River Avulsion Research Articles

Channel enlargement by avulsion-induced sediment starvation in the Saskatchewan River

Research Article| April 01, 2014 Channel enlargement by avulsion-induced sediment starvation in the Saskatchewan River Norman D. Smith; Norman D. Smith 1Department of Earth & Atmospheric Sciences, University of Nebraska, Lincoln, Nebraska 68588-0340, USA Search for other works by this author on: GSW Google Scholar Galina S. Morozova; Galina S. Morozova 2School of Environmental Sciences, University of Guelph, Guelph, Ontario N1G 2W1, Canada Search for other works by this author on: GSW Google Scholar Martin R. Gibling Martin R. Gibling 3Department of Earth Sciences, Dalhousie University, Halifax, Nova Scotia B3H 4R2, Canada Search for other works by this author on: GSW Google Scholar Author and Article Information Norman D. Smith 1Department of Earth & Atmospheric Sciences, University of Nebraska, Lincoln, Nebraska 68588-0340, USA Galina S. Morozova 2School of Environmental Sciences, University of Guelph, Guelph, Ontario N1G 2W1, Canada Martin R. Gibling 3Department of Earth Sciences, Dalhousie University, Halifax, Nova Scotia B3H 4R2, Canada Publisher: Geological Society of America Received: 01 Nov 2013 Revision Received: 27 Jan 2014 Accepted: 29 Jan 2014 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2014 Geological Society of America Geology (2014) 42 (4): 355–358. https://doi.org/10.1130/G35258.1 Article history Received: 01 Nov 2013 Revision Received: 27 Jan 2014 Accepted: 29 Jan 2014 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Norman D. Smith, Galina S. Morozova, Martin R. Gibling; Channel enlargement by avulsion-induced sediment starvation in the Saskatchewan River. Geology 2014;; 42 (4): 355–358. doi: https://doi.org/10.1130/G35258.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Can localized deposition resulting from river avulsion produce downstream sediment-starvation effects similar to those in channels downstream from dams? We explore this question by examining a 130 km reach of the Saskatchewan River (Canada) downstream of an active, 500 km2 avulsion belt initiated in the 1870s. Progradation history and measurements of bedload and channel-bed grain size indicate that little coarse sediment has escaped the avulsion belt since its inception. Comparisons of 17 channel cross sections surveyed in 1911, 1954–1956, and 2011–2012 show that bankfull areas enlarged by deepening and widening in the upper two-thirds of the study reach since 1911. Between 1911 and 1954–1956, average channel area increased by 35% in the upper 45 km, and as much as 44% locally. From 1954–1956 to 2011–2012, additional enlargement occurred in the upper 35 km, averaging 52% since 1911. These increases occurred despite a 24% decrease in mean annual discharge since 1913. These changes are interpreted as effects of sediment sequestration in the upstream avulsion belt. Net erosion by sediment-deficient outflows from areas of avulsive deposition may be an underappreciated process in evolving floodplains. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Is river avulsion style controlled by floodplain morphodynamics?

River relocation, or avulsion, is a fundamental process that fills alluvial basins. However, it is difficult to predict avulsion patterns because the landscape conditions associated with different avulsion styles are currently unknown. Two end-member avulsion styles have been documented in modern rivers: progradational avulsions, during which significant floodplain deposition occurs as a new channel is built, and incisional avulsions, during which floodplain erosion captures flow from a parent channel. Here we propose that avulsion style is related to the tendency for overbank flows to erode or deposit sediment (i.e., floodplain morphodynamics). We present a scaling comparison of floodplain erosion and deposition rates, test it with morphodynamic modeling, and show field data from ancient deposits that are consistent with modeling results. These results demonstrate that floodplain morphodynamics may determine how rivers relocate and that assessing the relative influence of floodplain erosion and deposition may be useful for predicting sedimentation patterns in avulsive systems.

