured levels of anthropogenic disturbances with the accompanying composition and abundance of fish species.

Use of morphometric analysis and self-organizing maps for alluvial fan classification: case study on Ostorankooh altitudes, Iran

The aim of this study is to classify alluvial fans formed by high-gradient braided streams and torrents that discharge into the Oshtorankook altitudes in the Lorestan province, Iran. The morphology of the fans and their watershed is quantitatively described through estimated morphometric parameters. For relationships between geomorphological features of the fans and their drainage basins, self-organizingmaps (SOM) were used. In SOM, according to both qualitative data and morphometric variables, the clustering tendency of alluvial fans was investigated using 15 alluvial fans parameters. The results of the analysis showed that several morphologically different fan types were recognizedbased on their geomorphological characteristics in the study area. A strong positive relationship was found between the drainage basin area and size of the fan with a simple power function. In addition, the relationship between fan slope and drainage area was found to be negative and moderately strong with a simple power function.

Using Self-Organizing Maps for Alluvial Fan Classification

Morphometry research used geography information system (GIS) to explore alluvial fan morphology for determination of fans location and fast and accurate analysis. The aim of this study is to classify alluvial fans formed by high- gradient braided streams and torrents that discharge into the Oshtorankook altitudes in the Lorestan province, Iran. The morphology of the fans and their watershed is quantitatively described through estimated morphometric parameters. For relationships between geomorphological features of the fans and their drainage basins, self-organizing maps (SOM) were used. In SOM, according to both qualitative data and morphometric variables, the clustering tendency of alluvial fans was investigated using 15 alluvial fans parameters. The results of the analysis showed that several morphologically different fan types were recognized based on their geomorphological characteristics in the study area. A strong positive relationship was found between the drainage basin area and size of the fan with a simple power function. In addition, the relationship between fan slope and drainage area was found to be negative and moderately strong with a simple power function.

Characterizing the primary material sources and dominant erosional processes for post-fire debris-flow initiation in a headwater basin using multi-temporal terrestrial laser scanning data

Wildfire dramatically alters the hydrologic response of a watershed such that even modest rainstorms can produce hazardous debris flows. Relative to shallow landslides, the primary sources of material and dominant erosional processes that contribute to post-fire debris-flow initiation are poorly constrained. Improving our understanding of how and where material is eroded from a watershed during a post-fire debris-flow requires (1) precise measurements of topographic change to calculate volumetric measurements of erosion and deposition, and (2) the identification of relevant morphometrically defined process domains to spatially constrain these measurements of erosion and deposition. In this study, we combine the morphometric analysis of a steep, small (0.01km2) headwater drainage basin with measurements of topographic change using high-resolution (2.5cm) multi-temporal terrestrial laser scanning data made before and after a post-fire debris flow. The results of the morphometric analysis are used to define four process domains: hillslope-divergent, hillslope-convergent, transitional, and channelized incision. We determine that hillslope-divergent and hillslope-convergent process domains represent the primary sources of material over the period of analysis in the study basin. From these results we conclude that raindrop-impact induced erosion, ravel, surface wash, and rilling are the primary erosional processes contributing to post-fire debris-flow initiation in the small, steep headwater basin. Further work is needed to determine (1) how these results vary with increasing drainage basin size, (2) how these data might scale upward for use with coarser resolution measurements of topography, and (3) how these results change with evolving sediment supply conditions and vegetation recovery.

Geomorphic signature of Antarctic submarine gullies: Implications for continental slope processes

Five quantitatively distinct gully types are identified on the Antarctic continental margin from swath bathymetric data of over 1100 individual features. The gullies differ in terms of length, width, depth, width/depth ratio, cross-sectional shape, branching order, sinuosity and spatial density. Quantitative analysis suggests that Antarctic gully morphology varies with local slope character (i.e. slope geometry, gradient), regional factors (i.e. location of cross-shelf troughs, trough mouth fans, subglacial meltwater production rates, drainage basin size), sediment yield and ice-sheet history. In keeping with interpretations of previous researchers, most gullies are probably formed by hyperpycnal flows of sediment-laden subglacial meltwater released from beneath ice-sheets grounded at the continental shelf edge during glacial maxima. The limited down-slope extent of gullies on the western Antarctic Peninsula is explained by the steep gradient and slope geometry at the mouth of Marguerite Trough, which cause flows to accelerate and entrain seawater more quickly, resulting in a reduction of the negative buoyancy effect of the sediment load. Due to pressure gradients at the ice-sheet bed caused by variations in ice thickness inside and outside palaeo-ice stream troughs, subglacial meltwater flow was generally focussed towards trough margins. This has resulted in gullies with larger cross-sectional areas and higher sinuosities at the trough margins. A unique style of gullying is observed off one part of the western Antarctic Peninsula, corresponding to an area in which the ice-sheet grounding line is not thought to have reached the shelf edge during the Last Glacial Maximum. We interpret the features in this area as the cumulative result of slope processes that operated over a long period of time in the absence of hyperpycnal meltwater flows.

Antarctic palaeo-ice streams

We review the geomorphological, sedimentological and chronological evidence for palaeo-ice streams on the continental shelf of Antarctica and use this information to investigate basal conditions and processes, and to identify factors controlling grounding-line retreat. A comprehensive circum-Antarctic inventory of known palaeo-ice streams, their basal characteristics and minimum ages for their retreat following the Last Glacial Maximum (LGM) is also provided. Antarctic palaeo-ice streams are identified by a set of diagnostic landforms that, nonetheless, display considerable spatial variability due to the influence of substrate, flow velocity and subglacial processes. During the LGM, palaeo-ice streams extended, via bathymetric troughs, to the shelf edge of the Antarctic Peninsula and West Antarctica, and typically, to the mid-outer shelf of East Antarctica. The retreat history of the Antarctic Ice Sheet since the LGM is characterised by considerable asynchroneity, with individual ice streams exhibiting different retreat histories. This variability allows Antarctic palaeo-ice streams to be classified into discrete retreat styles and the controls on grounding-line retreat to be investigated. Such analysis highlights the important impact of internal factors on ice stream dynamics, such as bed characteristics and slope, and drainage basin size. Whilst grounding-line retreat may be triggered, and to some extent paced, by external (atmospheric and oceanic) forcing, the individual characteristics of each ice stream will modulate the precise timing and rate of retreat through time.

Drainage area and incised valley fills in Texas rivers: A potential explanation

The largest rivers draining to the Gulf of Mexico in Texas are at more advanced stages of valley filling than rivers with smaller drainage areas, with corresponding differences in alluvial morphology and architecture, and avulsion style. This seems to imply that the larger fluvial systems either produce more sediment relative to the accommodation space in the alluvial valleys, or that they have undergone longer periods of valley aggradation. However, it is not clear that either of these is the case. This paper presents a theoretical analysis showing that fluvial systems with larger drainage areas can produce more advanced stages of infilling than those with smaller drainage areas (but with identical histories and sediment production per unit area) due to the nature of the scaling relationships between drainage basin area and incised valley size, and between basin area and sediment yield. Data for rivers in the region shows a strong linear relationship between valley width and drainage area. The nonlinear relationship between drainage area and proportion of the incised valley filled is somewhat weaker, but still significant, and major deviations are readily explained in terms of local inherited morphology or tectonic features. Results show that major differences in the degree or stage of valley filling could occur due to differences in drainage basin size alone. This indicates caution in interpreting incised valley fills of drainage basins of different sizes in terms of sediment supply.