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1.
Spatially sampled suspended sediment data from extra-channel surfaces of different ages during rainfall events were used to infer the timescale of landscape adjustment to Little Ice Age (LIA) deglacierization at Small River Glacier, Canada. Surface ages were determined by aerial photograph interpretation. Turbidimeters were installed in three ephemeral channels on “young,” “mature” and “old” surfaces; and their response to rainfall events of different magnitudes recorded. Results showed “young” ice proximal surfaces exposed since 1977 were increasingly vulnerable to mobilization during rainfall events. A suspended sediment response from “old” surfaces exposed since 1910 was recorded for only three events with 5- and 30-year return periods. The intensity and duration of rainfall events therefore had to increase in magnitude to mobilize sediment on surfaces of increasing maturity. Hysteresis in the channel suspended sediment response further confirmed the dependence of surface response on rainfall event magnitude. The rapid temporal decline in surface response indicated that surface armoring or sediment exhaustion is stabilizing surfaces within decades of exposure from the LIA maximum (ca. 1910) at Small River Glacier. However, further perturbation, for example by glacial advance or extreme climatic events, would likely modify the pattern of suspended sediment yields by changing terrain surface mobilization thresholds and reconfiguring channel sediment stores. We therefore suggest that it is difficult to determine when “paraglacial sedimentation” ceases to influence suspended sediment yields, and therefore significantly limiting the usefulness of the term to describe the period of suspended sediment yield adjustment following deglacierization. 相似文献
2.
Predicting landslides for risk analysis — Spatial models tested by a cross-validation technique 总被引：3，自引：3，他引：0
A landslide-hazard map is intended to show the location of future slope instability. Most spatial models of the hazard lack reliability tests of the procedures and predictions for estimating the probabilities of future landslides, thus precluding use of the maps for probabilistic risk analysis. To correct this deficiency we propose a systematic procedure comprising two analytical steps: “relative-hazard mapping” and “empirical probability estimation”. A mathematical model first generates a prediction map by dividing an area into “prediction” classes according to the relative likelihood of occurrence of future landslides, conditional by local geomorphic and topographic characteristics. The second stage estimates empirically the probability of landslide occurrence in each prediction class, by applying a cross-validation technique. Cross-validation, a “blind test” here using non-overlapping spatial or temporal subsets of mapped landslides, evaluates accuracy of the prediction and from the resulting statistics estimates occurrence probabilities of future landslides. This quantitative approach, exemplified by several experiments in an area near Lisbon, Portugal, can accommodate any subsequent analysis of landslide risk. 相似文献
3.
Namibian rangelands are encroached with Acacia mellifera, partially resulting from a poor understanding of vegetation dynamics. A conceptual state-and-transition model of vegetation dynamics in the semiarid Highland savanna in central Namibia, emphasising bush thickening by A. mellifera, is described. Two main states, a grassy and a bushy state, are identified. These are further subdivided, and 11 transitions are identified. The key transition initiating a change from grassy to bushy state can be termed a “leap” (an occasional, infrequent mass recruitment event) following a long “sleep” (no or little change in A. mellifera density). It is rare because it requires three consecutive years of above-average rainfall for seedling establishment. Fire, coinciding with seedling establishment, can interrupt it, while a low biomass grass sward facilitates it. The phenology and physiology of the encroaching species, seed predation and sapling herbivory influence this transition. The model proposes opportunistic management interventions, particularly the use of fire, to minimise the risk of further landscape-scale transitions to a bushy state. It highlights areas where understanding of vegetation dynamics is lacking and recommends crucial research foci. Conceptual models of bush-thickening processes need to account for differences in climate and phenological details of encroaching species. 相似文献
4.
In South Africa, “black” and “coloured” farmers engaged in several occupational activities are often considered as being “not interested in farming” or “not serious”. However, in Namaqualand the large majority of “coloured” farmers and especially the most successful ones are pluriactive and have off-farm incomes. Investigations were therefore carried out in that region to better understand the role of pluriactivity. Observations focus on middle-income households from the mining sector entering into small-scale commercial farming. Based on surveys in both non-farm and farm sectors, the results show that, for many households, access to sufficient farm assets depends strongly on the employment situation of the sector of origin and requires a level of investment which can be reached only progressively. This obliges these households to rely on off-farm incomes and communal land at least during the early transitional stage. These results stress the importance of taking into account the diversity of paths and of differentiating the various stages in the transitional period between two occupational activities. Finally, the paper emphasises the importance of taking these specific situations into consideration in the management of the commons. 相似文献
5.