One-dimensional modeling of a recent Ganga avulsion: Assessing the potential effect of tectonic subsidence on a large river

River avulsion as studied in small-sized and medium-sized rivers is partly explained by the water surface gradient advantage of a new channel course over the old course, caused by spatial differences in aggradation and compaction. Recently, the effect of meandering upstream of the avulsion node, or bifurcation, was shown to have an equally large effect on avulsion duration as gradient advantage. These effects remain poorly understood for the largest rivers on Earth, where gradients are very small, subtle gradient advantages are affected by tectonics, and often several anabranches remain active simultaneously. Our objective was to assess the relative importance of these factors in the River Ganga in determining the pacing of an avulsion. We used a combination of historical data, remote sensing, and one-dimensional modeling. The course of the Ganga in historical times was through the present Ganga–Bhagirathi system but then there was either a gradual or sudden shift to the present Ganga–Padma system. Historical evidence and remnant paleochannels, as observed in satellite sensor data, corroborate the changing pattern of the Ganga River system, but the exact causes of the shifting and of the short avulsion duration remain unclear. Based on generalized data, using a one-dimensional model we ran idealized scenarios bracketing different tectonic subsidence estimates for long-term morphodynamic evolution of the upstream channel and the two downstream bifurcates. The model predicts flow and sediment partitioning at the bifurcation node, and includes the effect of migrating meanders at the bifurcation and width adjustment of the bifurcates. Our modeling demonstrates that the old and the new branches can remain ‘open’ and morphologically active for a long time because of the large backwater effect and the high mobility of the sediment. The bifurcation stabilizes at an asymmetrical flow and sediment division, which in smaller rivers (such as the River Rhine) would be followed by residual channel filling but in the much larger Ganges results in morphologically active anabranches. The model results reveal that neither a gradient advantage nor a bend upstream of the bifurcation leads to an avulsion within centuries as has been observed in some large rivers in tectonically inactive regions. On the other hand, a realistic tectonic uplift of the old branch or subsidence of the new branch may force an avulsion to take place quickly, and historical data show that the study area is seismically active. The combination of these factors leads to a realistic modeled avulsion duration of less than three centuries. Historical data indicate that these general conclusions might also apply to other large rivers in this region, e.g. the Brahmaputra and the Teesta. We conclude that large rivers may avulse quickly in response to tectonics but attain an anabranching pattern because of the large dimension of the residual channel and backwater effects.

Wave reworking of abandoned deltas

River deltas and individual delta lobes frequently face reduction of sediment supply, either from the geologic process of river avulsion or, more recently, due to human activities such as river damming. Using a process-based shoreline evolution model, we investigate wave reworking of delta shorelines after fluvial input elimination. Model results suggest that littoral sediment transport can result in four characteristic modes of delta abandonment, ranging from diffusional smoothing of the delta (or delta lobe) to the development of recurved spits. A straightforward analysis of delta shape and wave characteristics provides a framework for predicting the mode of delta abandonment. The observed morphologies of historically abandoned delta lobes, including those of the Nile, Ebro, and Rhone rivers, fit within this framework. Our results provide quantitative insight into the potential evolution of active delta environments in light of future extreme reduction of fluvial sediment input.

Caspian sea-level changes during the last millennium: historical and geological evidence from the south Caspian Sea

Abstract. Historical literature may constitute a valuable source of information to reconstruct sea-level changes. Here, historical documents and geological records have been combined to reconstruct Caspian sea-level (CSL) changes during the last millennium. In addition to a comprehensive literature review, new data from two short sediment cores were obtained from the south-eastern Caspian coast to identify coastal change driven by water-level changes and to compare the results with other geological and historical findings. The overall results indicate a high-stand during the Little Ice Age, up to −21 m (and extra rises due to manmade river avulsion), with a −28 m low-stand during the Medieval Climate Anomaly, while presently the CSL stands at −26.5 m. A comparison of the CSL curve with other lake systems and proxy records suggests that the main sea-level oscillations are essentially paced by solar irradiance. Although the major controller of the long-term CSL changes is driven by climatological factors, the seismicity of the basin creates local changes in base level. These local base-level changes should be considered in any CSL reconstruction.

Mediative adjustment of river dynamics: The role of chute channels in tropical sand-bed meandering rivers

