Stream Courses Research Articles

Pore Water Pressure Contribution to Debris Flow Mobility

Problem statement: Debris flows are very to extremely rapid flows of saturated granular soils. Two main types of debris flow are generally recognized: Open slope debris flows and channelized debris flows. The former is the results of some form of slope failures, the latter can develop along preexisting stream courses by the mobilization of previously deposited debris blanket. The problem to be addressed is the influence of the mode of initiation on the subsequent mechanism of propagation. In particular the role of pore water pressure on debris flow mobility in both types was debated. Approach: Laboratory flume experiments were set up in order to analyze the behavior of debris flows generated by model sand slope failures. Failures were induced in sand slopes by raising the water level by seepage from a drain located at the top end of the flume, and by rainfall supplied by a set of pierced plastic pipes placed above the flume. Video recordings of the tests were performed to analyze debris flow characteristics. Results: In all the tests the sand water mixture flows were unsteady and non uniform and sand deposition along the channel bed was a relevant phenomenon. The flows were characterized by a behavioral stratification of the sand water mixture along the flow depth. Back analyzed pore water pressure were just in excess to the hydrostatic condition. The reliability of the experimental results was checked by comparison with other flume experiment data. Conclusion: Debris flow behavior was influenced by the mode of initiation, the inclination of the channel and grain size of the soils. These factors affected the attained velocities and the pore water pressure values. The mobility of debris flows was not always enhanced by high excess pore water pressure values.

Suspended sediment transport in a highly erodible catchment: The River Isábena (Southern Pyrenees)

Understanding and quantifying sediment load is important in catchments draining highly erodible materials that eventually contribute to siltation of downstream reservoirs. Within this context, the suspended sediment transport and its temporal dynamics have been studied in the River Isábena (445 km 2, south-central Pyrenees, Ebro basin) by means of direct sampling and turbidity recording during a 3-year dry period. The average flood-suspended sediment concentration was 8 g l − 1 , with maximum instantaneous values above 350 g l − 1 . The high scatter between discharge and suspended sediment concentrations (up to five orders of magnitude) has not permitted the use of rating curve methods to estimate the total load. Interpolation techniques yielded a mean annual sediment load of 184,253 t y − 1 for the study period, with a specific yield of 414 t km − 2 y − 1 . This value resembles those reported for small torrents in nearby mountainous environments and is the result of the high connectivity between the badland source areas and stream courses, a fact that maximises sediment conveyance through the catchment. Floods dominated the sediment transport and yield. However, sediment transport was more constant through time than that observed in Mediterranean counterparts; this can be attributed to the role of base flows that entrain fine sediment temporarily stored in the channel and force the river to carry high sediment concentrations (i.e., generally in the order of 0.5 g l − 1 ), even under minimum flow conditions.

A cut-and-closure origin for englacial conduits in uncrevassed regions of polythermal glaciers

AbstractOn uncrevassed regions of polythermal glaciers, englacial conduits can form by incision of supraglacial stream channels followed by roof closure. The origin and evolution of examples in Longyearbreen, Svalbard, and Khumbu Glacier, Nepal, were determined by speleological survey. The development of perennial incised channels requires that incision is significantly faster than glacier surface ablation, and thus will be favoured by high meltwater discharges in combination with cool climatic conditions or thick debris cover. Incised canyons can become blocked by drifted winter snow, refrozen meltwater, ice rafting from non-local sources (allochthonous breccias) and roof collapses (autochthonous breccias). Conduit closure can also occur in response to ice creep, particularly at depth. Following isolation from the surface, englacial conduits continue to evolve by vadose incision down to local base level. In the case of Longyearbreen, incision allowed the channel to reach the glacier bed, but on Khumbu Glacier deep incision is prevented because an effectively impermeable terminal moraine provides a high base level for the glacier drainage system. During our period of observations, deeper parts of the Longyearbreen conduit became blocked by a combination of ice accumulation and creep, causing the stream course to be re-routed to higher levels. On that glacier, incision, blockage and upward re-routing are cyclic. We conclude that ‘cut and closure’ is the dominant mechanism of englacial conduit formation on uncrevassed regions of polythermal glaciers.

