Large Waterfalls Research Articles

Expanding Karst Groundwater Tracing Techniques: Incorporating Population Genetic and Isotopic Data to Enhance Flow-Path Characterization

Karst aquifers are unique among groundwater systems because of variable permeability and flow-path organization changes resulting from dissolution processes. Over time, changes in flow-path connectivity complicate interpretations of conduit network evolution in karst hydrogeology. Natural and artificial tracer techniques have long provided critical information for protecting karst aquifers and understanding the potential impacts on ecosystems and human populations. Conventional tracer methods are useful in karst hydrogeologic studies for delineating flow paths and defining recharge, storage, and discharge properties. However, these methods only provide snapshots of the current conditions and do not provide sufficient information to understand the changes in interconnection or larger-scale evolution of flow paths in the aquifer over time. With advances in population genetics, it is possible to assess population connectivity, which may provide greater insights into complex groundwater flow paths. To assess this potential, we combined the more traditional approaches collected in this and associated studies, including artificial (dye) and natural (geochemistry, isotopes, and discharge) tracers, with the population genetic data of a groundwater crustacean to determine whether these data can provide insights into seasonal or longer changes in connections between conduits. The data collected included dye trace, hydrographs, geochemistry, and asellid isopod (Caecidotea bicrenenta) population genetics in Fern Cave, AL, USA, a 25 km-long cave system. Combined, these data show the connections between two separate flow paths during flood events as the downstream populations of isopods belonging to the same subpopulation were measured in both systems. Additionally, the sub-populations found in higher elevations of the cave suggest a highly interconnected unsaturated zone that allows for genetic movement in the vadose zone. Although upstream populations show some similarities in genetics, hydrologic barriers, in the form of large waterfalls, likely separate populations within the same stream.

Do zoo animals use off-exhibit areas to avoid noise? A case study exploring the influence of sound on the behavior, physiology, and space use of two pied tamarins (Saguinus bicolor).

Noise can be a known stressor but our understanding of its effects on animals living in zoo environments remains limited. Although exposure to loud, chronic noise may be expected to negatively impact welfare, providing access to quiet areas to escape loud noise may buffer these negative effects. In this study, we explored the benefits of access to quiet, off-exhibit areas for animals living in a chronically loud sound environment. Two pied tamarins (Saguinus bicolor) living near a large waterfall feature that emitted loud, chronic noise were experimentally exposed to varying sound levels during 2-week treatment conditions. Baseline conditions (waterfall feature on and access to quiet, off-exhibit areas), were followed by a Quiet sound condition (waterfall off), a Loud sound condition (waterfall on and a speaker in the off-exhibit area playing volume-matched white noise), and a final Baseline condition. During Baseline conditions, sound levels were greater than 10 dBA higher in the exhibit than in the off-exhibit area (i.e., roughly twice as loud). The number of visitor groups present during behavior observations were consistent throughout the study. Both tamarins modified their space use during treatment conditions. Under Baseline conditions, both individuals showed frequent usage of quiet, off-exhibit areas. During the Quiet and Loud conditions, where sound levels were generally consistent across spaces, both individuals used off-exhibit areas less and their use of exhibit and off-exhibit areas was not significantly different than would be expected by chance, given the size of the areas. Additional behavioral changes were minimal. No significant differences in fecal glucocorticoid metabolites were observed. Although the monkeys in this study appeared to avoid noise, the overall impact on welfare appeared to be minimal considering the limited behavioral and hormonal changes observed. This study highlights the potential benefits of off-exhibit spaces as a quiet refuge from noise in the zoo environment.

Waterfall low-frequency vibrations and infrasound: implications for avian migration and hazard detection.

Many researchers have suggested that birds may use natural infrasound sources for navigation and hazard avoidance. However, there is a need to define the sound levels and frequencies to characterize potential infrasound sources. This paper summarizes new measurements from Niagara Falls which define a stable, powerful infrasound source that could be detected by birds on a regional scale of over 400km. Measurements made in the vicinity of Niagara Falls show that exceptional infrasonic pressure levels can occur in the regions of large waterfalls (> 100Pa at a range of about 500m). This paper reviews investigator assessments of avian use of infrasound. A review of the results of Cornell researchers on pigeon hearing provides a basis for estimating avian detection ranges of waterfalls. It is possible that migrating birds use sounds from waterfalls as beacons- a component of their "navigation toolbox" as well as infrasound for hazard avoidance.