This paper examines processes of chute channel formation in four tropical sand-bed meandering rivers; the Strickland and Ok Tedi in Papua New Guinea, the Beni in Bolivia, and the lower Paraguay on the Paraguay/Argentina border. Empirical planform analyses highlight an association between meander bend widening and chute initiation that is consistent with recent physics-based modelling work. GIS analyses indicate that bend widening may be driven by a variety of mechanisms, including scour and cutbank bench formation at sharply-curving bends, point bar erosion due to cutbank impingement against cohesive terrace material, rapid cutbank erosion at rapidly extending bends, and spontaneous mid-channel bar formation. Chute channel initiation is observed to be predominantly associated with two of these widening mechanisms; i) an imbalance between cutbank erosion and point bar deposition associated with rapid bend extension, and ii) bank erosion forced by spontaneous mid-channel bar development. The work extends previous empirical analyses, which highlighted the role of bend extension (elongation) in driving chute initiation, with the observation that the frequency of chute initiation increases once bend extension rates and/or widening ratios exceed a reach-scale threshold. A temporal pattern of increased chute initiation frequency on the Ok Tedi, in response to channel steepening and mid-channel bar development following the addition of mine tailings, mirrors the inter- and intra-reach spatial patterns of chute initiation frequency on the Paraguay, Strickland and Beni Rivers, where increased stream power and sediment load are associated with increased bend extension and chute initiation rates. The process of chute formation is shown to be rate-dependent, and the threshold values of bend extension and widening ratio for chute initiation are shown to scale with measures of river energy, reminiscent of slope–ratio thresholds in river avulsion. Furthermore, Delft3D simulations suggest that chute formation can exert negative feedback on shear stress and bank erosion in the adjacent mainstem bifurcate, such that the process of chute formation may also be rate-limiting. Chute formation is activated iteratively in space and time in response to changes in river energy (and sediment load), predominantly affecting sites of rapid channel elongation, and thereby mediating the river response.

Precession‐scale cyclicity in the fluvial lower Eocene Willwood Formation of the Bighorn Basin, Wyoming (USA)

AbstractLittle is known about controls on river avulsion at geological time scales longer than 104 years, primarily because it is difficult to link observed changes in alluvial architecture to well‐defined allogenic mechanisms and to disentangle allogenic from autogenic processes. Recognition of Milankovitch‐sale orbital forcing in alluvial stratigraphy would provide unprecedented age control in terrestrial deposits, and also exploit models of allogenic forcing enabling more rigorous testing of allocyclic and autocyclic controls. The Willwood Formation of the Bighorn Basin is a lower Eocene fluvial unit distinctive for its thick sequence of laterally extensive lithological cycles on a scale of 4 to 10 m. Intervals of red palaeosols that formed on overbank mudstones are related to periods of relative channel stability when gradients between channel belts and floodplains were low. The intervening drab, heterolithic intervals with weak palaeosol development are attributed to episodes of channel avulsion that occurred when channels became super‐elevated above the floodplain. In the Deer Creek Amphitheater section in the McCullough Peaks area, these overbank and avulsion deposits alternate with a dominant cycle thickness of ca 7·1 m. Using integrated stratigraphic age constraints, this cyclicity has an estimated period of ca 21·6 kyr, which is in the range of the period of precession climate cycles in the early Eocene. Previous analyses of three older and younger sections in the Bighorn Basin showed a similar 7 to 8 m spacing of red palaeosol clusters with an estimated duration close to the precession period. Intervals of floodplain stability alternating with episodes of large‐scale reorganization of the fluvial system could be entirely autogenic; however, the remarkable regularity and the match in time scales documented here indicate that these alternations were probably paced by allogenic, astronomically forced climate change.

Development of spit–lagoon complexes in response to Little Ice Age rapid sea-level changes in the central Guilan coast, South Caspian Sea, Iran

The central Guilan coast along the Iranian Caspian coastline is characterized by sandy beaches and the development of spit–lagoon complexes, which are prone to preserve past sea-level fluctuations. The morphology of three spit–lagoon complexes along the central Guilan coast was studied using ground penetrating radar (GPR) and sediment sequences to understand the effects of past sea-level changes on spit–lagoon development. The results showed the prominent role of coastal setting in conditioning the development of spit–lagoon formation in response to sea-level change. When the Caspian Sea experienced a highstand in the Little Ice Age, the coast of central Guilan recorded fluctuations in sedimentation which are reflected, for example, by river avulsion and beach ridge formation depending on physical setting. In the western half of the central Guilan, eastward longshore currents and strong wave action on a W–E coastline coupled with sea-level changes shaped the Anzali spit–lagoon complex; while in the eastern part of the studied area river avulsion and changing the coastline orientation are responsible for development of the Amirkola and Kiashahr spit–lagoon complexes under the same sea-level fluctuations. Although sea-level change has a major role in spit–lagoon development, an increase in the frequency of storms, changes in sediment supply due to more precipitation, and river avulsion are other players in spit–lagoon development in the central Guilan during the Little Ice Age and more recent times.