Whalesong and Chants Democratic

Whalesong and Chants Democratic Joyce E. Chaplin (bio) Once upon a time, historians of science regarded the decades that stretched from Isaac Newton to Charles Darwin as a vast historical Sahara, a blank temporal expanse during which nothing much happened. But as it turns out, a lot did happen. As Graham Burnett has beautifully observed, for example, a New York court case over the natural, social, and commercial statuses of whales reveals a lively debate about the living world that took place in an era of supposed inconsequence. Rather than an arid Sahara, we have the teeming seven seas. Out of that salty cradle of life, I want to haul in yet another interesting marine specimen, this one from a slightly earlier period, the late 18th century, in order to clarify what science in early America signified before and after the American Revolution.1 I present to you not a whale, but a whaleman. Timothy Folger was a native of Nantucket, who, like many from that small island, took to the sea for his livelihood, eventually becoming a ship's captain. Posterity might have forgotten him were it not for his famous cousin, Benjamin Franklin, whose mother was from Nantucket. If Franklin's rise to fame and fortune is a matter of public history, it was also a matter of family pride—his relatives rejoiced in their kinsman's achievement in becoming the first scientific American, the first person born in the New World who was regarded as the equal of European men of science, and the first to enjoy cultural visibility and political power because of his scientific achievements. Wherever the eminent Franklin went, his waterborne Nantucket kin followed; they would call on him in Philadelphia, London, and even Paris.2 In 1768, while Franklin was living in London, he enjoyed a well-timed visit from Timothy Folger. The year 1768 marked the first stage of the troubled relations between American colonists and British administrators. Franklin himself felt stung by British criticism, both because he was a spokesman and lobbyist for American interests in London and because of his absenteeism from America, where he was supposed to be serving as Deputy Postmaster General. Serendipitously, as it would turn out, it was at this moment that British officials put a question to him: Why did it take longer to get a postal packet ship to New York than to Boston? Franklin in turn put the question to Timothy Folger, who was evidently amused at this evidence that British packet boat captains were perfect idiots. Little did they know that their travel to New York was delayed because they were navigating against the current of the Gulf Stream, which they avoided in the more northerly passage to Boston, hence the differential in travel time. To this explanation, Folger added an interesting fact: Nantucket mariners knew the Gulf Stream best because they sought it out—along its warm edges, whales went to catch fish, so that was where whalers went to catch whales. Click for larger view View full resolution Plotting the sea: Maury's "preliminary" whale chart of 1851 (mentioned by Melville in a footnote to Moby-Dick). Courtesy of the Library of Congress. Franklin now had a bit of maritime knowledge with which he could champion American know-how and defend himself against criticisms that, by living in London, he neglected his postal duties. He wrote up his cousin's testimony, sent it to his superior at the post office, and also asked Folger to mark on a chart of the Atlantic Ocean the extent and course of the Gulf Stream. Franklin forwarded this now-lost manuscript to the post office so that the chart of the Gulf Stream could be printed and distributed to all those clueless British packet boat captains. This was done and the result was the first map of the Gulf Stream, the joint handiwork of a landsman, Benjamin Franklin, eminent man of science, and his cousin, whom Franklin described as "an intelligent whaleman of Nantucket." Thus had some key knowledge about the fluid, whale-filled seas been captured and displayed on dry land. The charting of the open ocean was in fact just becoming a...

Geomorphology and geoarchaeology of the Paestum area: modifications of the physical environment in historical times