Trophic niches of native and nonnative fishes along a river-reservoir continuum

Instream barriers can constrain dispersal of nonnative fishes, creating opportunities to test their impact on native communities above and below these barriers. Deposition of sediments in a river inflow to Lake Powell, USA resulted in creation of a large waterfall prohibiting upstream movement of fishes from the reservoir allowing us to evaluate the trophic niche of fishes above and below this barrier. We expected niche overlap among native and nonnative species would increase in local assemblages downstream of the barrier where nonnative fish diversity and abundance were higher. Fishes upstream of the barrier had more distinct isotopic niches and species exhibited a wider range in δ15N relative to downstream. In the reservoir, species were more constrained in δ15N and differed more in δ13C, representing a shorter, wider food web. Differences in energetic pathways and resource availability among habitats likely contributed to differences in isotopic niches. Endangered Razorback Sucker (Xyrauchen texanus) aggregate at some reservoir inflows in the Colorado River basin, and this is where we found the highest niche overlap among species. Whether isotopic niche overlap among adult native and nonnative species has negative consequences is unclear, because data on resource availability and use are lacking; however, these observations do indicate the potential for competition. Still, the impacts of diet overlap among trophic generalists, such as Razorback Sucker, are likely low, particularly in habitats with diverse and abundant food bases such as river-reservoir inflows.

Поиск водопадов Эфиопии по данным дистанционного зондирования

Waterfalls are specific hydrological and hydrobiological objects that often serve as the natural obstacles for spreading of aquatic animal species, resulting as discontinuous range of these species. Waterfalls and rapids create special habitats for riverine aquatic organisms and cause microclimatic changes along the coast. The areas of the largest waterfalls in Ethiopia, high-mountainous Jinbar Waterfall and low-mountainous Blue Nile Falls (Tis Abay,Tis Issat), were considered as model. Up-to-date remote sensing and GIS for processing and storing satellite and field data makes it possible to identify new waterfalls and rapids, to correct and to supply the existing literature and cartographic data. ERS data obtained from the modern satellite Sentinel-2, designed to monitor the state of the environment, as well as data from radar satellite imagery (SRTM) were used mainly. Based on the results of the analysis of cartographic materials and remote sensing data, the localization of a number of large waterfalls and rapids on the rivers of the Ethiopian Highlands was determined and the parameters for the selection of remote sensing data were established. Images with a spatial resolution of 10–15 m/pixel and higher are suitable for detecting significant waterfalls (more than 30-m wide). According to the present study, the identifying the waterfall zones by the methods of GIS analysis of topographic maps at a scale of 1:200000 and larger, as well as from satellite topographic data is possible.

Diversity of EPT (Ephemeroptera, Plecoptera, Trichoptera) Along Streams Fragmented by Waterfalls in the Brazilian Savanna.

In stream ecosystems, waterfalls intercept the fluvial continuum and limit the dispersion of aquatic organisms. However, the barrier effect may be minimized or absent for insects whose life cycle is not restricted to the aquatic environment (i.e., winged adults), such as the orders Ephemeroptera, Plecoptera and Trichoptera (EPT). In this study, we investigated EPT diversity (abundance, richness, and composition) in savanna headwater streams (Cerrado Domain, Brazil) to investigate the hypothesis that large waterfalls have little effect on instream assemblage structure. During 2017 and 2018, we sampled EPTs and measured environmental conditions at reaches adjacent (downstream and upstream) to waterfalls (5 to 70m high). We collected 2532 immature specimens of 33 genera, including the first record of 14 genera of Ephemeroptera, 11 Trichoptera, and three Plecoptera for the Tocantins State. Hydro-geomorphic and streambed conditions varied among sites, but they were not related to the presence of waterfalls. Assemblage structure was similar between downstream and upstream reaches, with no significant difference in taxa richness, abundance, and composition. This pattern was consistently observed considering all data combined, and each order separately. These results support the hypothesis that physical barriers have weak effects on EPT assemblages at small spatial scales, where strong dispersal and habitat similarity probably homogenizes the structure of local assemblages in stream corridors adjacent to large waterfalls. The study provides important information about factors driving the distribution of biodiversity in savanna streams, which are relevant to support conservation and management plans in the Brazilian Cerrado.