Avulsion processes at the terminus of low-gradient semi-arid fluvial systems: Lessons from the Río Colorado, Altiplano endorheic basin, Bolivia

The Río Colorado dryland river system in the southeast of the endorheic Altiplano Basin (Bolivia) terminates on a very flat coastal plain at the edge of the Salar de Uyuni, the world's largest salt pan with an area of ca. 12,500km2. Since the Pleistocene the basin has experienced several lake expansion and contraction cycles in response to wetter and drier climate periods, respectively. At present the basin is in a dry climate period which results in a lake level lowstand and progradation of fluvial systems such as the Río Colorado onto the former lake bottom. The present field study of the terminus of the Río Colorado shows that the river experiences a gradual downstream decrease of bankfull width and depth. This bankfull decrease is caused by the combined effects of: (1) extremely low gradient of the lake bottom and, hence, loss of flow energy, and (2) downstream transmission losses due to high evaporation potential and river water percolation through the channel floor. Peak water discharge in seasonal, short-duration rain periods causes massive overbank flooding and floodplain inundation. On satellite images the morphology of the river terminus has a divergent pattern and resembles a network of coeval sinuous distributary channels. However, field observations show that only one channel is active at low flow stage, and at high-flow stage an abandoned, partially infilled channel may be active as well. The active channel at its termination splits into narrow and shallow anastomosing streams before its demise on the lacustrine coastal plain. The rest of the channels which form the divergent network are older sediment-filled abandoned sinuous river courses with multiple random avulsion points. These channel deposits, together with extensive amalgamated crevasse-splay deposits, form an intricate network of fluvial sand deposits. Successive stages of progressively deeper crevasse-channel incision into the floodplain are the result of waning-stage return flow of floodwaters towards the main channel. This results in increased crevasse-channel width, depth and length, and redeposition of eroded lacustrine coastal-plain sediment at the junction of the crevasse channel with the present-day river. A sedimentary model is proposed in which deep river bank incision by return flow in crevasse-channels creates the preferential locus for river avulsion. In a Lowstand Systems Tract with little vertical accommodation increase, the recurring avulsed-river paths result in a thin but laterally extensive network of amalgamated channel-fill, point-bar, crevasse-channel and crevasse-splay deposits. The resulting sedimentary architecture could easily be mistaken as produced by a distributary system of simultaneously active fluvial channels.

Compartimentação geomorfológica e processos deposicionais no megaleque fluvial do rio São Lourenço, Pantanal mato-grossense

The Pantanal wetland is an extensive depositional tract characterized by the presence of fluvial megafans, from which that of the Taquari River is the most extensive and well known. Located on the northern border of the Pantanal, the São Lourenço megafan is a large distributary fluvial system with an area of 16,000 km², but poorly known regarding its geomorphological and geological features. The Sao Lourenço and the Taquari are coalescent megafans, having the Piquiri interfan meander river in between them. Based on the interpretation of satellite images and field validation, it was possible to identify channel patterns, to interpret depositional and erosional processes, to recognize the phenomena of river avulsion and to map depositional lobes. Three geomorphological zones were recognized on the São Lourenço megafan: 1) abandoned depositional lobes located in the upper/intermediate fan, composed of Pleistocene fluvial deposits and exhibiting distributary paleochannels on their surfaces, which are currently being dissected by tributary channels; 2) an active Holocene confined meander belt formed by fluvial aggradation in a N65E incised-valley; 3) active depositional lobes placed on the distal part of the system, which are the main site of sedimentation and are characterized by frequent avulsion events, channel bifurcation, distributary drainage pattern and the presence of depositional lobated landforms. The active depositional lobes were formed due to an important event of river avulsion that caused the lower portion of the meander belt to be abandoned.

