Valley Widening Research Articles

Morphometric analysis of Arun River Valley, eastern Nepal

The Arun River shows antecedent characteristics but almost all tributaries show consequent characteristics. The drainage system of the Arun River Valley (ARV) is influenced by local geological structures in number of ways. The sharp bending, sudden high gradient, narrow valleys that suddenly become wide are common. In the ARV, major tributaries are of sixth order, as the order becomes higher, total number of stream segments becomes progressively smaller, but physical dimensions of the streams increase gradually. In some areas, the divides are low and indistinct, while in others they form high mountain systems. Valley deepening is affected by various factors such as- hydraulic action, corrosion/abrasion of the valley base and weathering of the stream bed increased by subsequent removal of weathered materials. Valley deepening is very high between Pisu la and the Barun Dovan; between the Num and Yamchung as well as Leguwaghat and Barachetra. Valley widening process is predominant near Kimathanka, between Tumlingtar and Surtibari and Sattare areas. The Arun River, flowing in the state of channel equilibrium condition partially modified after local recent tectonic events. In the near future there is very low risk of vast channel shifting phenomena in the valley. The paper describes major morphometrical analysis of the Arun River Valley, eastern Nepal.

Numerical modeling of fluvial strath-terrace formation in response to oscillating climate

AbstractMany river systems in western North America retain a fluvial strath-terrace rec ord of discontinuous downcutting into bedrock through the Quaternary. Their importance lies in their use to interpret climatic events in the headwaters and to determine long-term incision rates. Terrace formation has been ascribed to changes in sediment supply and/or water discharge produced by late Quaternary climatic fluctuations. We use a one-dimensional channel- evolution model to explore whether temporal variations in sediment and water discharge can generate terrace sequences. The model includes sediment transport, vertical bedrock erosion limited by alluvial cover, and lateral valley-wall erosion. We set limits on our modeling by using data collected from the terraced Wind River basin. Two types of experiments were performed: constant- period sinusoidal input histories and variable-period inputs scaled by the marine δ18O rec ord. Our simulations indicate that strath-terrace formation requires input variability that produces a changing ratio of vertical to lateral erosion rates. Straths are cut when the channel floor is protected from erosion by sediment and are abandoned—and terraces formed—when incision can resume following sediment-cover thinning. High sediment supply promotes wide valley floors that are abandoned as sediment supply decreases. In contrast, wide valleys are promoted by low effective water discharge and are abandoned as discharge increases. Widening of the valley floors that become terraces occurs over many thousands of years. The transition from valley widening to downcutting and terrace creation occurs in response to subtle input changes affecting local divergence of sediment-transport capacity. Formation of terraces lags by several thousand years the input changes that cause their formation.Our results suggest that use of terrace ages to set limits on the timing of a specific event must be done with the knowledge that the system can take thousands of years to respond to a perturbation. The incision rate calculated in the field from the lowest terrace in these systems will likely be higher than the rate calculated by using older terraces, because the most recent fluvial response in the field is commonly downcutting associated with declining sediment input since the Last Glacial Maximum. This apparent increase in incision rates is observed in many river systems and should not necessarily be interpreted as a response to an increase in rock-uplift rate.

Controls on the Geometry of Incised Valleys in the Basal Quartz Unit (Lower Cretaceous), Western Canada Sedimentary Basin