A detailed geoarchaeological study of the southern portion of the Sele river plain (Campania), and in particular of the area of Paestum which hosts the remnants of the famous Greek settlement Poseidonia, has been undertaken. Particular attention was paid to the reconstruction of the physical landscape at the moment when Poseidonia was founded, and to the environmental modifications which affected the vicinity of Paestum in historical times. Collected stratigraphic and archaeological data have allowed to reconstruct important landscape modifications related to the deposition of travertine by the Capodifiume stream in Holocene times. Archaeological data evidence that nearly all human activities were localised on travertine units whose deposition pre-dates the Greek settlement and which are represented, from north to south, by the Gaudo, Arcione, Paestum and Mancone units. Setting aside the Gaudo unit of pre-Holocene age and the Paestum unit which was also affected in Late Holocene times by further travertine deposition, yet to be investigated in detail, at least four distinct travertine units were deposited from the Middle Holocene onwards. These units are represented by the aforementioned Mancone and Arcione units (respectively of Middle Holocene and pre-Archaic age) and by the Spinazzo-Linora and Licinella units of Roman to sub-present age. Their distribution shows that, due to the deviations in the course of the Capodifiume stream, travertine deposition alternatively affected various zones located respectively north and south of the ancient city of Paestum, and the city itself, and significantly controlled man’s activities on the Paestum plain in prehistoric and historical times.

Delineation of paleolakes in the Sinai Peninsula, Egypt, using remote sensing and GIS

Well-stratified lacustrine deposits of clay, silt, and marl occur within the rugged mountainous triangle of igneous and metamorphic rocks of Sinai Peninsula, Egypt. These deposits occur only in two sites along Wadi Feiran channel (Feiran and El-Tarfa Oases), where very dense acidic dykes intersect their stream courses. Such dykes played an important role in damming water from torrential rainfall during the humid period to form lakes. The SRTM (90m) data and high-resolution images (IKONOS) have been utilized to reconstruct the paleolakes in terms of shape, size and water volume. Results show that lake deposits are located where acidic dykes cross narrow channel of high sinuosity. At their former heights, the dykes dammed the surface runoff, thus, three local freshwater paleolakes formed behind them. GIS analysis shows that the largest of these paleolakes was formed in the area of Feiran Oasis. Two other smaller paleolakes were formed at El-Tarfa Oasis due to the presence of two pronounced acidic dykes.These lake deposits were derived mainly from El-Tih Plateau via Wadi El-Akhdar. Another source of these deposits could have been a thin sedimentary cap over the weathered granites northeast of Feiran basin.

Measures of Physical Heterogeneity in Appraisal of Geomorphic River Condition for Urban Streams: Twin Streams Catchment, Auckland, New Zealand

This investigation created and tested a template to rapidly assess geomorphic river condition in urban settings. This extension to the River Styles Framework® (Brierley and Fryirs, 2005) entailed mapping the heterogeneity in bed material, habitat, and flow characteristics for different types of rivers, integrating parameters from geomorphology, ecology, and hydrology. Analysis was carried out at 27 sites in the Twin Streams catchment in West Auckland, New Zealand. The method successfully recorded the extent of degradation of physical structure following European settlement of the catchment. With the exception of one subcatchment, streams were found to be largely intact in the headwaters. Many of these headwater streams were found to be of exceptional quality, with high physical heterogeneity. Geomorphic condition is more degraded in downstream areas. Fine-grained sediment has smothered stream courses in the lower half of the catchment, covering bed material and creating homogenous structure and flow, decreasing quality of habitat for biota. In this more urbanized area, with more stormwater drains, riparian vegetation is limited and of poor to moderate quality. Understanding of geomorphic responses to human disturbance is critical in the design and implementation of effective management strategies that seek to improve the ecological condition of urban streams.

River Recovery in An Urban Catchment: Twin Streams Catchment, Auckland, New Zealand

This study examines the evolution of a small urbanizing catchment in Waitakere City, New Zealand over the period since European settlement. Geomorphic changes are interpreted from the use of historical documents complemented by sediment analysis at eight sites throughout the catchment. Differing forms and rates of geomorphic adjustment are characterized for five different types of stream. Reach sensitivity to change and trajectory of change are related to the capacity for adjustment of differing stream types and their position within the catchment. These relationships are analyzed using the recovery diagram developed by Fryirs and Brierley (2000). Profound disturbance has been experienced, and streams have responded to a series of land use changes over a relatively short time frame (around 160 years). The system responded quickly to forest clearance and subsequent phases of pasture, horticulture, viticulture, and urbanization. Significant geomorphic recovery is under way along most stream courses. While substantial increases in sediment yield are inferred, the fine-grained nature of these deposits has not significantly altered geomorphic features of headwater streams. In mid-catchment, benches have developed along overwidened channels. Prospects for geomorphic recovery are much more limited along lowland reaches, where cumulative impacts have brought about significant changes to river morphology, such that recovery to predisturbance conditions is no longer a realistic prospect over management time frames (50-100 years). The relative geomorphic resilience of this urbanizing catchment is considered to reflect the geologic imprint upon this landscape (a dissected volcano), the reforestation of headwater reaches within a short interval of initial clearance, and the location of urban impacts that are concentrated in the lower part of the river system.