Using acoustic perception to water sounds in the planning of urban gardens

Ecoacoustics explains how animal and human behavior are affected by environmental sounds. Many studies show the marked role that sound has on people's psychological health. Our world is more and more scarce in natural sounds that produce our well-being. In this sense, we explore what acoustic parameters are related to human comfort in cities. For this purpose, 16 water sounds, with very different acoustic characteristics in the number of harmonics, fundamental frequencies, spectral information and fractal dimension (=complexity), were sampled. Therefore, any previous psychological conditioning can be expected in the choice of these sounds, since they are all based on the same source. We evaluated responses of 135 students, teachers and administrative people of the University of Extremadura (UEx) in a comfort scale defined by five levels between −2 (very unpleasant) to +2 (very pleasant) being 0 indifferent. Relationships between sound parameters and comfort responses show that information related to harmonics is behind the preferences. We have defined some useful recommendations to use these results in the design of more comfortable and relaxing urban parks and gardens. In this sense, we demonstrated that fountains with large waterfalls or jets, produce a marked acoustic aversion to humans. It is to be expected that the same effect will happen with birds and other small vertebrates present in cities. Instead, we recommend in the design of urban parks and gardens, the adoption of artificial water channels with small jumps whose acoustic characteristics are ideal causing deep and sustained relaxation.

Phylogeny and biogeography of the Brazilian–Guiana Shield endemic Corymbophanes clade of armoured catfishes (Loricariidae)

AbstractNumerous rivers, interrupted by large waterfalls and extensive rapids, drain the geologically ancient Guiana Shield Highlands. We describe a new armoured catfish genus and two new species endemic to the upper Ireng and Kuribrong rivers, respective tributaries of the Amazon and Essequibo basins in western Guiana. Corymbophanes ameliae sp. nov. is distinguished by having vermiculations on the abdomen, bands on the caudal fin, the anal fin i,5 and narrow caudal peduncle. Yaluwak primus gen. & sp. nov. is distinguished by having evertible cheek odontodes, a plated snout, a tall caudal peduncle and absence of adipose fin and iris operculum. We present a new molecular phylogenetic analysis inclusive of these and several related genera that suggests that the Corymbophanes clade (Araichthys, Corymbophanes, Cryptancistrus, Guianancistrus, Hopliancistrus and Yaluwak) originated in the Guiana Shield with secondary dispersal to the Brazilian Shield. Within the Guiana Shield, relationships among Corymbophanes and Yaluwak are consistent with geodispersal between drainages via headwater capture, although an uplift-mediated relictual distribution cannot be ruled out. ND2 haplotype structure among C. ameliae populations suggests that ichthyofaunal diversity on the Guiana Shield escarpment is shaped not only by inter-, but also intrafluvial barriers to gene flow.

Waterfall formation at a desert river–reservoir delta isolates endangered fishes

AbstractUnforeseen interactions of dams and declining water availability have formed new obstacles to recovering endemic and endangered big‐river fishes. During a recent trend of drying climate and declining reservoir water levels in the Southwestern United States, a large waterfall has formed on two separate occasions (1989–1995 and 2001–present) in the transition zone between the San Juan River and Lake Powell reservoir because of deposited sediments. Since recovery plans for two large‐bodied endangered fish species, razorback sucker (Xyrauchen texanus) and Colorado pikeminnow (Ptychocheilus lucius), include annual stockings in the San Juan River, this waterfall potentially blocks upstream movement of individuals that moved downstream from the river into the reservoir. To quantify the temporal variation in abundance of endangered fishes aggregating downstream of the waterfall and determine population demographics, we remotely monitored and sampled in spring 2015, 2016, and 2017 when these fish were thought to move upstream to spawn. Additionally, we used an open population model applied to tagged fish detected in 2017 to estimate population sizes. Colorado pikeminnow were so infrequently encountered (<30 individuals) that population estimates were not performed. Razorback sucker captures from sampling (335), and detections from remote monitoring (943) showed high abundance across all 3 years. The razorback sucker population estimate for 2017 alone was 755 individuals and, relative to recent population estimates ranging from ~2,000 to ~4,000 individuals, suggests that a substantial population exists seasonally downstream of this barrier. Barriers to fish movement in rivers above reservoirs are not unique; thus, the formation of this waterfall exemplifies how water development and hydrology can interact to cause unforeseen changes to a riverscape.