Heavy metal pollution in urban, industrial, and mined watersheds of Europe is well documented, but less is known about metal contamination in agrarian watersheds or those with no history of mining. Along a 75-km reach of the Lahn River, central Germany, near-channel flood-plain sediments (<5 m from the active channel) have mean concentrations of Cd, Cu, Pb, and Zn that exceed background values. Vertically, metal concentrations are highest at 15 or 20 cm below the flood plain. Although mean metal concentrations in the watershed are below mean values found in more industrial watersheds of western Europe, individual near-channel sites along the Lahn River have metal concentrations approaching those found in more urbanized drainage basins. Several sites along the Lahn are “excessively contaminated” with Cd and “moderately/strongly” contaminated with Cu, Pb, and Zn. Metal concentrations are generally higher and more variable downstream from metal-producing locations and in the vicinity of industrial facilities. Topographic and geomorphic factors appear to have minimal influence on near-channel metal concentrations. The elevated concentrations of metals in geomorphically sensitive channel banks and near-channel sediments raise the possibility of future metal pollution in the Lahn River watershed even as metal emissions to the environment decline. 相似文献
6.
“Stone runs” is the Falklands vernacular term for openwork boulder accumulations, which include extensive blockstreams like the famous Darwin “stone-river” and associated features such as stone stripes. Since the early 20th century, they have been interpreted as the product of a suite of periglacial processes, including frost-wedging, gelifluction, frost heave, frost-sorting and snowmelt runoff. Following a literature review, the results of recent field investigations of the valley-floor blockstreams of East Falkland are presented. Access to the internal structure of these forms provides evidence for the existence of a three-fold profile, with clear vertical size gradation presenting striking similarities with an inverted weathering profile. Micromorphological analyses, SEM, XRD, thin sections and grain-size analyses lead to the hypothesis of an alternative model of stone run formation. It is suggested that the material forming the stone run profile lato sensu (including the superficial pavement) is not of periglacial origin, but derives directly from the stripping and accumulation downslope of a regolith, possibly Tertiary in age and formed under subtropical or temperate conditions. The valley-floor stone runs should, therefore, be considered as complex polygenetic landforms that may have formed according to a six-stage scenario, including in situ chemical weathering, regolith stripping by mass movements, soil formation, further regolith stripping, downslope accumulation and matrix washing-out (all phases possibly achieved by the Early Quaternary). Periglacial reworking of the stone run material would have operated at a “final” stage, i.e. during Quaternary cold stages, with boulder bioweathering and limonite-staining operating during the temperate intervals including the present one. The suggested antiquity of the Falklands blockstreams is in accordance with Caine's pioneer interpretation of Tasmania blockfields and with recent analyses and cosmogenic datings of blockfields from Scandinavia and North America. 相似文献
7.