Environmental geochemistry and mineralogy of lead at the old mine area of Baccu Locci (south-east Sardinia, Italy)

About a century of exploitation of the galena-arsenopyrite deposit of Baccu Locci in Sardinia (Italy) has caused a severe, persistent arsenic contamination that extends downstream from the mine for several kilometres. Differently from As, the contamination of lead in surface waters is only localised in the upper part of the mine despite very high Pb concentrations in geologic materials (waste rocks, tailings, stream sediments, soils) over the whole Baccu Locci stream catchment. The aqueous and solid speciation of Pb in various environmental media of the Baccu Locci system was determined by means of a combined analytical (ICP-MS, SEM-EDX, TEM-EDX, chemical extraction) and thermodynamic approach (PHREEQC). The study has pointed out that relatively little Pb (up to 30 µg/L) is initially released to surface waters (pH = 7–8, Eh = 0.4–0.6 V) very rapidly due to dissolution of anglesite that is the first product of galena oxidation. Subsequently, Pb is removed (down to 0.6 µg/L) by probable sorption onto hydrous ferric oxides (e.g. ferrihydrite) and/or possible precipitation of As-containing plumbojarosite that is the main secondary Pb-bearing phase in stream sediments/tailings along the Baccu Locci stream course. The latter hypothesis is controversial since it is reported from the literature that plumbojarosite is formed under acidic conditions, although there is contrary field evidence as well. Bearing in mind the uncertainties introduced from thermodynamic and analytical data, the solubility calculations indicate strong undersaturation of surface waters with respect to plumbojarosite (SI: − 19.9 to − 3.7). On the contrary arsenatian plumbojarosite is at or close to saturation (SI: − 0.6 to 3.2) in most surface waters and beudantite is clearly above saturation (SI: 4.1 to 12.7). This suggests that the incorporation of As might increase the stability of plumbojarosite, extending it up to near-neutral conditions. As a consequence, Pb is prevented from being released downstream to surface waters, and dissolved Pb concentrations remain definitely below the Italian and WHO limits for drinking waters (50 µg/L and 10 µg/L, respectively).

Landscape Effects on Growth of Age‐0 Arctic Grayling in Tundra Streams

AbstractThe growth of age‐0 Arctic grayling Thymallus arcticus in two adjacent watersheds with differing numbers of lakes was compared. Age‐0 Arctic grayling grew faster in the watershed with numerous lakes (eight) than in the watershed with only one lake. The presence of lakes along the stream courses in both watersheds decreased stream temperatures in the spring but increased temperatures in summer. The size of substrates was greater in streams below lakes than in inlet stream reaches. Growth rates of discrete populations of age‐0 Arctic grayling were positively related to stream temperature. No relationship between fish growth rate and the biomass of invertebrate drift was apparent. We conclude that the summer increase in stream temperatures attributable to the presence of lakes resulted in increased growth of Arctic grayling in the watershed with numerous lakes. A simulation model of the temperature‐dependent growth of age‐0 Arctic grayling indicated that the abundance of lakes in one of the watersheds enhanced the potential production of Arctic grayling almost sixfold.

