Development Of Drainage System Research Articles

Spatial Technologies in Deriving the Morphotectonic Characteristics of Tectonically Active Western Tripura Region, Northeast India

Employing integrated remote sensing and GIS technology the western most part of Tripura region (Northeast India) and adjoining Bangladesh region has been investigated in the light of its geomorphological characteristics. Nature of fold ridges, several streams and the respective drainage basins are well depicted in satellite images and digital elevation model providing meaningful information. Quantitative parameters such as stream sinuosity, drainage basin asymmetry, basin elongation ratio have been computed. Main rivers of the study area, namely the Gomti and Khowai follows extremely meandering path and crosses through the transversely faulted anticlinal ridges. Fluvial anomalies viz. shift in stream channel and the abandoned meandering loops have been inferred and mapped. The Haora river in the study area exhibits northward shift in some part. Development of drainage system towards north and south from the drainage divide along the latitude 23°45′N indicated up arching of the region which is also corroborated by the extracted topographic profiles. It has been observed that several tributary streams have gone dry and agricultural fields are developed along the dried up stream. These derived parameters remained useful to understand the nature of topographical modification attributed to the possible tectonic activity.

Miocene drainage reversal of the Amazon River driven by plate–mantle interaction

Northern South America experienced significant changes in drainage patterns during the opening of the South Atlantic. Numerical modelling of the influence of mantle processes on the South American continent indicates that mantle convection was partly responsible for the formation of the Amazon River, the largest river on Earth. Northern South America experienced significant changes in drainage patterns during the opening of the South Atlantic Ocean. Disappearance of a mega-wetland in the western Amazonian basins was followed by the formation of the eastward-draining Amazon River, which has been attributed to Andean uplift1,2,3,4,5. However, South America’s westward motion over cold, dense subducted slabs implies that regional subsidence and uplift east of the Andes may have been driven by mantle convection. Here we use a coupled model of mantle convection and plate kinematics to show that dynamic subsidence of up to 40 m Myr−1 initially formed the Amazonian mega-wetland. In our model, the sustained westward motion of continental South America over subducted slabs resulted in rebound of the Amazonian mega-wetland region at rates of up to 40 m Myr−1 after 30 million years ago, paired with continued subsidence of the eastern Amazonian sedimentary basins at 10–20 m Myr−1. The resulting progressive tilt of northern South America to the east enabled the establishment of the Amazon River, suggesting that mantle convection can profoundly affect the evolution of continental drainage systems.

Genealogy and palaeodrainage basins in Yunnan Province: phylogeography of the Yunnan spiny frog, Nanorana yunnanensis (Dicroglossidae)

Historical drainage patterns adjacent to the Qinghai-Tibetan Plateau differed markedly from those of today. We examined the relationship between drainage history and geographic patterns of genetic variation in the Yunnan spiny frog, Nanorana yunnanensis, using approximately 981 base pairs of mitochondrial DNA partial sequences from protein-coding genes ND1 and ND2, and intervening areas including complete tRNA(Ile), tRNA(Gln) and tRNA(Met). Two null hypotheses were tested: (i) that genetic patterns do not correspond to the development of drainage systems and (ii) that populations had been stable and not experienced population expansion, bottlenecking and selection. Genealogical analyses identified three, major, well-supported maternal lineages, each of which had two sublineages. These divergent lineages were completely concordant with six geographical regions. Genetic structure and divergence were strongly congruent with historical rather than contemporary drainage patterns. Most lineages and sublineages were formed via population fragmentation during the rearrangement of paleodrainage basins in the Early Pliocene and Early Pleistocene. Sympatric lineages occurred only in localities at the boundaries of major drainages, likely reflecting secondary contact of previously allopatric populations. Extensive population expansion probably occurred early in the Middle Pleistocene accompanying dramatic climatic oscillations.

