Holocene Avulsion Research Articles

Downstream Controls on Coastal Plain River Avulsions: A Global Study

AbstractThe avulsion frequency of coastal‐plain rivers is primarily governed by the rate at which channels become superelevated over neighboring plains, which is itself controlled by multiple factors. Notably, the importance of wave and tidal processes, the rates of relative sea‐level (RSL) change, and the bathymetry of the receiving basin are thought to affect channel morphodynamics and channel‐mouth progradation, thereby controlling streambed aggradation and influencing the avulsion frequency and drainage density of coastal plains and deltas. This work tests the significance of these downstream factors on the avulsion histories of 57 Holocene lowland river systems. A quantitative analysis is performed of relationships between variables that quantify downstream controls and estimations of avulsion frequency, based on the number of avulsion events, active or abandoned channel paths, and delta lobes; measures of spatiotemporal avulsion “density” are also derived by normalizing these metrics by the size of study areas and the number of distinct drainage systems. Relationships between avulsion‐frequency metrics and descriptors of process regime indicate that wave and tidal processes may stabilize coastal channel systems, but also that their influence may be modest. No consistent relationship is seen between avulsion‐frequency proxies and the offshore bathymetric gradient, which in the studied examples does not scale with the rate of shoreline progradation. No evident trend exists between measures of avulsion frequency and estimated rates of either eustatic or RSL fluctuations. Overall, the considered variables do not leave a clear statistical signature in Holocene avulsion histories, suggesting that upstream or intrabasinal factors may represent more important controls.

Geomorphic diversity of the middle Araguaia River, Brazil: A segment-scale classification to support river management

The complex mosaic of geomorphic units that characterizes the fluvial corridor of large rivers poses challenges to effectively describe and discriminate the landforms diversity, and is relationships with the hydrosedimentological dynamics and state of the system. Consequently, the classification of the diverse array of fluvial features into discrete segments can be a systematic and organized approach useful to sustain geomorphically-informed management strategies. Here we propose a methodological approach and characterize the geomorphic diversity of the middle Araguaia River by applying a classification of reaches into segments with a certain consistent level of morphological variability. A GIS framework developed for this purpose relied on the survey, analysis, and statistical treatment of morphological variables derived from remote sensing data. It allowed to differentiate 19 distinct reaches (R1-R19) that were further clustered into five major segments (SI-SV). We investigated the first-order controls of the fluvial style in each of the identified segments and found that changes in valley gradient influence in a major way the segments I, III, and IV, while the entrance of important tributaries and the nature of the valley floor are natural governing factors in segments II and V, respectively. The system exhibits a significant morphological diversity among segments, and it is strongly influenced by the late Holocene avulsion history of the river. Given the increasing pressure of human activities on the hydrological system, the present study can be of use to subsidize sustainable management plans aiming to safeguard the last large free-flowing river of the Cerrado biome, and our methodological proposal is valid for use in other large rivers worldwide.

First report on dendrochronological and radiocarbon studies of subfossil driftwood recovered across the Mureş/Maros Alluvial Fan

Abstract Visiting three gravel pits and three natural outcrops across the Mureş/Maros Alluvial Fan, 58 samples were collected from subfossil driftwood recovered from coarse-grained fluvial sediment layers, while no subfossil wood was found at three additional gravel pits. Dendrochronological and radiocarbon analysis of these relict wood can support the temporal extension of the regional dendrochronological reference datasets and their dating can provide a useful contribution to the reconstruction of the landscape evolution of the Mureş/Maros Alluvial Fan. The tree-ring widths of the subfossil samples were measured. Dendrochronological synchronization resulted in two oak chronologies which encompassed five, and two reliably cross-dated series covering 191 years (MURchr1) and 127 years (MURchr2), respectively. Based on the 14C ages the subfossil driftwood material represents Middle and Late Holocene ages. The occasionally up to 6 m-thick fluvial sediment covering relatively young, < 1000-yr-old wood, indicates intense accumulation at the apex of the Mureş/Maros Alluvial Fan, which explains the documented rapid and significant Holocene avulsions.