RivWidth: A Software Tool for the Calculation of River Widths From Remotely Sensed Imagery

RivWidth is an implementation in ITT Visual Information Solutions IDL of a new algorithm that automates the calculation of river widths using raster-based classifications of inundation extent derived from remotely sensed imagery. The algorithm utilizes techniques of boundary definition to extract a river centerline, derives a line segment that is orthogonal to this line at each centerline pixel, and then computes the total river width along each orthogonal. The output of RivWidth is comparable in quality to measurements derived using manual techniques; yet, it continuously generates thousands of width values along an entire stream course, even in multichannel river systems. Uncertainty in RivWidth principally depends on the quality of the water classification used as an input, though pixel resolution and the values of input parameters play lesser roles. Source code for RivWidth can be obtained by visiting http://pavelsky.googlepages.com/rivwidth.

Weichselian red tills in the Gardno Phase End Moraine (Dębina Cliff) – criteria for distinction, origin and stratigraphic position, and implications for the origin and course of the Baltic Ice Stream

Abstract. The paper presents the results of investigations of red tills from the South Baltic Middle Sea-Coast recorded in cliff exposures of Gardno Phase End Moraine. Distinctive features of these tills are: high percentage of Aland granitoids and porphyries identified in the 16-32 mm fraction, and large amounts of non-carbonate rocks, mostly red feldspars in relation to quartz, in the sandy fractions of 0,5-0,8 mm and 2-4 mm. Based on structural-textural features, the red till has been classified as flow till of local origin. The ice dynamism during Gardno Phase can be correlated with the Young Baltic Advance and most probably with the Belt Sea advance. On the basis of the petrographic properties of the Gardno red till and the results of radiocarbon dating, a new hypothesis regarding the origin, chronology and rate of flow of the Baltic Ice Stream is developed.

Bacterial Community Composition in Central European Running Waters Examined by Temperature Gradient Gel Electrophoresis and Sequence Analysis of 16S rRNA Genes

The bacterial community composition in small streams and a river in central Germany was examined by temperature gradient gel electrophoresis (TGGE) with PCR products of 16S rRNA gene fragments and sequence analysis. Complex TGGE band patterns suggested high levels of diversity of bacterial species in all habitats of these environments. Cluster analyses demonstrated distinct differences among the communities in stream and spring water, sandy sediments, biofilms on stones, degrading leaves, and soil. The differences between stream water and sediment were more significant than those between sites within the same habitat along the stretch from the stream source to the mouth. TGGE data from an entire stream course suggest that, in the upper reach of the stream, a special suspended bacterial community is already established and changes only slightly downstream. The bacterial communities in water and sediment in an acidic headwater with a pH below 5 were highly similar to each other but deviated distinctly from the communities at the other sites. As ascertained by nucleotide sequence analysis, stream water communities were dominated by Betaproteobacteria (one-third of the total bacteria), whereas sediment communities were composed mainly of Betaproteobacteria and members of the Fibrobacteres/Acidobacteria group (each accounting for about 25% of bacteria). Sequences obtained from bacteria from water samples indicated the presence of typical cosmopolitan freshwater organisms. TGGE bands shared between stream and soil samples, as well as sequences found in bacteria from stream samples that were related to those of soil bacteria, demonstrated the occurrence of some species in both stream and soil habitats. Changes in bacterial community composition were correlated with geographic distance along a stream, but in comparisons of different streams and rivers, community composition was correlated only with environmental conditions.

Movement Patterns of Japanese Fluvial SculpinCottus polluxin a Headwater Stream

AbstractMovements of the Japanese fluvial sculpinCottus polluxwere studied over a 2‐year period using a mark‐recapture technique in two reaches (100 and 300 m) of a small, temperate stream system, the Fujii River, Japan. Of 321 recaptures, 62 fish (19.3%) moved across at least one riffle (i.e., were “mobile”). Fish moved distances of up to 192 m (median = 4 m). Distance moved (m) by all fish correlated positively with the recapture interval (d) in both years, illustrating the importance of the temporal dimension for movements. Mean daily distance moved (m/d) in the high‐flow year was greater than that in the low‐flow year. The high‐flow year, which had a higher frequency of peak flow events, had a higher frequency of mobile fish than did the low‐flow year, suggesting that interannual variation in movements is related to streamflow. Fish size and growth did not differ between mobile fish and residents in the high‐flow year; however, mobile fish were larger and grew more slowly than residents in the low‐flow year. Patterns of movement from multiple recaptures showed evidence for round‐trip movements at channel unit and reach scales, but no fish completed round‐trip movements by crossing a small weir (<0.5 m in height). These results emphasize the importance of maintaining fish passage and pool‐riffle sequences throughout the stream course to prevent isolation of subpopulations by weirs, culverts, and road crossings. Physical barriers to Japanese fluvial sculpin movements could be mitigated by placing a ramp of cobbles downstream of the barriers.