Fluvial archive of peculiar avulsive fluvial patterns in the largest Quaternary intracratonic basin of tropical South America: The Bananal Basin, Central-Brazil

The Bananal Basin is the most important intracratonic Quaternary sedimentary basin of South America and preserves a good record of the paleohydrological conditions of Central Brazil at the Cerrado–Amazon ecotone. This area is currently a large tropical plain, seasonally flooded by rainfall and water table saturation that can be classified as a seasonal wetland.This paper presents a geological and geomorphological analysis supported with geochronological dating aiming to reconstruct the Pleistocene environmental history of the Bananal Basin. The basin was filled by the sediments sourced by the Araguaia–Mortes fluvial system. The deposits belong to the Araguaia Formation which covers ~106,000km2 of the Bananal Basin. TL and OSL chronologies of the Araguaia Formation show that sedimentation started at least in the Middle Pleistocene (MIS 7) but aggradation and avulsion processes were important in the Lower–Middle Pleniglacial between 70.5ka and 34.0±4.6ka (MIS 4 and 3) and in the Upper Pleniglacial between 24.5±3.1 and 17.2±2.3ka (MIS 2). River avulsions were more intense in the lower Bananal Basin (Bananal Island region). Remarkably, we did not record ages between 17ka and 10ka. Although avulsion seems to have been more active in the past, the abandonment by the Araguaia of a channel belt, now occupied by the underfit Javaés River, indicates that avulsion is still active. The avulsion processes are mainly related to palaeohydrological changes, although present-day seismic activity is also recorded.Additional data is needed to generate a more detailed scenario for the older Middle Pleistocene deposits. The data is remarkable, however, because it is the first Middle Pleistocene record described in a large alluvial tropical system in Brazil. The fluvial record is temporally in agreement with records from other large Brazilian fluvial systems such as the Amazon and Paraná basins. The Bananal plain, however, is unique because of its particular style of sedimentation and geomorphologic processes that generated a mega-scale anabranching-like complex pattern of paleochannels presently occupied by underfit streams.

Reorganization of a deeply incised drainage: role of deformation, sedimentation and groundwater flow

ABSTRACTDeeply incised drainage networks are thought to be robust and not easily modified, and are commonly used as passive markers of horizontal strain. Yet, reorganizations (rearrangements) appear in the geologic record. We provide field evidence of the reorganization of a Miocene drainage network in response to strike–slip and vertical displacements in Guatemala. The drainage was deeply incised into a 50‐km‐wide orogen located along the North America–Caribbean plate boundary. It rearranged twice, first during the Late Miocene in response to transpressional uplift along the Polochic fault, and again in the Quaternary in response to transtensional uplift along secondary faults. The pattern of reorganization resembles that produced by the tectonic defeat of rivers that cross growing tectonic structures. Compilation of remote sensing data, field mapping, sediment provenance study, grain‐size analysis and Ar40/Ar39 dating from paleovalleys and their fill reveals that the classic mechanisms of river diversion, such as river avulsion over bedrock, or capture driven by surface runoff, are not sufficient to produce the observed diversions. The sites of diversion coincide spatially with limestone belts and reactivated fault zones, suggesting that solution‐triggered or deformation‐triggered permeability have helped breaching of interfluves. The diversions are also related temporally and spatially to the accumulation of sediment fills in the valleys, upstream of the rising structures. We infer that the breaching of the interfluves was achieved by headward erosion along tributaries fed by groundwater flow tracking from the valleys soon to be captured. Fault zones and limestone belts provided the pathways, and the aquifers occupying the valley fills provided the head pressure that enhanced groundwater circulation. The defeat of rivers crossing the rising structures results essentially from the tectonically enhanced activation of groundwater flow between catchments.

Universal and local controls of avulsions in southeast Texas Rivers

This study addresses the relative importance of universal and local factors in river avulsions, from the perspective of the configurational complexity of the fluvial system as it relates to the potential for avulsions. Because local factors cannot be addressed without a specific context, several rivers of the southeast Texas coastal plain (Brazos, Navasota, Trinity, Neches, and Sabine Rivers) are studied based on field observations and surveys, digital elevation models, and geographical information system (GIS) analysis. Avulsions are influenced by a combination of universal factors relevant to any alluvial river, and local factors at least partly contingent on the environmental setting and history of the study rivers. The universal controls are factors that create conditions under which avulsions can occur: channel aggradation, banktop discharge, and cross-valley slope advantages. A rate of channel aggradation greater than that of floodplain accretion is an important setup factor, but superelevation (channel bed elevation greater than or equal to floodplain elevation behind the natural levee) is not required. Slope advantages are necessary, but not sufficient, for avulsions to occur. Local factors include abandoned channels on the floodplain, and basins or depressions associated with higher-discharge Pleistocene conditions. The local controls also include deflection factors that divert flow from the main channel during high discharges, and levee weaknesses. Relationships among the universal and local factors were represented as directed graphs, and the configurational complexity determined using connectance entropy. Results show that the relative importance of universal and local factors varies with the scale or scope of analysis. In the broadest context, universal factors contribute more than 75% of the connectance entropy, signifying the importance of identifying the key setup factors. Within a given aggradation or valley-filling context, however, local factors account for more than 80% of the connectance entropy, indicating that knowledge of the trigger factors and local floodplain morphology is the key to understanding and predicting avulsions. This study suggests a three-stage process for predicting avulsions. First, the universal factors can be used to identify reaches with the potential to avulse. Second, superelevation (a sufficient but not necessary condition) can be used to identify specific locations where avulsions are imminent, and (as a necessary but not sufficient condition) the absence of cross-valley slope advantages can be used to identify locations where avulsions cannot occur. The final stage—predicting avulsions at other locations within systems or reaches where the setup factors are present—requires specific consideration of local factors.