Timescales, mechanisms, and controls of incisional avulsions in floodplain wetlands: Insights from the Tshwane River, semiarid South Africa

Avulsion (relocation of a river course to a new position) typically is assumed to occur more frequently in rivers with faster sedimentation rates, yet supporting field data are limited and the influence of sedimentation rate on avulsion style remains unclear. Using analysis of historical aerial photographs, optically stimulated luminescence dating of fluvial sediments, and field observations, we document three avulsions that have occurred in the last 650years along the lower reaches of the semiarid Tshwane River in northern South Africa. Study of the modern river and abandoned reaches reveals that a downstream decrease in discharge and stream power leads to reduced channel size and declining sediment transport capacity. Bank erosion drives an increase in channel sinuosity, leading to a decline in local channel slope, and to a further decrease in discharge and sediment transport. Local sedimentation rates >10mma−1 occur within and adjacent to the channel, so over time levees and an alluvial ridge develop. The resulting increase in cross-floodplain gradient primes a reach for avulsion by promoting erosion of a new channel on the floodplain, which enlarges and extends by knickpoint retreat during periods of overbank flow. Ultimately, the new channel diverts the discharge and bedload sediment from the older, topographically higher channel, which is then abandoned. Our findings support the assumption that avulsion frequency and sedimentation rate are positively correlated, and we demonstrate that incisional avulsions can occur in settings with relatively rapid net vertical aggradation. The late Holocene avulsions on the semiarid Tshwane River have been driven by intrinsic (autogenic) processes during meander belt development, but comparison with the avulsion chronology along a river in subhumid South Africa highlights the need for additional investigations into the influence of hydroclimatic setting on the propensity for avulsion.

Holocene Avulsions of the Euphrates River in the Najaf Area of Western Mesopotamia: Impacts on Human Settlement Patterns

We present a study that reconstructs the ancient courses of the Euphrates in part of the Mesopotamian floodplain west and southwest of the ancient site of Babylon. The focus is on tracing paleochannel courses, determining when these paleochannels were active, and understanding the patterns of avulsion. The research was carried out using a combination of geological, geomorphological, remote sensing, historical, and archaeological approaches. Fieldwork included “ground truthing” of the remote sensing work, manually drilling boreholes (up to 7 m in depth), sedimentary and geomorphological documentation and sample collection for radiocarbon dating. As a result, five main courses of the Euphrates in five different periods have been mapped in this area, including four previously unidentified and/or unlocated migrations that linked the different periods. The main courses are the Purattum Course (before 3100–1000 BC), the Arahtum Course (1000–125 BC), the Sura Course (125 BC to AD 1258), the Hilla Course (13th to 19th century AD), and the Hindiya Course (19th to 20th century AD). There has been an overall migration of the main channel from east to west across the study area over time. The location of avulsion nodes changed along the length of the river, mainly downstream over time, but with a cluster of avulsion events near Babylon and a notable man‐made interference in the 20th century at the Hindiya Barrage.

Complex sediment deposition history on a wide continental shelf: Implications for the calculation of accumulation rates on the Great Barrier Reef

Understanding the key processes controlling the delivery, deposition and fate of sediments on continental shelves is critical to appreciate the evolution of coasts and estuaries and to interpret geological sequences. This study presents radiocarbon and Optically Stimulated Luminescence (OSL) ages of sediment cores collected from key locations offshore from the Burdekin River, Australia, the largest single source of sediment delivered to the Great Barrier Reef (GBR) lagoon. The ages show variable sediment accumulation rates at the different locations that coincide with the Holocene avulsion history of the Burdekin River. Our data show that most fine sediment (<63 μm) delivered from the Burdekin River is retained within 50 km of the mouth, a finding that contrasts previous studies which postulated that fine sediments are advected northwards via longshore drift processes. The pairing of radiocarbon and OSL ages provides insights on resuspension regimes operating on the inner shelf of the GBR. It was thought that turbidity on inshore GBR coral reefs and seagrass meadows has increased as a result of increased erosion in the adjacent catchment from agricultural development. Our data show that the age of the sediments in Cleveland Bay (derived from the radiocarbon ages from shell and organic material) can be several thousand years older than when the sediment was last deposited (OSL ages). However, the increased turbidity could conceivably be caused from ‘new biologically-produced sediment’ (i.e. particulate organic matter) as a result of increased nutrient export to the GBR. We suggest that the composition of sediment in resuspension events before and after the wet season be analysed to examine whether newly delivered organic-rich sediment can affect coral reefs and seagrass meadows.