Mass waisting in Song area of the Hawal Basement, NE Nigeria

A phenomenal mass waste incident was reported in Song area of Adamawa State in May 2003.The incident took place at a geomorphologic landmark called the Three Sisters Hills. The area is a metamorphic terrain, with Pan African granitoids occurring as intrusives. Basaltic flow is widespread with exposures along stream courses. Investigation reveals that the mass wasting is caused by a combination of lithological, structural, topographic, climatic and organic factors. Implications of the incident, and mitigation measures to avoid risks of loss of lives or property in the event of future occurrence are presented.Global Journal of Environmental Sciences Vol. 6 (1) 2007: pp. 65-70

Net local removal of floodplain sediment by river meander migration

Erosion from the banks of meandering rivers causes a local influx of sediment to the river channel. Most of the eroded volume is usually replaced in nearby point bars. However, the typical geometry of river bends precludes the local replacement of all eroded material because (i) point bars tend to be built to a lower elevation than cutbanks and (ii) point bars tend to be shorter than the eroding portion of cutbanks because of channel curvature. In a floodplain that is in equilibrium (i.e., neither increasing nor decreasing in volume), sediment eroded from cut banks must be replaced elsewhere on the floodplain. The local imbalance caused by differences in bank height should be balanced primarily by overbank deposition, while the local imbalance caused by curvature should be balanced primarily by deposition in abandoned stream courses or oxbow lakes. Estimates of these local imbalances based on remotely sensed measurements of bank geometry and channel migration have been made on four systems in the United States: a 91-km reach of the Pearl River in Louisiana and Mississippi, a 62-km reach of the Bogue Chitto River in Louisiana, a 35-km reach of the Neuse River in North Carolina, and a 2.7-km reach of the Vermillion River in Minnesota. For these systems, the total local imbalance, integrated over many bends, ranged from 0.36 to 2.27 m 3/yr/m of valley length (0.15 to 1.32 m 3/yr/m of channel length), or 7 to 45% of gross cutbank erosion, with a typical value of about 17%. When compared with gauged suspended sediment data, the measurements provide estimates of the relative importance of floodplains for storing material transported by a river system. The data suggest that even if the studied systems are near long-term mass balance equilibrium (as opposed to undergoing net deposition or erosion), almost all of the sand and in some cases much of the silt and clay in transit through these systems is likely to have spent some time stored in an upstream floodplain.

Diffuse pollution abatement – a key component in the integrated effort towards sustainable urban basins

Water bodies are highly stressed by overdrafts of water for many purposes upstream and in the cities, and effluent domination and excessive point and diffuse pollution downstream. Pollution is also caused by the urban landscape which prefers impervious rather than porous surfaces; fast-conveyance infrastructure rather than "softer" approaches like ponds and vegetation; and stream channelization instead of natural stream courses, buffers and floodplains, and development in the floodplains. In future, the comprehensive and complex problems of urban pollution must be solved within the framework of the total hydrological cycle concept. This provides a new impetus to diffuse pollution management in urban areas. The best management practices that have been developed in the past could become key components of the new urban total hydrological cycle paradigm for solving the water shortage and pollution problems in an integrated manner, and making the urban systems hydrologically and ecologically sustainable. The paradigm will include landscape changes (less imperviousness, more green space used as buffers and groundwater recharge) as well application of the best management practices that provide water conservation, storage and reuse.