Seasonal variability in the dynamics of marine-terminating outlet glaciers in Greenland

AbstractRecent studies indicate that the dynamics of fast-flowing, marine-terminating outlet glaciers of the Greenland ice sheet may be sensitive to climate and ocean forcing on sub-annual timescales. Observations of seasonal behavior of these glaciers at such high temporal resolution, however, are currently few. Here we present observations of front position, flow speed, near-surface air temperature and ocean conditions for six large marine-terminating glaciers in the Uummannaq region of West Greenland, to investigate controls on short-term glacier dynamics. As proposed by other studies, we find that seasonal front advance and retreat correlates with the formation and disappearance of an ice melange. Our data suggest that high sea-surface temperature, anomalously low sea-ice concentration and reduced melange formation in early 2003 have triggered multi-year retreat of several glaciers in the study area, which is consistent with other regions in Greenland. Of the stable glaciers, only Rink Isbræ exhibits a seasonal speed variation that correlates with variations in front position, with the others undergoing mid-summer deceleration that indicates the effects of subglacial meltwater discharge and drainage system evolution. Drainage of supraglacial lakes and water-filled crevasses results in substantial decreases in speed (40–60%) on fast-flowing glaciers. Our results demonstrate that attempts to model ice-sheet evolution must take into account short-timescale flow dynamics resulting from drainage events and oceanographic conditions.

Mechanisms of englacial conduit formation and their implications for subglacial recharge

Ideas about the character and evolution of englacial drainage systems have been deeply influenced by the theoretical model developed by Shreve [1972. Movement of water in glaciers. Journal of Glaciology 11(62), 205–214]. The Shreve model is based on three main assumptions: (1) englacial drainage is in steady state; (2) englacial water will flow along the steepest hydraulic gradient within the glacier; and (3) pressure head equals the pressure of the surrounding ice minus a small component due to melting of the walls. The Shreve model has been widely adopted as a fundamental component of englacial drainage theory. There is no evidence, however, that the model provides a realistic picture of actual glacial drainage systems.To evaluate Shreve's theory, we used speleological techniques to directly survey englacial conduits. We mapped a total of 8.25km of passage in 27 distinct englacial conduits in temperate, polythermal, cold-based and debris-covered glaciers between 2005 and 2008. New information reported here is supplemented by published data on 40 other englacial conduits located worldwide and surveyed to ice depths of 176m using speleological techniques. In all cases, englacial drainage systems consisted of a single unbranching conduit. Englacial conduit morphologies were found to be intimately linked to the orientation of a glacier's principal stresses or the presence of pre-existing lines of high hydraulic conductivity. If a sufficient supply of water is available, hydrofracturing forms vertical conduits in zones of longitudinal extension and subhorizontal conduits where longitudinal stresses are compressive. On unfractured glacier surfaces, relatively shallow subhorizontal conduits with migrating nickpoints form by cut-and-closure provided channel incision is significantly faster than surface lowering. Conduits can also form along permeable debris-filled crevasse traces that connect supraglacial lake basins of different potential. Our results suggest that Shreve-type englacial drainage systems do not exist and implies that englacial conduits can only penetrate through thick ice to recharge the bed where supraglacial water bodies either intersect, or are advected through, zones of acceleration.

Response of drainage systems to Neogene evolution of the Jura fold-thrust belt and Upper Rhine Graben

The eastern Jura Mountains consist of the Jura fold-thrust belt and the autochthonous Tabular Jura and Vesoul-Montbeliard Plateau. They are drained by the river Rhine, which flows into the North Sea, and the river Doubs, which flows into the Mediterranean. The internal drainage systems of the Jura fold-thrust belt consist of rivers flowing in synclinal valleys that are linked by river segments cutting orthogonally through anticlines. The latter appear to employ parts of the antecedent Jura Nagelfluh drainage system that had developed in response to Late Burdigalian uplift of the Vosges-Back Forest Arch, prior to Late Miocene-Pliocene deformation of the Jura fold-thrust belt. The following stages are recognized in the evolution of the Jura Mountain drainage systems: 1) middle to late Tortonian (10–7.2 Ma) folding-related overpowering and partial reversal of the south-directed Jura Nagelfluh drainage system, 2) Messinian to early Pliocene (7.2–4.2 Ma) Aare-Danube and proto-Doubs stage, 3) early to middle Pliocene (4.2–2.9 Ma) Aare-Doubs stage, 4) late Pliocene to early Quaternary (2.9–1.7 Ma) Aare-Rhine and Doubs stage and 5) Quaternary (1.7–0 Ma) Alpine-Rhine and Doubs stage. Development of the thin-skinned Jura fold-thrust belt controlled the first three stages of this drainage system evolution, whilst the last two stages were essentially governed by the subsidence of the Upper Rhine Graben, which resumed during the late Pliocene. Late Pliocene and Quaternary deep incision of the Aare-Rhine/Alpine-Rhine and its tributaries in the Jura Mountains and Black Forest is mainly attributed to lowering of the erosional base level in the continuously subsiding Upper Rhine Graben. Incision of the Doubs and Dessoubre canyons reflects uplift of the Franches-Montagnes and Franche-Comte in response to thick-skinned deformation of the Jura fold-thrust belt, which had commenced around 3 Ma. Geodetic data indicate that uplift of the Jura Mountains, relative to the Tabular Jura, presently continues at very low strain rates whilst the Upper Rhine Graben subsides very slowly and the Black Forest is relatively stable.