Late Glacial and Holocene avulsions of the Rio Pastaza Megafan (Ecuador–Peru): frequency and controlling factors

The geomorphological study by means of remote sensing imagery of the Rio Pastaza Megafan (Ecuador and northern Peru) reveals the traces of numerous avulsions. One hundred and eight avulsion sites have been defined. The location of these sites, the available radiocarbon ages as well as historical maps of the seventeenth century, enable us to propose an evolution history of the migration and avulsions of the Rio Pastaza since the Last Glacial Maximum. The first avulsions of the Río Pastaza occurred after the LGM in a zone close to and roughly parallel to the sudandean front, where the developed avulsion gave a distributive pattern to the ancient stream of the Río Pastaza in an area located between the modern Río Morona and Pastaza, where they caused the Rio Pastaza to develop a fan-like distributary pattern. This is interpreted as a response to thrust-related forelimb tilt, progressively shifting eastward the Rio Pastaza and the apex of the megafan. This sequence of events ended with the Great Diversion of the Rio Pastaza towards the modern Rios Corrientes and Tigre. Avulsions occurred in the Tigre-Corrientes Area between 9200 and 8,500 years Cal BP. Afterwards, the Río Pastaza was diverted to its present-day north–south course. This last significant avulsion occurred before AD 1691. In the area located between the modern Río Morona and Pastaza, avulsion frequency—probably overestimated—ranges between 100 and 200 years. In the Ríos Tigre and Corrientes area, avulsion frequency—probably underestimated—ranges from 300 to 400 years. Regional tectonics is likely to have triggered most of the avulsions in the Morona Pastaza area but its influence is restricted to this area. The factors controlling the avulsions in the Tigre-Corrientes area are less clear because the frequently described “hydrologic”-driven avulsion as observed in areas characterized by contrasted hydrologic cycles are inconsistent with the characteristics of the hydrologic cycles of the Rio Pastaza.

A Middle to Late Holocene avulsion history of the Euphrates river: a case study from Tell ed-Dēr, Iraq, Lower Mesopotamia

Geoarchaeological research was performed to reconstruct the floodplain history in the surroundings of two ancient Mesopotamian cities: Tell ed-Dēr and Sippar. The mapping of the floodplain is based on facies analyses of the sedimentary succession of 225 hand-operated boreholes. The archaeological sites Tell ed-Dēr and Sippar are closely linked to a palaeochannelbelt of the Euphrates, located in the western part of the study area. Channel activity started at least in ca 3100 BC/5050 cal BP, until ca 1400–1000 BC/3350–2950 cal BP. The channel belt was part of an avulsion driven multiple Euphrates channel network that gradually became abandoned from the second half of the 2nd millennium BC. A second mapped Euphrates, Tigris or Joint Euphrates -Tigris palaeochannel belt became abandoned well before 3100 BC. Examples of natural processes as well as human interactions triggering avulsion are given. Moreover, textual, archaeological and geological data show clearly that flood-control techniques and the construction of large-scale dikes seemed to be a common practice.

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.