Sediment provenance of late Quaternary morainic, fluvial and loess‐like deposits in the southwestern Verkhoyansk Mountains (eastern Siberia) and implications for regional palaeoenvironmental reconstructions

AbstractA provenance analysis of late Quaternary deposits from tributaries of the Aldan and Lena rivers in Central Yakutia (eastern Siberia) was carried out using analysis of heavy minerals and clay mineralogy. Cluster analysis revealed one assemblage that is characterized by relatively high proportions of amphibole, orthopyroxene and garnet as well as pedogenic clay minerals, reflecting a sediment provenance from the wide catchment area of the Lena and Aldan rivers. In contrast, the three other clusters are dominated by stable heavy minerals with varying amounts of clinopyroxene, apatite and garnet, as well as high percentages of illite and chlorite that are indicative of source rocks of the Verkhoyansk Mountains. Glacial moraines reveal the local mountain source signal that is overprinted by the Lena‐Aldan signal in the oldest moraines by reworking processes. Alluvial sediments in the Verkhoyansk Foreland show a clear Lena source signal through intervals of the middle and late Pleistocene, related to a stream course closer to the mountains at that time. Loess‐like cover sediments are characterized by the dominant Lena provenance with increasing proportions of local mountain sources towards the mountain valleys. Aeolian sands in an alluvial terrace section at the mountain margin covering the time between 30 ka and 10 ka BP reflect temporarily dominant inputs of aeolian materials from the Lena Plains. Copyright © 2007 John Wiley & Sons, Ltd.

Distribution of Precambrian iron and gold deposits on the southwestern East European Platform reflected in underlying transcrustal structure and current river systems

The East European Platform is underlain by Archaean and Proterozoic complexes of the East European Craton. In the southwest these are locally exposed in the Ukrainian Shield and the Voronezh Massif on either side of the ca. 2000 km long ESE-striking late Palaeozoic Pripyat–Dniepr–Donets rift. Evaluation with Landsat imagery of 1 : 1,000,000 scale published maps of the Precambrian complexes [Zaritsky, A.I., Galetsky, L.S. (Eds.), 1992. Geology and Metallogeny of the Southwest of the East-European Platform Map Series, 1 : 1,000,000, Ukrainian State Committee on Geology and Utilization of Mineral Resources, Kiev.] is largely obstructed by a cover of post-Palaeozoic sediments and soils of variable thickness. This obstruction is aggravated by an almost continuous patchwork of farmlands. However, analysis of the current drainage patterns in the Dniepr River basin and surrounding regions reveals a spatial coincidence of numerous stream courses and watersheds with previously inferred steep, transcrustal discontinuities of most probably Precambrian age. Transcrustal dislocations constituted important pathways for heat and fluids as is indicated by the distribution of a large proportion of assumed Early Proterozoic hydrothermal iron and gold deposits along them. This distribution is underpinned by the spatial coincidence of mineralization and elongate areas of highly irregular magnetization attributed to uneven distribution of hydrothermal magnetite in banded iron formation. In view of the extent of these dislocations, both vertically and laterally, the generation of hydrothermal fluid flow, emplacement of mantle-sourced magma and associated mineral potential away from banded iron formation complexes is likely. A second group of gold deposits, of Archaean age, is known to occur in association with still recognizable volcanic edifices in greenstone complexes. It is not known if and to what extent such Archaean gold deposits are related to these major transcrustal discontinuities. The kinematics and dynamics of these dislocations and pathways appear largely unknown and deserve high-priority investigation. The geological longevity of the transcrustal dislocation framework till the present day inferred from the current drainage systems is corroborated, however, by repeated regional topographical levelling surveys.

Biological Control of Tropical Weeds: Research Opportunities in Plant–Herbivore Interactions1