Katharine Guiles Ellis Henry T. Mullins William P. Patterson 《Journal of Paleolimnology》2004,31(3):343-361
Climate change in the northeastern United States has been inferred for the last deglaciation to middle Holocene (∼16,600 to 6000 calendar years ago) using multi-proxy data (total organic matter, total carbonate content, δ18 O calcite and δ13 C calcite) from a 5 m long sediment core from Seneca Lake, New York. Much of the regional postglacial warming occurred during the well-known Bolling and Allerod warm periods (∼14.5 to 13.0 ka), but climate amelioration in the northeastern United States preceded that in Greenland by ∼2000 years. An Oldest Dryas climate event (∼15.1 to 14.7 ka) is recognized in Seneca Lake as is a brief Older Dryas (∼14.1 ka) cold event. This latter cold event correlates with the regional expansion of glacial Lake Iroquois and global meltwater pulse IA. An increase in winter precipitation and a shorter growing season likely characterized the northeastern United States at this time. The Intra-Allerod Cold Period (∼13.2 ka) is also evident supporting an “Amphi-Atlantic Oscillation” at this time. The well-known Younger Dryas cold interval occurred in the northeastern United States between 12.9 and 11.6 ka, consistent with ice core data from Greenland. In the Seneca Lake record, however, the Younger Dryas appears as an asymmetric event characterized by an abrupt, high-amplitude beginning followed by a more gradual recovery. Compared to European records, the Younger Dryas in the northeastern United States was a relatively low-amplitude event. The largest amplitude and longest duration anomaly in the Seneca Lake record occurs after the Younger Dryas, between ∼11.6 and 10.3 ka. This “post-Younger Dryas climate interval” represents the last deglacial climate event prior to the start of the Holocene in the northeastern United States, but has not been recognized in Greenland or Europe. The early to middle Holocene in the northeastern United States was characterized by low-amplitude climate variability. A general warming trend during the Holocene Hypsithermal peaked at ∼9 ka coincident with maximum summer insolation controlled by orbital parameters. Millennial- to century-scale variability is also evident in the Holocene Seneca Lake record, including the well-known 8.2 ka cold event (as well as events at ∼7.1 and 6.6 ka). Hemispherical cooling during the Holocene Neoglacial in the northeastern United States began ∼5.5 ka in response to decreasing summer insolation. 相似文献
8.
Hydrophytes of the oases in the Sierra de la Giganta of Central Baja California Sur, Mexico: Floristic composition and conservation status 总被引：1，自引：0，他引：1
One of the features of the Sierra de la Giganta mountain range is the existence of many small riparian wetlands (oases) along drainage channels. There are no geohydrological studies from which to determine their dynamics in the arid environment, but these areas confront increasing water extraction to satisfy demands of small local settlements. This study compiled a checklist of the hydrophytes from 12 representative oases, and discusses their current status on the basis of composition, richness, and species characteristics. Floristic composition (absence–presence of 57 hydrophytes) at each location was classified by using UPGMA to represent a dendrogram of group cohesivity. Two major kinds of locations are present. By analysing individual site characteristics, we found that one group occurs in places under “good conservation conditions” and the other composed of locations that are “impacted” in several ways. The former group contains oases that are relatively rich in species, including most of the aquatic forms, which are sensitive to disturbance. Geographical affinity of the floristic list reveals that most species have a broad distribution, mostly in tropical America. 相似文献
9.
The Negro River, which flows through the north central Amazon Basin, is one of the largest tributaries of the Amazon. The name “Negro” comes from the colour of its water, which reflects the large quantity of dissolved humic acids and iron oxides that also gives the water its characteristic acid pH. The river is estimated to have the fifth largest water discharge in the world, about 30,000 m3/s. The Negro River is characterized by a high dissolved load but a low energy system. Neotectonics and water quality are the principal factors that control the modern sedimentation in the Lower Negro River. The Lower Negro River is controlled largely by a NW–SE tectonic lineament, that is a segment of a major tectonic transcurrent dextral megasystem of the Amazon Basin. Neotectonism in this area is responsible for the depth of the river and for the occurrence of steep “falésias” (cliffs), along some parts of its borders. It also seems that neotectonics have influenced the origin of the Anavilhanas Islands, which are a series of anastomosed, elongated silty clayey channel bars, with internal round or long narrow lakes. The “igapó”, which is the forested area flooded during part of the year, appears to have a neotectonic origin as well. Igapós are located on intermediate blocks between the high blocks of the “terra firme” and the low blocks of the channel. The absence of clastic sediments carried in suspension is related to the rare appearance of floodplains, which are limited to very thin layers of fine sediments, located on the abrasion shelfs carved in clastic deposits of the Alter do Chão Formation. Sand bars occur in places along the base of the cliffs and along the edges of the channel system. These sand bars are composed of quartz sand, derived from the reworking of the sand of the Alter do Chão Formation. 相似文献
10.