Rapid glacier sliding, reverse ice motion and subglacial water pressure during an autumn rainstorm

AbstractMeasurements of basal water pressure from 15 boreholes located at both local (tens of meters) and regional (kilometers) length scales were used to elucidate the pressure/sliding relationship during an autumn rapid motion event on Bench Glacier, Alaska, USA. The 8 day event had two distinct phases, each with a ten-fold speed-up with respect to winter velocity. The water pressure in all 15 boreholes varied synchronously during the speed-up. The first phase of rapid sliding began after a peak in basal water pressure and continued while the pressure was elevated and stable, or decreasing. The second phase of rapid sliding occurred when the basal water pressure was low but increasing, and terminated before the pressure peaked. Pressure and velocity do not appear unrelated, but the pressure/sliding relationship was not consistently linked to increasing, decreasing or a critical water pressure. The pressure variations and sliding accelerations are a response to a warm rainstorm, although equally large input events occurred in weeks prior with no apparent response. Drainage system evolution therefore appears to play a key role in both the acceleration and the pressure/velocity relationship. Basal cavity dynamics are likely responsible for three episodes of reverse (up-valley) motion observed after enhanced sliding.

Modeling the influence of horizontal advection, deformation, and late uplift on the drainage development in the India‐Asia collision zone

We present results of a new model in which we couple a mechanical model to describe continental indentation in plan view with a landscape evolution model to describe drainage development. We apply the model to investigate aspects of the development of drainage systems, erosion, and exhumation in the India‐Asia collision zone. It is shown that the orogen‐scale distribution of erosion and exhumation can be well matched by the model. However, the elevation of the Tibetan Plateau and the steep topographic gradients at its margins are difficult to reproduce. They appear to require an additional cause of uplift. We show that some of the major observed river capture events can be reproduced by invoking such a late uplift in much of the orogen, except the region of the Sichuan Basin. This includes the inferred capture of the Yarlong‐Tsangpo by the Brahmaputra and that of the Upper Yangtze by the Lower Yangtze (thereby deserting the Red River).

Morphotectonic control on drainage network evolution in the Perachora Peninsula, Greece

In tectonically active areas drainage systems are often influenced by the type, geometry, and recent activity of regional and local faults. In the Perachora peninsula, eastern Gulf of Corinth (Greece), most drainage networks are influenced by neotectonic processes and eustasy. Two major east–west trending fluvial systems (Perachora and Pissia) in the southwestern part of the peninsula are affected by faults of similar orientation. Detailed geomorphological mapping was performed focusing on knickpoints, gorges, planation and depositional surfaces, alluvial fans and talus cones. Longitudinal river profiles and hypsometric curves of their basins were constructed utilizing GIS technology. The two drainage systems exhibit opposite network asymmetries. The Pissia network is well developed south of the main channel while the Perachora extends to the north. The analysis of the longitudinal profiles of the main stream channels denotes that they exhibit different evolutionary trends. The Pissia stream has a “transverse” drainage crossing the active Loutraki fault, while the Perachora stream has a “parallel” evolution as it has the same orientation as the Loutraki fault. The drainage system of Pissia is much older than the Perachora as a large part of the latter was underwater in Late Pleistocene times. The combination of eustacy and vertical tectonic movements has led to the development of four marine terraces corresponding to previous high sea-level stands during Oxygen Isotope Stages (OIS) 5e, 7e, 9c and 11c. Following the analysis of the longitudinal profiles and the hypsometric curves two depositional surfaces were verified at Perachora village. The first surface at 280–360 m and the second one between 120 and 160 m are probably related to marine terraces 11c and 7e respectively. The development of the present drainage systems of Perachora and Pissia in the Late Quaternary depends mostly on the fault tectonism of the two main offshore fault systems of Xylokastro and Loutraki resulting in the uplift of marine terraces to heights of more than 300 m, but also due to the inland faults (Pissia and Alepochori) which created depositional surfaces and knickpoints. Sea-level changes have played a secondary role in the development of the drainage systems.