In southern Alberta, multiple episodes of fluvial erosion occurred during Lower Mannville, Basal Quartz time (Early Cretaceous), creating a series of incised-valley systems. This study examines two of these valley networks, in order to illustrate the geomorphic and genetic complexity of incised-valley systems. Both valley networks are filled by a transgressive succession of fluvial to estuarine deposits. Isopach mapping of these deposits over an area of approximately 10,000 km 2 in southern Alberta shows that both systems are dendritic to rectilinear in character, with the trunk valley flowing from south to north along the length of the Western Canada Sedimentary Basin. These systems were fed from the west, south, and east by a network of tributaries. Eustatic sea-level changes may have played a role in initiating river incision, but regional tectonic movements had an important influence on the history of accommodation change. The periods of erosion appear to have been relatively brief because the valleys are narrow, although confinement within cohesive, mudstone-rich paleosols of the underlying incised-valley deposits may have slowed valley widening. A relatively wet paleoclimate may also have contributed to the formation of narrow valleys by favoring the existence of single-channel rivers. The general morphology of the Alberta foreland basin, together with the distribution of unfilled relief associated with a preceding valley system and/or of less resistant, slightly older deposits, determined the location of the trunk valleys. Uplift of the Bow Island Arch (Sweetgrass Arch) to the east may have been responsible for a shift in the valley headwaters between the formation of the two valley systems. Synerosional block faulting in response to flexural loading had a significant influence on the geometry of the valley network through its control on the location and geometry of major tributary valleys. The trunk valley is also significantly narrower and more deeply incised where it crosses the most pronounced uplifted block. Enhanced erosion at tributary junctions and valley bends produced localized areas (ca. 2-3 km in diameter) where the valley fill is up to five times thicker and considerably coarser grained than the adjacent valley deposits. Such localized scour fills represent prime petroleum-exploration targets.

Quaternary landscape evolution: a framework for understanding contemporary erosion, southeast Spain

AbstractRecent research into the long‐term landscape development of a tectonically active terrain in arid SE Spain has revealed the significance of river capture in understanding current landscape instabilities (badlands and landslides). The river capture was initiated at c.100 ka BP and effected a 90 m base‐level change at the point of capture. This stimulated a wave of incision to propagate through the landscape to 20 km upstream of the capture site. The net effect of the associated increase in erosion has been to change valley shapes from broad and shallow to narrow and deep. The associated unloading and steepening of valley sides has led to a focus of landslide activity in lithologies with more unconfined compressive strength (limestones) and a dominance of gullying, piping and badland development in the lithologies with lower unconfined compressive strengths (marls and sands). Post‐capture rapid valley widening was initially achieved through landslide development. This form of slope degradation was sustained in the more resistant, joint‐controlled lithologies. In weaker lithologies it was superseded by badland development. The elevated sediment fluxes associated with the c.100 ka BP base‐level perturbation will continue into the near future, but are expected to decay, assuming that no additional environmental disturbances occur. The patterns of landscape instability witnessed today are controlled by (1) proximity to the areas affected by the base‐level change and (2) the robustness of the local geology. Understanding of this long‐term temporal context of the landscape provides a valuable spatial and temporal framework for land system management, facilitating the prediction of future natural trends in landscape stability. Copyright © 2002 John Wiley & Sons, Ltd.

Middle–late Holocene river terraces in the Erhjen River Basin, southwestern Taiwan—implications of river response to climate change and active tectonic uplift

We reconstruct the Holocene river history of the Erhjen River (area: 140 km 2) by correlating river terraces aided by 28 radiocarbon dates. Multiple terraces developed in the lower Erhjen River since the middle Holocene; they converge downstream to the Coastal Plain. The rates of channel incision into bedrock calculated from these terraces at Yuehshihchieh are 7–8 mm/year during ca. 5.7–2.5 ka, 5 cm/year during 1.5–1.3 ka, and 1 cm/year since 1 ka; the last is close to the average incision rate since middle Holocene. Meanwhile, only a single but wide middle–late Holocene paleo-floodplain was developed in the upper Erhjen River; it was completely abandoned only after 0.8 ka, likely following an episode of base-level fall starting from the Coastal Plain. Based on the apparent downstream and upstream convergence of these dated terraces, we identify a doming structure or anticline within the basin, which results in a tilt rate of 10 −6 to 10 −7 per year in the lower Erhjen River valley. The major terraces here had different initial long profiles, which implies that a critical (graded) long profile may not be a prerequisite for formation of a wide erosional terrace surface. Instead, we propose that these terraces were initiated by a series of catastrophic rainfall events, probably climatic-related, which brought a large amount of bedload from hillslopes to prevent the channel from incising when valley widening was facilitated by high-discharge runoff. We find that not only terrace-surface formation but also channel incision can be strongly controlled by climatic-driven discharge and bedload conditions, as suggested by the contrast of bedrock incision rates we observe at Yuehshihchieh. Apparently, the climate and its effects on the landscape cannot be regarded as constant during the Holocene even in a humid tropical area like Taiwan. However, such a fluctuation of climate could only be recorded in a setting where rivers have a high tendency to incise so that multiple terraces can be created. The lower Erhjen River that is characterized by active tectonic tilting is an example of this setting.