Landslide inventory in a rugged forested watershed: a comparison between air-photo and field survey data 总被引：2，自引：0，他引：2
Landslide inventories are routinely compiled by means of aerial photo interpretation (API). When examining photo pairs, the forest canopy (notably in old-growth forest) hides a population of “not visible” landslides. In the present study, we attempt to estimate how important is the contribution of landslides not detectable from aerial photographs to the global mass of sediment production from mass failures on forested terrain of the Capilano basin, coastal British Columbia. API was coupled with intensive fieldwork for identification and measurement of all landslides. A 30-year framework was adopted. We show that “not visible” landslides can represent up to 85% of the total number of failures and account for 30% of the volume of debris mobilised. Such percentages display high sub-basin variability with rates of sediment production varying by one order of magnitude between two sub-basins of the study area. This is explained qualitatively by GIS-based analysis of slope frequency distributions, drainage density, and spatial distribution of surficial materials. Such observations find further support in the definitions of transport-limited and supply-limited basins. As a practical consideration to land managers, we envisage that supplementary fieldwork for landslide identification is mandatory in transport-limited systems only. Fieldwork has demonstrated that gully-related failures have a greater importance than one could expect from API. 相似文献
11.
12.
Complexity in a cellular model of river avulsion 总被引：2，自引：1，他引：2
We propose a new model of river avulsion that emphasizes simplicity, self-organization, and unprogrammed behavior rather than detailed simulation. The model runs on a fixed cellular grid and tracks two elevations in each cell, a high elevation representing the channel (levee) top and a low one representing the channel bottom. The channel aggrades in place until a superelevation threshold for avulsion is met. After an avulsion is triggered a new flow path is selected by steepest descent based on the low values of elevation. Flow path depends sensitively on floodplain topography, particularly the presence of former abandoned channels. Several behavioral characteristics emerge consistently from this simple model: (1) a tendency of the active flow to switch among a small number of channel paths, which we term the active channel set, over extended periods, leading to clustering and formation of multistory sand bodies in the resulting deposits; (2) a tendency for avulsed channels to return to their previous paths, so that new channel length tends to be generated in relatively short segments; and (3) avulsion-related sediment storage and release, leading to pulsed sediment output even for constant input. Each of these behaviors is consistent with observations from depositional river systems. A single-valued threshold produces a wide variety of avulsion sizes and styles. Larger “nodal” avulsions are rarer because pre-existing floodplain topography acts to steer flow back to the active channel. Channel stacking pattern is very sensitive to floodplain deposition. This work highlights the need to develop models of floodplain evolution at large time and space scales to complement the improving models of river channel evolution. 相似文献
13.
Climate variability and rapid urbanization have influenced the sand environments in the northern Coachella Valley throughout the late 20th century. This paper addresses changes in the spatial relationships among different sand deposits at northern Coachella Valley between two recent time periods by using satellite data acquired from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER). The approach employed here, involving multispectral thermal infrared (TIR) data and spectral mixture analysis, has shown that the major sand deposits can be spatially modeled at northern Coachella Valley. The “coarse-grained (quartz-rich) sand” deposit is associated with active eolian sand, and the “mixed sandy soil” and “fine-grained (quartz-rich) sand” deposits are associated with inactive eolian sand. The fractional abundance images showed a significant decrease between 2000 and 2006 in the percentage of active sand in the major depositional area for fluvial sediment, the Whitewater River, but also in two downwind areas: the Whitewater and Willow Hole Reserves. The pattern of the active sand appears to be related to variations in annual precipitation (wet and dry years) and river discharge in the northern Coachella Valley. We suggest here that recent human modifications to the major watercourses that supply sand affect the capability of fluvial deposition areas to restore sediments over time and consequently the responses of the sand transport system to climate change, becoming more sensitive to dry years where areas of active sand may shrink, degrade, and/or stabilize faster. The approach utilized in this study can be advantageous for future monitoring of sand in the northern Coachella Valley for management of these and similar environments. 相似文献
14.
Theoretical constraints and chronological evidence of Holocene coastal development in central and southern New South Wales, Australia 总被引：3，自引：0，他引：3
Holocene coastal evolution in New South Wales has been interpreted essentially as the unfolding of the impact of marine transgression. Sea level on this coast supposedly reached its present height at 6–6.5 ka, and varied < 1 m since then. The early Holocene rise of the sea has been considered the key factor (“forcing function”) in dune migration, coastal sand barrier development, and the evolution of estuaries. Episodic storminess during the late Holocene has been seen as an important, though secondary, factor in beach erosion and dune mobilisation. An alternate interpretation presented here challenges the concept of the marine transgression as the primary “forcing function”. It (a) attributes early Holocene dune mobilisation to climate rather than the rising sea; (b) shows that the sea reached its present level by 7 ka and rose to at least + 2 m until 1.5 ka; (c) links late Holocene dune activity to local disruption of vegetation rather than to regional episodic storminess; (d) demonstrates a fall of 2°C in sea surface temperature after 3 ka that coincides with the onset of barrier erosion; (e) recognises the imprint of at least three tsunamis in the coastal sedimentary record. 相似文献
15.