Example of integrated use of surface and groundwater resources in irrigation in Croatia

Irrigation in Croatia was until recently a neglected measure in food production, especially in continental part of the country. Development of drainage system in the last fifty years was more important due to the problems caused by floods and excess water in the fields. In the last decade the hydrological regime has been changed and drought events became as frequent as flood events, causing even more damage. Future development of agriculture in the northern counties of Croatia depends on the introduction of new, profitable crops which imply irrigation as an essential factor of future social and economic growth. The first step in the implementation of irrigation was the development of National Irrigation Master Plan as a framework for future activities. According to the recommendations of the National Master Plan all counties have created County Irrigation Plans considering local natural conditions, social and economic background. This paper is going to present how is that process of integrated water resources management developing in the continental part of Croatia on the example of Osijek County Irrigation Plan located in the Danube river basin.

Late Cenozoic paleogeographic evolution of northeastern Nevada: Evidence from the sedimentary basins

Field and geochronologic studies of Neogene sedimentary basins in northeastern Nevada document the paleogeographic and geologic evolution of this region and the effects on major mineral deposits. The broad area that includes the four middle Miocene basins studied—Chimney, Ivanhoe, Carlin, and Elko, from west to east—was an upland that underwent prolonged middle Tertiary exposure and moderate erosion. All four basins began to retain sediments at ca. 16 Ma. Eruption of volcanic flows in the Chimney and Ivanhoe basins produced short-lived (ca. 2 Ma), lacustrine-dominated basins before the dams failed and the streams drained to the southwest. In contrast, early, high-angle, normal faulting induced fluvial to lacustrine sedimentation in the Carlin and Elko basins, and volcanic flows further blocked drainage in the Carlin basin until the basin drained at ca. 14.5 Ma. The Elko basin, with continued synsedimentary faulting, retained sediments until ca. 9.8 Ma and then drained west into the Carlin basin. Sediment buildup in all basins progressively buried existing highlands and created a subdued landscape. Relatively minor post-sedimentation extension produced early north-northwest–striking normal faults with variable amounts of offset, and later east-northeast–striking normal faults with up to several kilometers of vertical and left-lateral offset. The earlier faults are more pronounced east of the Tuscarora Mountains, possibly reflecting a hanging-wall influence related to uplift of the Ruby Mountains-East Humboldt core complex on the east side of the Elko basin. The later faults are concentrated along the north-northwest–trending northern Nevada rift west of the Tuscarora Mountains. The area west of the rift contains major tilted horsts and alluvium-filled grabens, and differential extension between this more highly extended region and the less extended area to the east produced the intervening east-northeast–striking faults. The Humboldt River drainage system formed as the four basins became integrated after ca. 9.8 Ma. Flow was into northwestern Nevada, the site of active normal faulting and graben formation. This faulting lowered the base level of the river and induced substantial erosion in upstream regions. Erosion preferentially removed the poorly consolidated Miocene sediments, progressively reexposed the pre-middle Miocene highlands, and transported the sediments to downstream basins. Thus, some ranges in the upstream region are exhumed older highlands rather than newly formed horsts. In addition, the drainage system evolution indicates that northern Nevada has become progressively lower than central Nevada since the middle Miocene. Mineral belts with large Eocene gold deposits are exposed in uplands and concealed beneath Neogene basin units in the study area. Also, numerous epithermal hot-spring deposits formed at and near the paleosurface in the Chimney, Ivanhoe, and Carlin basins as those basins were forming. The Neogene geologic and landscape evolution had variable effects on all of these deposits, including uplift, weathering, supergene enrichment, erosion, and burial, depending on the events at any particular deposit. As such, this study documents the importance of evaluating post-mineralization processes at both regional and local scales when exploring for or evaluating the diverse mineral deposits in this area and other parts of the Basin and Range region.