Characteristics of Seismicity in the Areas of Large Water Reservoirs and Waterfalls: The Role of Effects from Additional Load and Permanent Vibration

The characteristics of seismicity in the near vicinity of five large water reservoirs and three large waterfalls from different regions of the Earth are considered. It is found that in some cases induced seismicity manifests itself during the filling of reservoirs at quite large depths: in the lower crust and even in the upper mantle. There is negative correlation between the maximum magnitudes Мmax of the earthquakes recorded near water reservoirs and waterfalls and the water discharge in these objects (V p ). The largest values of Мmax are characteristic of earthquakes that occurred near Sarez Lake (Tajikistan) and the Koyna Reservoir (India), which have the lowest V p ; in contrast, the smallest magnitudes are reported for earthquakes in the areas of the Khone Falls (Laos) and Niagara Falls (United States, Canada), where there are no large artificial water reservoirs, but huge water discharge takes place. The available data indicate that permanent vibration caused by falling water reduces the level of seismicity.

Reduced Gene Flow in Two Common Headwater Fishes in the Ouachita Mountains: A Response to Stream Drying and In-stream Barriers

Stream drying and in-stream barriers can influence migration and recolonization, yet we know little about how these impact population genetics of fishes. We analyzed microsatellite DNA from two common headwater fishes, Longear Sunfish (Lepomis megalotis) and Highland Stoneroller (Campostoma spadiceum), to compare population genetics of fishes in streams with varying degrees of connectivity. We sampled perennial and intermittent streams including examples with and without anthropogenic structures (road crossings). An additional perennial stream with a large waterfall was added to examine potential effects of a natural structure. We collected between 24 and 30 individuals of each species from three equidistant locations along each of five study streams. We found that Longear Sunfish and Highland Stoneroller populations experienced greater reductions in gene flow along the intermittent streams than along the perennial streams. However, the greatest reduction in gene flow between sub-populations generally coi...

Ancient Lake Maxima and Substrate-dependent Riverine Migration Have Defined the Range of the Mudpuppy (<i>Necturus maculosus</i>) in Southern Ontario Following the Wisconsinan Glaciation

The Mudpuppy (Necturus maculosus) is an entirely aquatic salamander whose geographic range is thus defined by immigration routes in watersheds that permit feasible travel. Significant barriers, such as large waterfalls, effectively bar this species from further colonization upstream. We compared the contemporary distribution of Mudpuppies in southern Ontario with varying post-glacial ancient lake maxima and riverine outlet-flow conditions. Topography does not appear to be a range-limiting factor, but the type of river grade (waterfalls versus riffles) does. The distribution of modern records of this species in Ontario aligns closely with maxima from the Nipissing phase occurring 4000–5000 years ago, leading us to suggest that this is when Mudpuppies invaded and proliferated in the Great Lakes Basin.