The absence of “rim gravels” north of Grand Canyon and of “Canaan Peak-type” gravels south of Grand Canyon suggests that a paleocanyon, which intersected the transport of these gravels north and south, may have begun forming in the Laramide in approximately the same position as today's central Grand Canyon. This Laramide-age canyon is envisioned as having flowed generally from the SW to NE; from the Peach Springs Canyon area to Mile 197 where it was captured by karst; then along a N. 60°E joint system to the Kanab Point area where it converged with drainage coming off the west side of the Kaibab arch. From there it flowed north along the west flank of the Kaibab arch to Paleogene Lake Claron. The critical idea suggested by this proposed model is that the modern Colorado River utilized Laramide paleotopography in establishing its course through the central Grand Canyon, with younger sections of the canyon integrating with it later, in the middle to late Miocene. This paleocanyon route, in association with headward erosion from the Grand Wash Cliffs toward the Kaibab arch after 16–17 Ma, helps account for the total volume of rock eroded from Grand Canyon, which cannot be explained by present-day incision rates. 相似文献
16.
Jansson and Glasser (Jansson, K.N., Glasser, N.F., 2008. Modification of peripheral mountain ranges by former ice sheets: the Brecon Beacons, southern UK. Geomorphology 97, 178–189.) have recently provided unconventional interpretations of selected glacial erosional and depositional landforms in the Brecon Beacons, UK, based on remotely sensed imagery. These new interpretations contradict well-established and reliable evidence for the origins and ages of certain glacial landforms of this upland area and elsewhere. They suggest that during a post-Last Glacial Maximum (LGM) ice-sheet event ice flowed up supposed, essentially “fluvial” valleys producing “glacial lineations” and depositing marginal moraines at the valley heads and on cirque floors. We argue that their interpretations of some key landforms are incorrect and that they have ignored much of the previous dating and field geomorphological evidence. Sedimentary and morphological evidence (e.g., lack of erratic content; convex planform with respect to the headwall; relatively large height range of moraines; and close association with headwall extent, height, and steepness) all indicate that higher level cirque-floor and valley-head moraines in the Brecon Beacons (> c. 400 m) were formed by cirque glaciers. Available dating evidence indicates a Younger Dryas age. We demonstrate that the supposed “fluvial” valleys, comprising trough heads with steep headwalls, have more nearly parabolic than V-shaped cross profiles indicating substantial glacial modification. Field evidence shows that proposed key exemplar post-LGM glacial lineations are in fact debris flow deposits. We conclude that whilst the adoption of a macroscale approach can shed new light on large-scale, ice-sheet movements, this approach should not be undertaken without consideration of the associated field evidence. 相似文献
17.
Tectonic geomorphology of the San Andreas Fault zone from high resolution topography: An example from the Cholame segment 总被引：7，自引：0，他引：7
High resolution topographic data along fault zones are important aids in the delineation of recently active breaks. A 15 km-long portion of the south-central San Andreas Fault (SAF) along the southern Cholame segment contains well preserved tectonic landforms such as benches, troughs, scarps, and aligned ridges that indicate recurring earthquake slip. Recently acquired LiDAR topographic data along the entire southern SAF (“B4” project) have shot densities of 3–4 m− 2. Computed from the LiDAR returns, Digital Elevation Models (DEMs) of 0.25 to 0.5 m resolution using local binning with inverse distance weighting and 0.8 m or larger search radii depict the tectonic landforms at paleoseismic sites well enough to assess them confidently. Mapping of recently active breaks using a LiDAR-only based approach compares well with aerial photographic and field based methods. The fault zone varies in width from meters to nearly 1 km and is comprised of numerous en echelon meter to kilometer-length overlapping sub parallel fault surfaces bounding differentially moving blocks that elongate parallel to the SAF. The semantic variations of what constitutes “active” and the importance of secondary traces influence the breadth and complexity of the resulting fault trace maps. 相似文献
18.