Avulsion: The relative roles of autogenic and allogenic processes

Avulsion, the abandonment of all or part of a channel belt in favor of a new course, is controlled by both autogenic (intrabasinal) and allogenic (extrabasinal) processes. However, their relative roles are not known well. The Rhine–Meuse delta is without doubt the best studied delta regarding Holocene avulsion processes. Based on studies of this delta, an overview is given of (1) allogenic and autogenic processes influencing different aspects of avulsion, (2) the relative importance of these processes in time and space, and (3) observations that can be used to differentiate autogenic and allogenic causes of avulsion. The following avulsion parameters are quantified, and their allogenic and/or autogenic nature are discussed: period of activity of channel belts (period between beginning and end of overbank sedimentation by the river channel), interavulsion period (time period between successive avulsions of a channel belt), avulsion frequency (number of avulsions per time interval in a given area), and avulsion location. In the Rhine–Meuse delta, the period of activity of the Holocene channel belts varies considerably, but shows no significant trend over time, whereas extrabasinal factors (climate, base-level, and tectonics) changed. This suggests that this parameter is mainly autogenic controlled. The average interavulsion period increased from 8000 cal yr BP to 2800 cal yr BP, and decreased since then. Significant fluctuations occurred on a shorter time scale. A maximum variability in the interavulsion period occurred between 3200 and 1800 cal yr BP. This was a time when large channel belts gradually came into existence. During this period the avulsion frequency also reached a maximum. The long-term increasing trend in interavulsion period is related to the decreasing rate of sea level rise. The decreasing interavulsion period since 2800 cal yr BP can be explained by increased discharge and sedimentation, resulting in an increase in the number of avulsions. Seven avulsion sequences (as predicted by the Mackey and Bridge [Mackey, S.D., Bridge, J.S., 1995. Three-dimensional model of alluvial stratigraphy: theory and application. Journal of Sedimentary Research B65 (1), pp. 7–31.] model) may be recorded in the Holocene dataset of the Rhine–Meuse, suggesting a periodicity of ∼ 500–600 yr. Each avulsion sequence coincides with a peak in the avulsion frequency. The periodicity of ∼ 500 yr in the avulsion frequency is proposed to be autogenic. However, there is also a long-term trend in the avulsion frequency that can be related to sea level rise, and (after 3000 14C yr BP) to climate change and human influence. Avulsion locations are not randomly distributed over the delta, and are related to the allogenic factors: relative sea level rise, local tectonics, and changes in discharge and sediment load.

Causes of River Avulsion: Insights from the Late Holocene Avulsion History of the Mississippi River, U.S.A.--Discussion

Our understanding of river avulsions is still in its infancy, especially if we are to rationally predict how, when, and where avulsions will occur. The recent paper by Aslan et al. (2005) sheds new light on the late Holocene evolution of the Lower Mississippi Valley, with particular emphasis on the avulsion that led to the formation of the Atchafalaya River. Although we welcome their contribution and agree with most of their conclusions, we also feel that a key point raised in their paper is somewhat misleading. Aslan et al. (2005) contend that factors other than gradient advantage (notably the presence of buried sand bodies and/or floodplain topography associated with residual channels) are at least as important in determining the occurrence …

Causes of River Avulsion: Insights from the Late Holocene Avulsion History of the Mississippi River, U.S.A.--Reply

Causes of River Avulsion: Insights from the Late Holocene Avulsion History of the Mississippi River, U.S.A.--Reply

Causes of River Avulsion: Insights from the Late Holocene Avulsion History of the Mississippi River, U.S.A.

Abstract The emphasis on gradient advantages in studies of avulsion is misleading. While gradient advantages are necessary for an avulsion to occur, the late Holocene avulsion history of the Mississippi River in Louisiana suggests that factors such as substrate composition and floodplain channel distributions are more important. Cross-valley to down-valley slope ratios of the modern floodplain range from 16 to 110 and are typically > 30. The slope ratio is 35 at the location of the Mississippi–Atchafalaya diversion (Old River) yet slope ratios are 83 to 110 immediately upvalley of Old River. All values of Mississippi River floodplain slope ratios are significantly larger than values of avulsion threshold calculated by numerical models. Shallow floodplain cores, 14C dating of organic remains, and geologic mapping show that the Mississippi River has avulsed only four times over the past 5 ky in the southern Lower Mississippi Valley (LMV). Gradient advantages are widespread, yet avulsions are rare. These observations indicate that factors other than gradient advantage control Mississippi River avulsion. Several examples of Mississippi and Red River avulsion by channel reoccupation support the idea that channel distributions and substrate compositions are primary influences on avulsion. Incipient Mississippi River avulsion and development of the Atchafalaya River involved reoccupation of abandoned Mississippi River channels and a Red River crevasse-splay complex. The modern Atchafalaya River also incises buried Mississippi River channel-belt sands. Abandoned channel belts and crevasse-splay complexes consist of sandy substrates that facilitate scour and the development of channels capable of capturing the Mississippi River. Abandoned channels provide ready-made conduits for Mississippi River flow that can efficiently develop into avulsive channels. Multi-storied sheet sandstones in ancient fluvial deposits may provide additional support for the idea that erodible substrates and floodplain channel distributions are critical influences on avulsion. These features record episodic reoccupation of channel belts, which at least in some cases, may simply reflect successive avulsions rather than major changes in aggradation rate or extrabasinal factors such as climate.