Landform and Vegetation Change in the Greaves Creek Basin: An asymmetric hanging valley in the Blue Mountains, New South Wales

Greaves Creek has cut a hanging valley through the entire Triassic sandstone sequence near Blackheath in the western Blue Mountains, New South Wales. Downstream of Beauchamp Falls, it cuts into Permian strata in the Grose Gorge. The hanging valley has a valley-in-valley structure. The narrow inner valley is bounded by high cliffs and its floor is cut by a deep narrow slot canyon where stream incision has occurred without valley widening. The course of the creek is related to joint directions. Intense jointing, minor faulting and sapping influence the stability of cliffs but up to 30m of incision has occurred without valley widening in the slot canyon. Topographic asymmetry expressed as unequal slopes of the valley sides is related to differential insolation, erosion, vegetation cover, bioturbation and fire intensity. In the western Blue Mountains and elsewhere in the Sydney Basin asymmetric slopes occur in many other valley-ridge systems, particularly those whose long axes are oriented between about east-west and north-east-south-west. Vegetation structure and floristics within Greaves Creek valley are related to physiography of the valley and to aspect through their effects on fire, moisture availability, light availability, soil depth and temperature.

Geomorphology, Sediment Budget, and Relief Development in Austdalur, Austfirdir, East Iceland

This process-geomorphological study analyzes the present sediment budget in a subarctic oceanic periglacial environment in East Iceland. By a combined investigation of slope denudation and Streamwork in the Austdalur drainage basin (23 km2), located in the mountains of the Icelandic Eastern Fjords (Austfirðir), information on the intensity of the present geomorphological processes, the relative importance of the different processes for slope and valley formation, and recent trends of relief development is presented. In annual mass transfer (t m yr−1), the fluvial sediment transport in the main channels clearly dominates over slope processes. The fluvial transport of solids is more important than the fluvial transport of dissolved salts. The most important slope process with the highest annual mass transfer is aquatic slope denudation (slope and rill wash), followed by geochemical denudation, ground avalanches, rock- and boulder falls, creep, debris slides and flows, and deflation. By the retreat of rock walls and rock ledges and the continued formation of talus cones located below the rock walls and rock ledges, slope processes cause gradual elimination of knickpoints in the slope long profiles and valley widening. The main channels incise into the bedrock of the trough valley floors. There is only a fluvial mass transfer from the slope systems into the main channels in small tributaries. The incision of the tributaries does not keep up with the incision of the main channels. At present the slope systems and the main channel systems are largely decoupled. Postglacial modification of the glacial relief is minor. Because of the short time since the deglaciation (10,000–12,000 yr), the high resistance of the Miocene and Pliocene plateau basalt layers in the area, and the low intensity of the operating periglacial processes, there has been no adjustment of the larger Pleistocene glacial landforms to the geomorphological processes active in the present periglacial morphoclimate.

Geomorphology, Sediment Budget, and Relief Development in Austdalur, Austfirðir, East Iceland