Observation of rapid drainage system development by thermal erosion of ice wedges on Bylot Island, Canadian Arctic Archipelago

AbstractRapid development of a new drainage system was observed on Bylot Island. A 750‐m long gully system was eroded in four years. The process was initiated by the formation of sinkholes eroded in ice wedges by runoff flowing into open frost cracks. The sinkholes evolved into underground tunnels cut in the ice‐wedge network and the ice‐rich permafrost. Widening of tunnels was followed by subsidence and collapse of their roofs and the development of open gullies. The drainage generally developed as the shortest line along the regional slope with some deviations caused by collapse of blocks of soil which temporarily obstructed the water flow. Retrogressive scarps exposed to flowing water retreated at maximum rates of up to 5 m/day for a total of 15 to 50 m during the summer. Scarps exposed to atmospheric heat and solar radiation retreated between 2.5 and 40 m over four summers with a mean of 15.5 m. Such slopes had nearly stabilised after four years with a retreat rate of only a few centimetres per year in the last year of observation. Copyright © 2007 John Wiley & Sons, Ltd.

Cenozoic uplift of Variscan Massifs in the Alpine foreland: Timing and controlling mechanisms

The European Cenozoic Rift System (ECRIS) and associated fault systems transect all Variscan Massifs in the foreland of the Alps. ECRIS was activated during the Eocene in the foreland of the Pyrenees and Alps in response to the build-up of collision-related intraplate stresses. During Oligocene and Neogene times ECRIS evolved by passive rifting under changing stress fields, reflecting end Oligocene consolidation of the Pyrenees and increasing coupling of the Alpine Orogen with its foreland. ECRIS is presently still active, as evidenced by its seismicity and geodetic data.Uplift of the Massif Central and the Rhenish Massif, commencing at the Oligocene–Miocene transition, is mainly attributed to plume-related thermal thinning of the mantle–lithosphere. Mid-Burdigalian uplift of the SW–NE-striking Vosges–Black Forest Arch, that has the geometry of a doubly plunging anticline breached by the Upper Rhine Graben, involved folding of the lithosphere. Late Burdigalian broad uplift of the northern parts of the Bohemian Massif reflects lithospheric buckling whereas late Miocene–Pliocene uplift of its marginal blocks involved transpressional reactivation of pre-existing crustal discontinuities. Crustal extension across ECRIS, amounting to no more than 7 km, was compensated by a finite clockwise rotation of the Paris Basin block, up warping of the Weald–Artois axis and reactivation of the Armorican shear zones. Intermittent, though progressive uplift of the Armorican Massif, commencing in the Miocene, is attributed to transpressional deformation of the lithosphere.Under the present-day NW-directed compressional stress field, that came into evidence during the early Miocene and further intensified during the Pliocene, the Armorican Massif, the Massif Central, the western parts of the Rhenish Massif and the northern parts of the Bohemian Massif continue to rise at rates of up to 1.75 mm/y whilst the Vosges–Black Forest arch is relatively stable.Uplift of the Variscan Massifs and development of ECRIS exerted strong controls on the Neogene evolution of drainage systems in the Alpine foreland.

Morphology of the Itapeva to Tramandai transgressive dunefield barrier system and mid‐ to late Holocene sea level change

AbstractThe surface morphology of the transgressive dunefield barrier extending from Itapeva to Tramandai along the northern littoral of Rio Grande do Sul, is examined and an attempt is presented to link morphological changes across the barrier to Holocene sea level changes. The 4·5 km wide Holocene barrier displays two typical morphologies: an inner part dominated by large‐scale, continuous alongshore, overlapping dunefield phases comprising sand sheets, dunes, deflation plains and precipitation ridges; and an outer part dominated by discontinuous, medium‐ to small‐scale, triangular to lobate transgressive dunefield phases, cut by both active and relict (palaeo‐) creeks or washouts. Holocene sea level in the region rose to c. +1 to +3 m above present reaching a maximum around 5100 years bp and then progressively fell to the present level. We argue that the effect on barrier development was to suppress the development of a drainage system during the rising and maximum stages, and encourage the development of an organized drainage system in the form of regularly spaced washouts during the falling period, and that this change in sea level from rising to falling therefore produced the large‐scale differences in barrier morphology. Copyright © 2006 John Wiley & Sons, Ltd.