Identify Potentiality of Establishing an Ecotourism Region by Linking Horton Plains, Babarakanda and Lanka Water Falls

Tourism is one of the biggest service industry with different aspects in the world. Ecotourismis present tendency with the concept of sustainability. It is a purposeful travel to natural areasto understand the culture and natural history of the environment taking care not to alter theintegrity of the ecosystem while producing economic opportunities that make conservation ofnatural resources beneficial to local people.This study conducted to identify potentiality of creating ecotourism region by connectingHorton plains, Babarakanda and Lanka waterfalls. As sub objectives identify resourcesneeded to establish ecotourism region and ways of tourism service can improve in this region.In each district, Badulla and Nuwara Eliya two G.N divisions were selected as study areanamely Kalupahana, Weerakoongama and Loinan, Pattipola respectively. To tackle downabove objectives questionnaire survey was carried out for both villagers and local as well asforeign tourists. Sample size was 50 for villagers than 50 and 20 for local and foreign touristsrespectively. Chi square test and weighted overlay technique in ArcGIS used to analysecollected data. According to Pearson’s chi square test following relationships were identified:development in tourism have a relationship with economic development and regionaldevelopment in the area. As well to enhance opportunities of employment and income waysand also for develop infrastructures in study area with 0.001, 0.036, 0.000, 0.000 and 0.002significance levels respectively under the 95% confidence level. Roadmap, vegetation covermap and tourist site map were used to generate suitable ecotourism regions map. It revealedNorth, West and Southwest of study area were more suitable as ecotourism region because ofmany small waterfalls with large waterfalls such as Babarakanda and Lanka waterfallsbesides its vegetation cover and Horton plain’s biodiversity.77% of tourism attraction siteslocated this region while remaining locations scattered around the lower suitable areas. Studyarea support for tourist activities such as hiking, plant investigation, mountain climbing etc.Interconnectivity between tourist hot spots must develop and same time improvement ofinfrastructures and services must provide in order to have a productive ecotourism region.

A Previously Undocumented Locality of Eastern Hellbenders (Cryptobranchus alleganiensis alleganiensis) in the Elk River, Carter County, TN

Abstract Cryptobranchus alleganiensis alleganiensis (Eastern Hellbender) is a large, imperiled aquatic salamander found in rocky upland streams from New York to Alabama. Although widespread, many Hellbender populations are now highly fragmented by impoundments and degraded habitats. Hellbenders likely require specific stream habitats with relatively low anthropogenic impacts in order to maintain population viability. The Elk River is a small (5th order), high-gradient tributary of the Watauga River drainage that originates in Avery County, NC and flows northwest to Watauga Reservoir in northeastern Tennessee. Although the Elk River's headwaters are heavily impacted by development in the resort towns of Banner Elk and Sugar Mountain; its lower reaches flow through portions of the Pisgah and Cherokee National Forests and over several large waterfalls before reaching the reservoir. Hellbender presence was undocumented in the Elk River prior to 2010. We learned that Hellbenders were likely present in the lowe...

1932, a watershed year in nuclear physics

The consequences, for good and ill, of that annus mirabilis of discovery and invention are still very much with us.

A study of the Reptilian faunal diversity in Kukulugala isolated hill Forest, Ratnapura District in Sri Lanka