Shredding of environmental signals by sediment transport

Landscapes respond to climate, tectonic motions and sea level, but this response is mediated by sediment transport. Understanding transmission of environmental signals is crucial for predicting landscape response to climate change, and interpreting paleo‐climate and tectonics from stratigraphy. Here we propose that sediment transport can act as a nonlinear filter that completely destroys (“shreds”) environmental signals. This results from ubiquitous thresholds in sediment transport systems; e.g., landsliding, bed load transport, and river avulsion. This “morphodynamic turbulence” is analogous to turbulence in fluid flows, where energy injected at one frequency is smeared across a range of scales. We show with a numerical model that external signals are shredded when their time and amplitude scales fall within the ranges of morphodynamic turbulence. As signal frequency increases, signal preservation becomes the exception rather than the rule, suggesting a critical re‐examination of purported sedimentary signals of external forcing.

Delta distributary dynamics in the Skagit River Delta (Washington, USA): Extending, testing, and applying avulsion theory in a tidal system

Analysis of historical aerial photos shows that Skagit Delta (Washington, USA) distributary dynamics are consistent with the Slingerland and Smith model of avulsion dynamics where the ratio of the water surface slopes of the two branches of a bifurcation predicts avulsion stability. This model was extended to predict distributary inlet (upstream) width and bankfull cross-sectional area. The water surface gradient ratio for a bifurcation pair predicted distributary width well; the lowest R 2 was 0.61 for the 1937 data points, but R 2 ranged from 0.83 to 0.90 for other year-specific regression lines. Gradient ratios were not constant over the historical record; from 1937 to 1972 the mainstem river channel lengthened by 1250 m in the course of marsh progradation, while distributary lengthening was comparatively negligible. Consequently, the gradient advantage of the distributaries increased and their channels widened. After the mainstem river terminus stabilized from 1972 to the present, the distributaries continued to lengthen with marsh progradation, so that distributary gradient advantage steadily declined and the distributaries narrowed. While distributary cross sections were not available for the historical period, they were surveyed in 2007 near the distributary inlets. Gradient ratio was more closely related to distributary inlet bankfull cross-sectional area ( R 2 = 0.95) than to minimum distributary width for any photo year examined. Applying this form of analysis to Skagit Delta distributaries that have been dammed in the course of agricultural development suggests that their restoration to stabilize eroding marshes at their outlets and recover salmon migration pathways would be feasible without significant risk of full river avulsion.

Early Prehistoric Landscape and Landuse in the Fier Region of Albania

Little was known until recently about regional patterns of early prehistoric occupation in Albania, making it difficult to situate the Albanian record within existing, general models of early prehistoric landuse. An intensive regional survey, the Mallakastra Regional Archaeological Project (MRAP), carried out in the Fier region of central Albania from 1998-2003, gathered widespread evidence for human occupation during the Palaeolithic and Mesolithic periods, from the Myzeqe Plain to the Mallakastra hills. The Pleistocene and early Holocene landscape of Fier differed considerably from the present landscape, and at times the Adriatic shoreline was much farther inland than it is now. As a consequence, Palaeolithic and Mesolithic foragers were able to exploit coastal-wetland and estuarine environments that have been buried only recently by alluvial sediment as a result of river avulsion and soil erosion. The landscape studied by MRAP is only a fragment of the total landscape once exploited by early humans in this part of the Balkans, whose home range may have included much of soutwest Albania and parts of northwest Greece. Our reconstruction of early prehistoric landuse in central Albania, based on collection and mapping of the locations of 1593 lithic artifacts from a 35 sq km area, indicates patterns of logistical foraging that span the Palaeolithic and Mesolithic and match those modeled in other Mediterranean countries, such as Greece.