Terrain Pattern Recognition and Spatial Decision Making for Regional Slope Stability Studies 总被引：1，自引：0，他引：1
A terrain partition scheme is presented that allows the identification of regions with high landslide risk in natural terrain zones on the basis of geomorphometric criteria from moderate resolution DEMs. The key factor being the terrain segmentation to aspect regions (regions formed by points preserving the same aspect direction) instead of using an artificial regular-grid terrain partition scheme. The study area is in western Greece (NW Peloponnesus) whereas a moderate resolution digital elevation model with spacing 75 m is used. Landslide inventory analysis and knowledge conceptualization identified that the landslide susceptibility of a particular aspect region is high, if the mean elevation is low and the mean gradient is high. Each aspect region was parametrically represented on the basis of its mean gradient and elevation. The domain of each parameter was divided to seven slices (classes) on the basis of the observed density. Subsequent knowledge based mapping identified aspect regions with high landslide susceptibility for the following spatial rule: (a) “mean slope in class 6 or 7” and (b) “mean elevation in class 1 to 5”. Alternatively the rule is expressed as mean slope to be equal or greater than 15∘ whereas mean elevation to be in the range 0 to 750 m. These identified zones correspond to regions where historical landslides occurred (populated coastal areas in the North) as well as to south regions (natural terrain zone) where no landslide record is available, because of the limitations posed by the natural terrain landslide mapping program in Greece. The presented terrain segmentation technique combined to the spatial decision-making process, provided both an object framework for integrating geomorphometric parameters and a method for landslide risk analysis in natural terrain zones. 相似文献
19.
The relationship between geomorphological features and water geochemistry was studied for a group of mountainous rivers (from ~ 900 to ~ 2200 m a.s.l.) with similar geology and climate, in the Sierras Pampeanas of Córdoba (Argentina, 31° 30′, 32° 00′S, and 64° 30′, 65° 10′W). A multivariate approach was used to identify three morphological domains that describe the set of sampled rivers, namely “size” dominance, slope dominance, and drainage density dominance. The links between physicochemical and geomorphological variables show that “size” dominance is mostly related to major ions, conductivity, and pH, which tend to increase downstream. Slope dominance is associated with the total concentration of heavy metal (i.e., high heavy metal concentrations are associated with relatively flat areas with slightly acid to circumneutral pH, which promotes desorption). The drainage density dominance results in an association between well-drained catchments and low Cl− concentration (i.e., preserving the chemical signature of atmospheric precipitation). 相似文献
20.
DEM-based morphometry of range-front escarpments in Attica, central Greece, and its relation to fault slip rates 总被引：4，自引：3，他引：4
In this paper, we apply current geological knowledge on faulting processes to digital processing of Digital Elevation Models (DEM) in order to pinpoint locations of active faults. The analysis is based on semiautomatic interpretation of 20- and 60-m DEM and their products (slope, shaded relief). In Northern–Eastern Attica, five normal fault segments were recognized on the 20-m DEM. All faults strike WNW–ESE. The faults are from west to east: Thriassion (THFS), Fili (FIFS), Afidnai (AFFS), Avlon (AVFS), and Pendeli (PEFS) and range in length from 10 to 20 km. All of them show geomorphic evidence for recent activity such as prominent range-front escarpments, V-shaped valleys, triangular facets, and tilted footwall areas. However, escarpment morphometry and footwall geometry reveal systematic differences between the “external” segments (PEFS, THFS, and AVFS) and the “internal” segments (AFFS and FIFS), which may be due to mechanical interaction among segments and/or preexisting topography. In addition, transects across all five escarpments show mean scarp slope angles of 22.1°±0.7° for both carbonate and metamorphic bedrock. The slope angle equation for the external segments shows asymptotic behaviour with increasing height. We make an empirical suggestion that slope angle is a function of the long-term fault slip rate which ranges between 0.13 and 0.3 mm/yr. The identified faults may rupture up to magnitude 6.4–6.6 earthquakes. The analysis of the 60-m DEM shows a difference in fault patterns between Western and Northern Attica, which is related to crustal rheology variations. 相似文献