The Kukulugala Forest (KF) is situated within the Western boundary of the Ratnapura district, Sabaragamuwa Province; and is located 15 km away from Bulathsinhala town. The study area is located between 60 30’-6031’ Northern latitudes and 800 -800 15’ Eastern longitudes. The forest ecosystem, which covers an area of 6000 acres within the Ayagama secretariat division, can be categorized as a low land evergreen rain forest. The dominant tree species are Dipterocarpus sp., Mesua sp., Doona sp., Schumacheria castaneifolia, Artocarpus nobilis, Calophyllum inophyllum, Mangifera zeylanica, Humboldtia laurifolia, Oncosperma fasciculatum and Canarium zeylanicum species. Kukulugala mountain, also known as “Horanae Kanda” in sinhala, is 705m a.s.l. The area consists of a rich hydrological network which includes two large waterfalls called “Ritigas Ella” and “Miyunu Ella”. Additionally, two large streams that start from this mountain, “Thaberum Ela” and “Era-Hadapana Ela”, are the major tributaries that flow throughout the year. The average annual rainfall is around 3849 mm, with heavy rainfalls occurring from December to May. The weather gradually becomes dry from July to October with highest temperatures recorded during the month of August. The mean annual temperature in KF is 28.70C with maximum of 320C and minimum of 21.30.The KF is well known for its great faunal diversity within the Ratnapura District and is thus considered a very important forest area. It is particularly considered a hot spot for reptiles due to its availability of microhabitats. The present study was carried out between November 2001 and February 2002. Fieldwork was conducted for a total of 20 days (9 hrs / day) over the 4 month period. One hour was spent at each of the 72 randomly selected transects that were located within the three habitat types found in the area. Surveys were conducted both day and night, with flashlights being used at night. The diversity and abundance of reptiles were investigated in KF by using the belt transect (2×100 m) sampling method. During the study period we recorded a total of 708 individuals under 58 species of reptiles, which is about 31.5 % of the total Sri Lankan reptiles described to date. They belonged to 41 (51.2%) genera and 12 (57.1%) families. The endemic Sri Lankan relict genera; Aspidura, Balanophis, Cercaspis, Lyriocephalus, Ceratophora, Lankascincus and Nessia were also encountered in KF. Out of the 58 recorded species, 25 (43.1%) are threatened and 21 (36.2%) are endemic.The cutting of forests and the slash and burn technique of shift cultivations are the main causes of habitat loss of several endemic reptiles. Many human activities such as cutting trees inside the forest contribute to the decline of arboreal reptiles such as, Calotes liolepis, Lyriocephalus scutatus, Ahaetulla pulverulenta, Chrysopelea ornata, Dendrelaphis bifrenalis and Trimeresurus trigonocephalus species. Habitat loss is the major threat to KF Reptile populations. Despite the short term nature of the present study, it is evident that there is a need for further, long term studies of this nature. In addition it is recommended that awareness programmes on managing the forest and its resources are conducted for the local communities which will in turn contribute to the protection of these species.

Entropic Steps: Rocks, Ruins, and Increase in John Ruskin, Robert Smithson, and Per Kirkeby

In Chamounix in 1849, during a brief break in inclement weather, John Ruskin (1819-1900) hurried up to the glacial source of the river Arveron, hoping for a quick draft of scenic beauty before dinner. He was surprised by the dramatic after-effects of a recent avalanche. Ellipsoids and balls of ice - up to the size of travelling trunks - hurtled through space as if overtaking gravity in their wild rush from the edge of the Mer de Glace directly into the bed of a large waterfall and rocks below. When the frozen chunks hit the turbulent stream, it formed a surging mass that thundered on its course, bumping against stream bed and bridge pilings. Ruskin likened the agitated throng to twisting porpoises, and to 'some thick paste of plums, or ill made pineapple ice with quantities of fruit in it'. The sight made him giddy.

Fish community and bioassessment responses to stream network position

If organisms move beyond the boundaries of local sampling units, regional metacommunity dynamics could undermine the ability of bioassessment studies to characterize local environmental quality. We tested the prediction that fish dispersal influences local fish community structure and bioassessment metrics as a function of site position within stream networks. We evaluated fish community data from the US Environmental Protection Agency’s Regional Environmental Monitoring and Assessment Program in West Virginia, USA, to compare the influences of stream network position, ecoregion, basin, and stream size on local fish community composition. We assigned sites to 1 of 3 stream network positions: 1) main channels (MC, n = 12) encompassed streams with upstream catchment areas >200 km2, 2) mainstem tributaries (MT, n = 43) flowed into MC-sized confluences within 15 fluvial km, 3) headwater tributaries (HT, n = 31) lacked such riverine confluences within 15 fluvial km. MT and HT sites had similar upstream catchment sizes and landuse gradients, but species richness was greater in MT sites than HT sites, whereas MT and MC sites were not different in this regard. Three bioassessment metrics were greater in MT sites than HT sites (intolerant species richness, cyprinid species richness, benthic species richness), but a multimetric index of biotic integrity did not differ among stream network positions. Ordinations revealed that fish community composition was organized primarily by zoogeographic basin (Monongahela River basin, New River basin, Ohio River basin), ecoregion (Central Appalachian Plateau, Western Appalachian Plateau, Ridge and Valley), and stream size. Riverine specialists were more abundant in MT than HT sites and were more abundant in basins connected to the Ohio River than in basins isolated from the Ohio River by a large waterfall (New River). Our results suggest that contemporary dispersal among streams influences fish community composition over small spatial scales (101 km), historical dispersal constrained by zoogeographic barriers influences community structure over larger areas (102 km), and contemporary dispersal by fishes influences certain metrics commonly used in bioassessment programs.