A review of Holocene avulsions of the Tigris and Euphrates rivers and possible effects on the evolution of civilizations in lower Mesopotamia

The role of avulsion in the evolution of civilization in lower Mesopotamia is widely recognized. Ancient settlements are closely associated with abandoned courses of the Euphrates and Tigris Rivers. This paper discusses avulsion history, controls of avulsions, their effects on civilization evolution from 7000 to 1000 yr B.P., and interactions between human activity and channel-network evolution based on previous archaeological and geomorphological surveys, analysis of cuneiform texts, maps, satellite photos, and limited geological data. Settlement distribution in ancient Mesopotamia is modeled in relation to avulsion styles, as well as channel and sedimentation patterns. Avulsion belts are suggested as places where urban settlements emerged and were sustained. Multiple channel networks and avulsion belts created large, naturally irrigated areas, fostering the efficient agriculture needed to sustain dense rural and urban settlements. After channel networks were abandoned during delta evolution, largescale canal construction was required to sustain settlements but still could not prevent their decline. © 2005 Wiley Periodicals, Inc.

Book Review: Holocene avulsions in the Rhine-Meuse delta, The Netherlands

Book Review: Holocene avulsions in the Rhine-Meuse delta, The Netherlands

Avulsion Frequency, Avulsion Duration, and Interavulsion Period of Holocene Channel Belts in the Rhine-Meuse Delta, The Netherlands

ABSTRACT Avulsion parameters greatly influence alluvial architecture, because they determine channel density and interconnectedness. Reliable data on avulsion parameters is still scarce. From a detailed reconstruction of the Holocene avulsion history of the whole Rhine-Meuse delta we determined quantitative values for the avulsion parameters: avulsion frequency, avulsion duration, and interavulsion period. In the Rhine-Meuse delta the number of coeval channels is related to the avulsion frequency. Calibrated 14C intensity histograms are used to analyze 14C dates of beginning and ending river activity. The histograms show that instantaneous and gradual avulsions were almost equally important in the Rhine-Meuse delta. A high avulsion frequency occurred from 8000 to 7300 cal yr BP (a total of 17 avulsions, i.e., 2.43 avulsions/100 years). During this period the avulsion frequency was related to the high rate of sea-level rise, which induced fluvial sedimentation in the present near-coastal area. After 7300 cal yr BP avulsion frequency decreased as a result of a continuously decreasing rate of sea-level rise. From 7300 to 3200 cal yr BP avulsion frequency was low; 35 avulsions took place within 4100 years (0.85 avulsions/100 years). Approximately 5000 cal yr BP the coastal barriers became closed and large-scale peat formation occurred. This resulted in fixation of the river channels and low cross-valley gradients, reducing the chances for avulsion. Between 3200 and 1400 cal yr BP avulsion frequency was high again (a total of 34 avulsions, i.e., 1.89 avulsions/100 years) as a result of increased discharge and/or within-channel sedimentation, which enhanced chances for avulsion. The period of activity of channel belts shows no significant trend on the time scale of the Holocene. It is highly variable and averages 1280 ± 820 cal yr. The avulsion duration fluctuates between less than 200 and 1250 cal years and averages 335 cal years. The avulsion duration shows no significant trend through time and remained constant until at least 1900 cal yr BP. Avulsion duration seems not to be influenced by aggradation rate. Average interavulsion period is shorter than the period of activity of channel belts, and is approximately 945 cal years. In the Rhine-Meuse delta, on the time scale of the Holocene, interavulsion period and avulsion duration are on average constant. Therefore, the number of coeval channels is directly related to avulsion frequency. Available evidence from other rivers and deltas around the world, however, suggests that the relationship between these avulsion parameters is not everywhere the same.