This process-geomorphological study analyzes the present sediment budget in a subarctic oceanic periglacial environment in East Iceland. By a combined investigation of slope denudation and Streamwork in the Austdalur drainage basin (23 km2), located in the mountains of the Icelandic Eastern Fjords (Austfirðir), information on the intensity of the present geomorphological processes, the relative importance of the different processes for slope and valley formation, and recent trends of relief development is presented. In annual mass transfer (t m yr−1), the fluvial sediment transport in the main channels clearly dominates over slope processes. The fluvial transport of solids is more important than the fluvial transport of dissolved salts. The most important slope process with the highest annual mass transfer is aquatic slope denudation (slope and rill wash), followed by geochemical denudation, ground avalanches, rock- and boulder falls, creep, debris slides and flows, and deflation. By the retreat of rock walls and rock ledges and the continued formation of talus cones located below the rock walls and rock ledges, slope processes cause gradual elimination of knickpoints in the slope long profiles and valley widening. The main channels incise into the bedrock of the trough valley floors. There is only a fluvial mass transfer from the slope systems into the main channels in small tributaries. The incision of the tributaries does not keep up with the incision of the main channels. At present the slope systems and the main channel systems are largely decoupled. Postglacial modification of the glacial relief is minor. Because of the short time since the deglaciation (10,000–12,000 yr), the high resistance of the Miocene and Pliocene plateau basalt layers in the area, and the low intensity of the operating periglacial processes, there has been no adjustment of the larger Pleistocene glacial landforms to the geomorphological processes active in the present periglacial morphoclimate.

Middle Pleistocene lacustrine deposits in eastern Essex, England and their palaeogeographical implications

Investigations in quarry exposures in the Asheldham Gravel and related deposits of southeast Essex are described. Section logging, mapping and borehole investigations are supported by clast lithological, heavy and clay mineralogical determinations. The sediments are derived from reworking of local Thames basin materials, fine sediment being predominantly from the London Clay. The sequence is shown to represent an aggradation that began as the fluvial infilling of the River Medway valley. The River Thames, diverted into this valley by glaciation further west, overwhelmed the Medway, reworking the deposits. The valley was subsequently drowned and fine laminated lake sediment was initially deposited. This was during a period when the valley was drowned by the glacial lake ponded in the southern North Sea basin by the Anglian/Elsterian ice sheet. Progradation by a braid-delta complex advanced along the valley and subsequently fluvial deposition returned. Valley widening and straightening accompanied the delta progradation. The deposits were dissected by deep fluvial valleys infilled by Hoxnian interglacial sediments. The Asheldham Gravel is therefore placed in the Anglian/Elsterian Stage.

Middle Pleistocene lacustrine deposits in eastern Essex, England and their palaeogeographical implications

Investigations in quarry exposures in the Asheldham Gravel and related deposits of southeast Essex are described. Section logging, mapping and borehole investigations are supported by clast lithological, heavy and clay mineralogical determinations. The sediments are derived from reworking of local Thames basin materials, fine sediment being predominantly from the London Clay. The sequence is shown to represent an aggradation that began as the fluvial infilling of the River Medway valley. The River Thames, diverted into this valley by glaciation further west, overwhelmed the Medway, reworking the deposits. The valley was subsequently drowned and fine laminated lake sediment was initially deposited. This was during a period when the valley was drowned by the glacial lake ponded in the southern North Sea basin by the Anglian/Elsterian ice sheet. Progradation by a braid-delta complex advanced along the valley and subsequently fluvial deposition returned. Valley widening and straightening accompanied the delta progradation. The deposits were dissected by deep fluvial valleys infilled by Hoxnian interglacial sediments. The Asheldham Gravel is therefore placed in the Anglian/Elsterian Stage.

Quaternary Climate Change and the Formation of River Terraces across Growing Anticlines on the North Flank of the Tien Shan, China

Nested stream terraces, warped upward over actively growing anticlines along the north flank of the Tien Shan in western China, appear to record alternating phases of valley widening and incision. Differences of relative heights between remnants of four separate strath terraces along one river and between two such terraces along another reach 100 to 120 m over the crests of the anticlines. We infer that this spacing is due to alternating stages of valley widening and rapid incision associated with climate changes with a periodicity of 100 kyr. The crests of the anticlines appear to emerge from the aggrading flanks of the anticlines at an average rate of about 1 mm/a. The maximum heights of 25 and 35 ( ± 10) m for the lowest terraces above their projected initial profiles imply ages of roughly 25 kyr and 35 kyr ( ± 10 kyr). Hence, they suggest that flood plains, which were abandoned to form the terraces, developed adjacent to active stream beds during the last glacial period, when climates were relatively cold and dry. We presume that they were incised during deglacial periods when discharges and stream power increased. Apparent durations of exposure, obtained from $$^{10}Be$$ in quartz cobbles lying on the surface of the lower terrace from one anticline, concur with abandonment and deep (~150 m) incision of the flood plain during the last global deglaciation (ca. 20 to 13 kyr B.P.). A minimum carbon-14 date of 33.9 kyr B.P. from deposits on the lowest terrace sequence from the other anticline, however, implies that such abandonment and incision of this flood plain occurred before the most recent global glacial maximum, about 20 kyr B.P. We infer that incision of this second anticline's floodplain began during an earlier deglacial epoch within the last glacial period (between about 70 and 20 kyr, and perhaps near 35 kyr B.P.).