Scarp retreat versus pinned drainage divide in the formation of the Drakensberg escarpment, southern Africa

The dramatic escarpment bounding the Drakensberg-Maluti mountainland has long been regarded as a classic example of a topographic feature formed by scarp retreat processes. However, this view has recently been challenged on the basis of apatite fission track (AFT) and cosmogenic isotope data, which are argued to be inconsistent with a uniform rate of scarp retreat from the original position of continental break-up. It was suggested rather, that the evolution and present position of the escarpment was controlled primarily by a pre-existing inland drainage divide. Numeric surface process models have been used to support this interpretation, although these rely on a large number of unconstrained variables, and thus do not provide unique solutions. However, several lines of direct geological field evidence support the scarp retreat model for the evolution of the Drakensberg escarpment. The Drakensberg-Maluti Mountains in southern Africa are formed by a ~1000 m thick sequence of Karoo basalts, capping Karoo sediments. This mountainland has an approximately rectangular shape, surrounded by orthogonal escarpments, except in the southwest, where it is dissected by the deeply incised Orange River. In detail, these marginal scarps are everywhere double topographic features, reflecting two resistant layers that form prominent cliffs: the thick upper basalt flows, and the Clarens Formation sandstone at the base to the lava sequence. Moreover, in the Drakensberg-Maluti example, the presence of inland-facing escarpments reveals that escarpment formation is not exclusively related to processes that occur at or adjacent to the site of continental break-up. These direct geomorphological and geological observations demonstrate that resistant units are the dominant influence in escarpment formation. Headward retreat of large waterfalls on major rivers over considerable distances (10–100 km) provides clear field evidence that scarps formed by resistant lithologies will not invariably degrade as a result of the existence of an inland drainage divide, as has been argued on the basis of surface process modelling. Several complementary factors provide a ready explanation for apparent inconsistencies in the scarp retreat model that were identified from the AFT and cosmogenic isotope studies. The Drakensberg-Maluti mountains are surrounded by a dense network of dolerite dykes that were contemporary with, and supplied, the Karoo basalts. The massive lava flows capping the modern escarpment therefore probably originally extended at least to the edge of the dyke network. The advance inland of an erosion front, initiated by continental break-up, would have slowed dramatically where such massive lava flows were encountered. Tectonic processes, climate changes, and possibly plant evolution may have compounded this decrease in erosion rates. Although these latter factors are not readily quantified, they were probably subordinate to the dominant lithological control that we invoke. Escarpments may thus form and persist over long periods, extending from the time of continental break-up to the present, independently of inland drainage divides. These field observations must be taken into consideration when assigning values to the numerous unconstrained variables used in surface process models.

Translocation and Spread of Piscivorous Fishes in the Burdekin River, North-eastern Australia

The distribution of the biogeographically distinctive fish fauna of the Burdekin River, north-eastern Australia, is largely determined by the presence of a large waterfall located at the lower quarter of the river’s length. Downstream of the falls, assemblages are characterised by the presence of piscivorous fishes whereas such species are largely absent from upstream reaches. Sleepy cod (Oxyeleotris lineolatus), a large piscivorous gudgeon, was first introduced into the upper reaches of the Burdekin River in 1980 and other releases, both official and unofficial, have occurred subsequently. The population remained small and restricted to the site of introduction for a decade, but expanded in size and distribution after the occurrence of a large flood and entry into a prolonged period of drought. This gudgeon is now present in every tributary system of the Burdekin Basin. Despite the occurrence of substantial temporal variation in fish abundance due to a highly variable flow regime, negative impacts on one species, a small gudgeon (Mogurnda adspersa), are evident. Both deliberate and accidental releases of other species into the upper Burdekin River have also occurred, often to satisfy recreational fishing demand. Such species are typified by large size and piscivorous habit, characteristics alien and inimical to the native fish fauna. It is hypothesised that these piscivorous species may have even greater impact than O. lineolatus in some tributary systems of the upper Burdekin River.