Factors Controlling the Holocene Avulsion History of the Rhine-Meuse Delta (The Netherlands)

ABSTRACT The avulsion history of the Holocene Rhine-Meuse delta (The Netherlands) was reconstructed, on a timescale of millenia, using detailed paleogeographic maps based on approximately 200,000 lithological borehole descriptions, over 1150 14C ages, and 36,000 archaeological artifacts. Gradient lines were constructed for all channel belts. These allowed determination of gradients, paleo-flow direction, relative age of channel belts, and time correlation of undated channel-belt fragments. Avulsion sites were inferred from the paleogeographic reconstruction. At least 91 avulsions occurred over the past 10,000 years, of which 82 could be dated with an accuracy of ± 200 14C years. The location and shifting of Holocene avulsion sites in the Rhine-Meuse delta in space and time are related to: (1) Relative sealevel rise. In the Early Holocene, avulsions could not take place, because rivers were still incised. Around 7500 yr BP, avulsions occurred in the western part of the present delta as a result of backfilling of the Late Weichselian valley. Between 7500 and 3700 yr BP, the zone where avulsions occurred shifted inland as a result of relative sealevel rise. (2) Neotectonics. Between 4900 and 1700 yr BP, the location of avulsion sites seems to have been influenced by neotectonic movements of the upthrown Peel Horst. Four out of six avulsion nodes in the Rhine-Meuse delta were located in the Peel Horst fault zones. (3) Increased discharge and/or within-channel sedimentation. From 2800 until about 1500 yr BP, avulsion sites were located all over the delta. During this time, the number of channels was high, and avulsion frequency reached a maximum, at a time when aggradation rate decreased with a reduction in the rate of sealevel rise. After 2000 yr BP meander wavelength of alluvial channels increased considerably. The increased meander wavelength and the high avulsion frequency are attributed to increased bankfull discharge or within-channel sedimentation (leading to channel widening), or both. (4) Human influence. Between 1000 and 650 yr BP, all the rivers were embanked, and avulsions could no longer take place. The few that occurred were induced by humans.

Holocene avulsion styles and sedimentation patterns of the Saskatchewan River, Cumberland Marshes, Canada

Abstract This paper describes the styles and sedimentation patterns of Holocene avulsions of the Saskatchewan River at the Cumberland Marshes predating the most recent (1870's) avulsion. Holocene avulsions involved two major styles: (1) progradation of lacustrine deltas and splay complexes, and (2) local reoccupation or appropriation of preavulsive channels. Most avulsions were progradational and occurred in floodplain lakes occupying floodbasin areas between elevated alluvial ridges. The two most recent avulsions (Old Channel and 1870's) diverted mainly into inundated peatland. Progradational avulsions were accompanied by deposition of avulsion-belt sediments associated with multiple channel networks. The characteristics of avulsion-belt deposits include: (1) sediments dominated by poorly sorted silt and very fine sand, (2) lateral continuity in transverse and longitudinal directions with subhorizontal upper and lower boundaries, (3) predominantly nonerosional contacts with underlying sediments, (4) coarsening-upward successions, (5) consistent radiocarbon dates for the tops of preavulsive substrates, and (6) occurrence beneath levee and overbank deposits of the associated major channel belt, indicating that avulsion-belt deposition occurs prior to the development of a new master channel. The width of ancient avulsion belts reaches 18 km, with typical thicknesses of 2–3 m. Avulsion belt deposition accompanied by many small channels was followed by flow reversion to a few larger channels and, eventually, to one single channel, accompanied by deposition of levee and channel deposits. Reoccupational avulsions were not accompanied by deposition of widespread avulsion-belt sediments and led mainly to the modification of pre-existing channel belts. The sedimentation patterns of Holocene avulsions of the Saskatchewan River reveal that avulsion-belt sediments represent major components of the floodplain deposits, supporting the model developed from observations of the 1870's avulsion.

Middle and late Holocene avulsion history of the River Rhine (Rhine-Meuse delta, Netherlands)

At present, little evidence is available on avulsion rates, avulsion frequencies, and interavulsion periods of aggrading fluvial systems over time scales of millenia. The large number of high-quality 14C ages from the Holocene Rhine-Meuse delta (central Netherlands) enables a detailed assessment of such avulsion parameters and an evaluation of the factors controlling avulsion frequency. 14C histogram analysis indicates that instantaneous avulsion (coexistence of successive channel belts for less than a few centuries) has dominated in this area. Interavulsion periods were typically 1000 yr, although values of 500 or 5000 yr have also been found. Avulsion frequency was especially large prior to 4300 cal. yr B.P., indicating a strong control by aggradation rate, in turn controlled by sea-level rise. In addition, periods with high avulsion frequency appear to correlate with time intervals characterized by large-scale transgressive tendencies. The time interval with the largest number of coexisting distributaries corresponds to the period with dominance of anastomosing fluvial style.