A discussion of hirano and Aniya's (1988, 1989) explanation of glacial‐valley cross profile development

AbstractIn A rational explanation of cross‐profile morphology for glacial valleys and of glacial valley development, Hirano and Aniya (1988) present an explanation for the apparent catenary form of glaciated‐valley cross profiles based on the assumption that glacial erosion works towards minimizing friction between a glacier and its channel. However, as there is as yet no convincing justification for the application of such extremum principles to this problem, it is not clear how useful this type of analysis might be. In more detail, Hirano and Aniya's analysis includes assumptions about patterns of basal water pressures and the controls on friction inconsistent with other work in this area. Furthermore, as Hirano and Aniya (1989) point out, the catenary solution they derive is the maximum rather than the minimum friction solution. The underlying reason for this surprising result is that the variational principle Hirano and Aniya use is inappropriately structured for the problem being addressed, as it does not involve a cross‐section area, i.e. ice discharge, constraint.Hirano and Aniya also discuss the progressive development of glacial‐valley cross profiles, but their analysis is hindered by the fact that most of the data they present are purely morphological, and thus cannot be used to assess form development as a function of either time or extent of glacial erosion. The only possible exceptions are Graf's (1970) data, which give morphological parameters as a function of valley order, but these do not unequivocally support Hirano and Aniya's conclusions, and are used with implicit assumptions about the relationship between valley order and time of glacial occupation or magnitude of glacial erosion that are incorrect or uncorroborated, respectively. However, if we accept the common assumption that glacial erosion results in a progression from initial V‐shaped to broadly U‐shaped (parabolic or catenary) forms, Hirano and Aniya's argument that there are two distinct types of form development (incision in alpine situations versus valley widening under ice sheets) is justified, and is an important observation that needs to be addressed in future models of glacial‐valley development.

The Geomorphic Evolution of the Taylor Black Prairie Between the Trinity and Colorado Rivers: ABSTRACT

The Taylor Black Prairie is an interesting and complex area that has been, heretofore, relatively ignored as a distinct geomorphic unit. Therefore, the purpose of this study is to describe the region as it exists at the present time and to speculate on its overall evolutionary development. Based on descriptive geomorphic variables including geology, landform morphology, soil type and distribution, surface gravure, vegetation type and distribution, and land use, the Taylor Black Prairie may be subdivided into three north-to-south trending geomorphic areas. These are, from west to east, the Lower Taylor Prairie, the Wolfe City scarp, and the Upper Taylor Prairie. There is also present, in the southern region of the study area, a relatively distinct small exposure of high gravels located between Little River and Brushy Creek. Because these gravels are geographically restricted within the study area, and because they are dissimilar petrologically from the Cretaceous strata upon which the north-to-south prairies are developed, they constitute a distinct geomorphic area. The dominant active processes in the Taylor Black Prairie are soil erosion and mass wasting. These are acting under present climatic conditions to shape and modify and topography of this region. The geomorphic evolution of the Taylor Black Prairie is related to the deposition of Tertiary Uvalde Gravel in the central Texas region. These sediments were carried by major rivers from the edge of the southern High Plains eastward through valleys entrenched in Paleozoic and Comanchean rocks. The less-resistant Gulfian and Tertiary rocks allowed valley widening, thus forming braided alluvial streams that deposited humid alluvial fans. End_Page 517------------------------------ Topographic development was further enhanced by changes in climate and fluvial response during glacial and interglacial periods. Glacial periods were characterized by increased rainfall which resulted in downcutting and erosion of the divides by major rivers. Interglacial periods were characterized by a rise in regional base level accompanied by fluvial aggradation. The net effect has been a reversal of topography in which the Uvalde Gravel, which occupied the bottom of river valleys at the time of deposition, now caps the highest divides. Studies of modern erosion rates indicate that man's activities, mainly agricultural, have greatly increased the rate of landscape evolution. End_of_Article - Last_Page 518------------