INVASIVE EXOTIC SPECIES ARE A MAJOR ECOLOGICAL PROBLEM in many of the world's ecosystems (Sala et al. 2000). With some notable exceptions, however, most tropical moist forests appear to be resistant to the impacts of invasive exotic plants (Rejmainek 1996, Denslow & DeWalt in press). Sala et al. (2000), for example, identified land use change, but not biotic exchange, as the major threat to tropical ecosystems. However, the impacts of invasive exotic plants are a leading threat to native ecosystems on oceanic islands, where naturalized exotic plants have doubled the sizes of floras (Sax et al. 2002). On tropical islands, exotic species invade intact forests, alter successional trajectories, impede restoration of endangered plant communities, and contribute to the degradation of fragmented ecosystems (Denslow & DeWalt in press). These impacts are of special concern because tropical islands are repositories of great biodiversity and are characterized by high levels of endemism and large numbers of threatened and endangered species. Furthermore, continental tropical forests that are fragmented or otherwise disturbed also are vulnerable to the impacts of exotic invaders (e.g., Humphries et al. 1992), and control of invasive exotic plants is a major cost for many land managers. Unfortunately, the options available for management of established invasive species are few and costly and carry risk of unintended consequences. A tool with great promise is the introduction of exotic hostspecific arthropods or pathogens to reduce the abundance, rate of spread, and/or habitat distribution of the target plant in its invasive range. Classical biological control is based on the premise that the population explosions of some exotic species in their introduced ranges are due in part to their escape from their native herbivoresthe ecological release hypothesis (Keane & Crawley 2002, DeWalt etal. 2004, Levine etal. 2004). Historically, the primary objective of weed biocontrol has been the direct reduction in density, cover, and range of target weeds that suppress desirable forage species, are toxic or unpalatable to livestock or that impede waterways. Most of this experience has developed in nonforest ecosystems (Coombs et al. 2004) and at temperate as opposed to tropical latitudes; among weed genera targeted by biocontrol worldwide (except Australia which has both tropical and nontropical ecosystems), 51 percent have been exclusively nontropical and only 25 percent exclusively tropical (Julien & Griffiths 1998). In some cases, this approach has met with dramatic success, e.g., St. John's wort: Hypericumperforatum L. (Harris et al. 1969), leafy spurge: Euphorbia esula L. (Harris 1993), various cactus species: Opuntia spp. (e.g., Moran & Zimmermann 1991), water hyacinth: Eichornia crassipes (Martius) Solms-Laubach (Center et al. 1999b), and giant salvinia: Salvinia molesta D. S. Mitchell (Room et al. 1981). The use of introduced arthopods and pathogens to control exotic wildland weeds is more recent but is showing promise in the control of paperbark: Melaleuca quinquenervia (Cav.) Blake in the Florida Everglades (Center et al. 2000), purple loosestrife: Lythrum salicaria L. in the northern (U.S.) wetlands (Blossey 2002), saltcedar: Tamarix spp. along western (U.S.) stream courses (DeLoach & Carruthers 2004), and several Acacia species in South Africa (Impson et al. 2000). Concern has been expressed over the use of biocontrol agents because of their potential to cause'both direct and indirect nontarget impacts (Simberloff & Stiling 1996; Louda et al. 1997, 2003; McEvoy & Coombs 2000). Nontarget impacts of introduced biocontrol agents for weeds can take the form of direct feeding by the agent on nontarget plant species (Pemberton 2000) or indirect interactions such as competition with native herbivores (Louda et al. 1997) and alteration of the community trophic structure (Henneman & Memmott 2001). Recent reviews provide evidence that the risk of direct nontarget impacts is greatest in closely related species and that the agents released following modern, widely accepted host-range testing protocols and review processes rarely affect nontarget species (Pemberton 2000). However, only a small proportion of agents released are effective at reducing the target species, and our understanding of indirect interactions of introduced biocontrol agents is rudimentary (McEvoy & Coombs 2000). As our appreciation grows of the harm caused by invasive plants in native ecosystems, the use of biological control agents in wildlands becomes increasingly attractive. Extensive use of chemical and mechanical control methods in native ecosystems also can have substantial secondary effects, including impacts on nontarget species and ecosystem processes. In addition to their high host specificity, biocontrol agents can be effective over large areas of poorly accessible terrain. In many tropical countries, where available resources-both financial and human-for the long-term control of invasive species in natural areas are small, cost-effectiveness makes biological control an attractive conservation tool. 1 Received 3 March 2005; revision accepted 17 June 2005. 2 Corresponding author; e-mail: jdenslow@fs.fed.us