Phylogeographic patterning in a freshwater crab species (Decapoda: Potamonautidae:Potamonautes) reveals the signature of historical climatic oscillations

AbstractAim The phylogeographic relationships among populations of the common Cape River crab,Potamonautes perlatus, are examined to investigate whether the contemporary population genetic structure is congruent with the hypothesized hydrographic evolution of drainage systems established during the Pliocene, or whether it reflects an older Miocene climatic amelioration.Location 139 samples ofP. perlatuswere collected from 31 populations distributed among the five major perennial drainage systems and a number of smaller catchments in the Western and Eastern Cape, South Africa.Methods Phylogeographic analysis using parsimony, maximum likelihood, minimum evolution and Bayesian inferences was employed for the 16S rRNA mtDNA gene region, while bootstrapping and posterior probabilities were used to assess the robustness of clades. In addition, nested clade analysis was performed in an attempt to disentangle the contemporary and historical factors that have sculpted genealogical relationships among conspecific populations ofP. perlatus.Results Phylogenetic topologies were congruent irrespective of the evolutionary method employed. Two highly distinct reciprocally monophyletic clades characterized by marked levels of corrected sequence divergence were present, with no shared haplotypes between the two major phylogroups. Phylogroup one comprises the populations of the westward‐flowing drainages (mainly the Berg and Olifants drainages), and phylogroup two comprises all of the southward‐flowing drainages and can further be divided into two subclades – one containing the Breede River populations, and the other containing the Gamtoos and Gourits drainage systems. The nested clade analysis demonstrated restricted gene flow and long‐distance dispersal for a number of higher clade levels. The higher‐level groups and results for the total cladogram suggest either fragmentation or isolation by distance.Main conclusions Freshwater crabs are generally highly philopatric, and dispersal, although not common, has occurred historically. The westward‐flowing drainages (Berg, Olifants, Eerste, Liesbeeck and Tokai) are isolated from the southward‐flowing drainages by the Cape Fold Mountains, while the southward‐flowing drainages have a number of tributaries that extend into the low‐lying regions, allowing for gene flow between these three major drainages systems (Breede, Gamtoos and Gourits). Among the westward‐flowing drainages, a more intensive sampling regime is required to understand evolutionary relationships. Our molecular results suggest that the observed patterns pre‐date the formation of contemporary hydrographic patterns in the Cape. This suggests that an older Late Miocene event has severely impacted the contemporary population structure in this species, as recent Pliocene hydrographic boundaries do not correspond to the phylogeographic pattern observed. Conservation efforts for aquatic taxa should clearly be directed at the catchments, in an attempt to conserve biological diversity.

Late Miocene–Quaternary volcanism, tectonics and drainage system evolution in the East Carpathians, Romania

Middle Miocene (Sarmatian) convergence created the fold and thrust belt of the Eastern Carpathians of Romania, which subsequently experienced post-collisional crustal deformation combined with calc-alkaline and alkalic-basaltic volcanism in late Miocene–Quaternary time. This deformation led to the rise of the Cǎlimani–Gurghiu–Harghita volcanic mountains and to the subsidence of the N–S-oriented intramontane Borsec/Bilbor–Gheorgheni–Ciuc and Braşov pull-apart basins, and the E-oriented monocline-related Fǎgǎraş basin. The regional drainage network is the composite of: (1) Older E-, SE- and S-flowing rivers, which cross the Carpathians, radiate towards the foreland and were probably established during the Middle Miocene (Sarmatian) collision event. (2) A more recent drainage system related to the contemporaneous development of the volcanoes and intramontaneous basins, which generally drains westward into the Transylvanian Basin since late Miocene time and has been capturing the older river system. The older river drainage system has also been modified by Late Pliocene–Quaternary folding, thrusting and monoclinal tilting along the Pericarpathian orogenic front and by reactivated transverse high angle basement faults, which cross the Eastern Carpathian foreland.

Diurnal variability of subglacial drainage conditions as revealed by tracer experiments

The morphology of the drainage system of Unteraargletscher, Switzerland, and the diurnal variability of drainage conditions were investigated by conducting a series of tracer tests over a number of discharge cycles during the ablation season 2000. Dye injections into a moulin were repeated at intervals of a few hours and were accompanied by simultaneous measurements of discharge of supraglacial meltwater draining into the moulin and bulk runoff in the proglacial stream. Fast transit velocities in conjunction with low dispersion values suggest that the tracer was routed through a hydraulically efficient, channelized drainage system. However, detailed analyses reveal a large diurnal variability in terms of transit velocity and dispersion coefficient. This finding underlines the difficulty of detecting a possible drainage system evolution based on single tracer tests conducted at coarse temporal intervals. Furthermore, the obtained velocity‐discharge relationships display pronounced hysteresis. We suggest that the evolution of the cross‐sectional area of an ice‐walled conduit and the modulation of inflow at the junction of a tributary moulin to a main subglacial channel are responsible for the observed behavior.