Geomorphological Dating of Cave Openings at Makapansgat, Sterkfontein, Swartkrans and Taung

Cave opening at several of the South African hominid sites occurred through incision and valley widening in one current erosion cycle. Measurements of rates of cyclic nickpoint migration and valley flank recession make possible time-place transpositions, and the approximate date of cavern opening can be determined for each site. In each case this date represents a terminus post quem for the accumulation of hominid-bearing deposits.

PIEDMONT INTERSTREAM SURFACES OF THE COLORADO SPRINGS REGION, COLORADO

The Colorado Piedmont, adjacent to the Front Ranges, contains broad interstream surfaces which stand above the modern valley bottoms, but in striking contrast to the steep mountain front and the aligned hogbacks of the foothills. The interstream flats occur both in abutting and detached relationship with the mountain front. Often a multiple arrangement is displayed, and several steplike levels are preserved. Merging of adjacent levels is common, particularly upstream. Remnants of higher surfaces, as well as ridges of resistant rock, project above the surfaces. On many, old stream channels converge toward a major canyon in the mountain front. The piedmont surfaces are fanlike terraces floored with coarse fanglomerate deposits. The alluvium overlies bedrock erosion surfaces which truncate the tilted strata of the foothills belt, often extending onto the rocks of the mountain mass. Ordinarily the erosion surfaces disregard variations in rock type. The chief irregularities in the bedrock surfaces are broad, shallow depressions and small, channel-like incisions. Wide, thinly alluviated valleys with relatively flat bedrock floors occur below the piedmont interstream surfaces. The width of these valleys correlates with the resistance of the rock types traversed. Some of the valleys originate in the mountain mass and extend through the piedmont, whereas others, tributary to the mountain-born drainage lines, begin in the piedmont. Most are dry throughout the year, except during occasional summer storms. Valley segments in the mountain area are deep and narrow, except locally where small, thinly alluviated, flat-floored basins are present (Tator, 1949, p. 1774–1783[1][1].) The piedmont valleys contain flights of rock-floored terraces which form steps below the interstream surface levels. The origin of the piedmont levels is a major key to the diastrophic history of the Rocky Mountain front region. This present work includes a brief summation of concepts of origin of similar land forms in other areas, detailed descriptions of piedmont interstream surfaces in a selected portion of the piedmont, a brief analysis of the processes of valley widening by sidewall weathering and rock planation by vertical corrasion now active in the modern valleys (Tator, 1949), an adaption of these processes to development of terrace and piedmont interstream surfaces in the region, an interpretation of the physiographic history of the area studied, and tentative establishment of middle or late middle Pleistocene and Recent as the intervals during which development of the various surfaces took place. [1]: #fn-1

VALLEY WIDENING PROCESSES IN THE COLORADO ROCKIES

Studies of local widening in canyons of the Colorado Front Range near Colorado Springs show detailed evidences of the processes involved. The streams of the region are typically ephemeral, subject to repeated floods from torrential runoff. These conditions are believed to have been uniform for a long period of time. The widening results in small, flat-floored basins usually associated with tributary drainage junctions. The basin floors are thinly alluviated except for numerous bedrock exposures along modern and old channels. The processes involved are:(1) retreat of valley sides by weathering and local corrasion by streams and (2) shifting of channels by plugging the main channel at the basin head and by tributary deposition on the basin floor. Repeated shifting of the channel over a long period and flood scour to an average depth produces a flat bedrock floor. These valley widening processes are believed to be significant in the development of prominent piedmont flats in the region.