Communities of knowledge: Science and flood management in Bangladesh

This paper traces the integration of a particular set of knowledge claims into flood management in Bangladesh following the instigation and collapse of the Flood Action Plan. Using the work of Goodbred and Kuehl (1998) as an entry point and partially in response to Nicholls and Goodbred's (2004) call for integrated assessments to improve understanding and management, we explore the incorporation of sedimentation and subsidence knowledge claims into flood management. We approach this issue from a ‘scholarly’ perspective, tracing the citations and cross-references within academic publications, and from a ‘government’ perspective, exploring recent policy to determine the degree of consideration for sedimentation, subsidence and their related phenomena (i.e. lateral river erosion, river avulsions, river conveyance). Despite mutual recognition of relevance and widespread support for holistic or interdisciplinary knowledge management, our findings suggest an isolation between the natural and social science communities concerned with flood management. Furthermore, the exploration suggests a similar isolation between government and the sciences. Drawing upon Barry et al.'s (2008) analysis on interdisciplinarity, the findings suggest that different ‘logics’ might account for the isolation and associated sequestration of knowledge, raising the possibility of improved communication and collaboration among those interested in this increasingly complex and important issue.

Wave climate, coastal sediment budget and shoreline changes for the Danube Delta

Since its formation, approximately 12 000 years ago, the coastal zone of the Danube Delta experienced alternating periods of accretion and erosion related to the natural process of lobe abandonment caused by river avulsion. Over the last 150 years, the effects of human interventions modified these natural trends. Regulation of the Danube River (e.g. the Iron Gates barrages facilitating navigation and controlling river discharge), construction and extension of the Sulina entrance jetties and dredging of the Sulina mouth bar for navigational purposes are examples of human interventions that influenced and still influence the actual state of the Danube Delta coastal zone. Most importantly, these interventions diminished the quantity of sediments reaching the coast and, consequently, led to a severe retreat of the shoreline, especially in the Sulina–Sf. Gheorghe coastal sector. To investigate and quantify the recent sediment budget and associated shoreline changes along the Danube Delta coastal zone we constructed a hindcast of the wave climate, the alongshore transport capacity and the resulting sediment budget using a numerical coastline model. On a local scale, it was found that overwash plays an important role; i.e. the observed evolution of Sahalin spit island shows a cross-shore landward migration that is much larger than could be explained by the calculated alongshore sediment transport gradients. On a delta-scale however, the resulting alongshore sediment transport gradients and associated sediment budgets comply very well with the observations, confirming the dominant role of wave-induced alongshore transport and giving confidence in our predictions regarding the shoreline position for the next 25 and 50 years. The results of this study indicate the potential for broader applicability of the model to other deltas, especially those in wave-dominated settings.

Complexity in a cellular model of river avulsion

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.

The influence of valley aggradation and listric normal faulting on styles of river avulsion: A case study of the Brazos River, Texas, USA

The Holocene avulsion history of the lower Brazos alluvial valley of east Texas, USA, was studied using 10 drill cores, 26 radiocarbon dates, aerial photos, and a digital elevation model. This study shows that two long-term processes, aggradation and localized valley tilting (along a normal listric fault), are responsible for generating two styles of avulsion. The first process precedes avulsion-by-progradation, while the second process precedes avulsion-by-annexation. As valley aggradation migrated updip over the last 7.5 ka, three regional backstepping avulsions occurred along the lower 140 km of the valley and each generated sizable deposits. A pattern emerges of landward stepping progradational avulsions tracking the locus of valley aggradation and of valley aggradation migrating inland even after the rate of sea level rise diminishes. At the same time, several local nodal avulsions occurred between 50 and 55 km updip of the current highstand shoreline but generated no observable deposits. Geomorphic evidence indicates that, since the late Pleistocene, active movement along a previously undocumented normal listric fault has occurred at the location of the nodal avulsion. These two long-term processes do not operate mutually exclusive of each other to promote avulsions; rather, they operate concurrently. Only aggradation promotes avulsions that affect floodplain alluviation, although the total volume of these deposits comprises a small portion of the valley fill.