The role of hydrologically‐driven ice fracture in drainage system evolution on an Arctic glacier

Observations from Ellesmere Island suggest that the connection between surface and subglacial drainage on a predominantly cold glacier is made abruptly by hydrologically‐driven propagation of fractures from the surface to the bed. Where ice is 150 m thick, water ponded to a depth of 6.9 m within a supraglacial stream system before establishing a permanent bed connection. Multiple premonitory drainage events preceded the final drainage of ponded water, implying that fracturing is necessary, but insufficient, to establish a permanent link between surface and subglacial drainage. Refreezing of water that penetrates the first fractures to form may reseal the connection, while flow resistance within the subglacial system may delay the onset of continuous through‐flow. A large volume of ponded water is required to enlarge fractures sufficiently by melting to maintain continuous drainage, while feedbacks between subglacial hydrology and ice dynamics may assist in maintaining the connection and initiating subglacial outflow.

Impact of an extreme melt event on the runoff and hydrology of a high Arctic glacier

AbstractOn 28–30 July 2000, an extreme melt event was observed at John Evans Glacier (JEG), Ellesmere Island (79° 40′N, 74° 00′W). Hourly melt rates during this event fell in the upper 4% of the distribution of melt rates observed at the site during the period 1996–2000. Synoptic conditions during the event resulted in strong east‐to‐west flow over the northern sector of the Greenland Ice Sheet, with descending flow on the northwest side reaching Ellesmere Island. On JEG, wind speeds during the event averaged 8·1 m s−1 at 1183 m a.s.l., with hourly mean wind speeds peaking at 11·6 m s−1. Air temperatures reached 8°C, and rates of surface lowering measured by an ultrasonic depth gauge averaged 56 mm day−1. Calculations with an energy balance model suggest that increased turbulent fluxes contributed to melt enhancement at all elevations on the glacier, while snow albedo feedback resulted in increased melting due to net radiation at higher elevations. The event was responsible for 30% of total summer melt at 1183 m a.s.l. and 15% at 850 m a.s.l. Conditions similar to those during the event occurred on only 0·1% of days in the period 1948–2000, but 61% of events occurred in the summer months and there was an apparent clustering of events in the 1950s and 1980s. Such events have the potential to impact significantly on runoff, mass balance and drainage system development at high Arctic glaciers, and changes in their incidence could play a role in determining how high Arctic glaciers respond to climate change and variability. Copyright © 2003 John Wiley & Sons, Ltd.

Influence of subglacial drainage system evolution on glacier surface motion: Haut Glacier d'Arolla, Switzerland

The relationship between the evolution of subglacial drainage system morphology and spatial patterns of glacier surface velocity was investigated using dye tracing experiments and ground surveying throughout the 1995 melt season at Haut Glacier d'Arolla, Switzerland. With the onset of high and variable melt season discharges, subglacial drainage changed from a predominantly distributed system to a predominantly channelized system. The change occurred later farther up glacier. During the period of drainage evolution the glacier was subjected to three periods of rapidly rising meltwater discharge. The magnitude and spatial pattern of the glacier's velocity response differed between these periods and can be explained in terms of the impact of the evolving drainage system morphology on the amplitude and spatial distribution of basal hydrological forcing. Increasing discharge through a distributed drainage system caused widespread basal forcing and high glacier velocity. Increasing discharge through incipient channels below moulins, not yet connected to the main channel, caused more localized basal forcing and slightly increased glacier velocity. Increasing discharge through a fully channelized drainage system caused no significant basal forcing and glacier velocity was not significantly different from the annual deformation flow pattern. Empirical orthogonal function analysis of flow patterns defined two distinct spatial modes of surface velocity which corresponded closely with the drainage system morphologies inferred to be present during each event. The relative importance of these modes changed through the melt season, suggesting a temporal change in the spatial pattern of hydrologically induced basal forcing.