When do stream terraces form

However, since the very first researches, several efforts were also made to chronologically constrain fill-terrace staircase, basing on both indirect inferences and artefacts found on terraces e.

At present, since the only Upper Pleistocene-Holocene sediments can be profusely dated with 14 C, only the youngest terraces provided a most assuredly chronology. Conversely, only a few isotopic and numerical ages are available for older terraces e. Nonetheless, a reliable terrace chronology has been recently proposed by K. Pazzaglia also based on a notable amount of numerical ages. It is the scheme on which O. Nesci et al. Nonetheless, the progress of research took both number and hierarchy of alluvial terraces to be repeatedly discussed, rearranged and re-assessed.

As regards the oldest fill terraces, it has been stressed Nesci et al. Such array of Pleistocene terraces has been firstly introduced by O. Given the relatively old age of the earlier fill terraces, tectonic deformation may be responsible for more or less localised height discrepancies Nesci et al.

Anyhow, it is only in relatively recent times that the intrinsic difference of genetic mechanisms and morphoevolutive significance between such terraces and older ones has been definitely assessed Ciccacci et al. More in detail, studies of outlet areas substantiated by subsurface data and radiometric dating demonstrated that the alluvial suites rapidly thicken seaward and consist of Pleistocene alluvium disconformably overlain by thick Holocene deposits at least in part related to eustatic sealevel rise e.

Furthermore, anthropogenic causes proved to be effective in triggering both Holocene aggradation and terrace development e. In addition, with a special reference to inland areas, other causes have been claimed as responsible in the Holocene for the development of, often local, terraces throughout the northern Marche trunk-valleys; among such causes there are stream adjustment following meander cutoff Nesci and Savelli, a , complex responses sensu Schumm, ; Calderoni et al.

Accordingly with more general achievements, such configuration has been usually retained a result of the general decreasing rate of crustal uplift from the chain axis towards the Adriatic.

Regardless of overall downwalley decreasing in heights, major fill terraces are found above the present thalweg at heights ranging from few meters recent terraces in downstream areas up to m, with maxima of m that are reported for the oldest units of upper Metauro valley by O.

Anomalies in both downstream and transverse valley-profiles are well known, and usually related with tectonic displacement e. Also noticeable is the unequal altimetric distribution of the same terrace order to retain as roughly contemporaneous given the climate-driven origin; Bull, within adjacent valleys fig. Location of transepts in fig. Position des profils sur la fig. Lipparini, , p. Savelli , highlighted a systematic recurrence of peculiar erosional-depositional steps in each cycle of fill terrace development.

Despite some obvious limitations due to the lesser degree of preservation of the older terraces, such steps have been recognised to repeat quite unvaried into the four main cycles of aggradation-dissection T 1a , T 1b , T 2 , and T 3 according to O. The cold-climate driven aggradation of main valleys has been acknowledged to consist of two steps fig.

Hence, sceneries for the alluvial plains during the glacial stages fig. The cyclical repetition of alluvial environments and facies in the main fill terraces makes detailed analyses of terraced deposits of the last climatic cycle effective for unraveling the oldest terraced suites, as already highlighted for the Marche region Nesci and Savelli, ; Nesci et al.

In this regard, the mid sectors of the main trunk-valleys have been deemed as representative area to unravelling the evolution stages Nesci and Savelli, and following , since influenced neither by sea level effects nor upstream climato-tectonic controls on fluvial system behavior see also Blum and Tornqvist, ; Bridgland and Westway, ; Lewin and Gibbard, In this reading, the main achievements can be summarised as follows.

According to D. Savelli et al. Savelli , b , the alluvial deposits of fill terraces usually rest unconformably above deeply-incised pre-Quaternary bedrock both in inland areas and in coastal sectors, where, as discussed in next sections, the last aggradation cycle accounts for incised valleys deeply entrenched below present sea level Calderoni et al.

Savelli, , thus predates any major trunk-valley aggradation. According to a majority of authors e. The onset of the aggradation regime - also based on radiocarbon datings from Conca Calderoni et al. Such behavior accounts for a shift from incision to aggradation during glacial stages because of cold climate-driven increase of sediment supply at the basin scale. It matches similar mechanisms already proposed for both Marche river valleys Nesci and Savelli, ; Coltorti et al.

For palaeoclimatic and chronostratigraphic assumptions see fig. After Savelli et al. A: Upper Pleistocene braided-stream alluvium downstream Fossombrone, Metauro valley. C: Upper Pleistocene braided alluvial deposits A unconformably overlain by a thin sheet of early Holocene meandering-stream alluvium, hinting at ongoing re-incision leading to underfit streams; relete to fig.

Valley reaches where channels developed relatively high-sinuosity patterns have also been identified, as along the Cesano River Nesci and Savelli, b, and along the upper Esino River Calderoni et al. Anastomosed patterns have also been pointed out in the upper Esino River Calderoni et al. Certain variability in both alluvial lithofacies and channel patterns comes even from this short summary, and can be related to extension, relief, hydrology, lithology, and physiography of the basins.

In addition, a vertical complexity of the alluvial suites has been also established, underlining that the overall braided-stream aggradation Savelli et al. Two major findings are worth to be here emphasised in this regard. Such lithofacies, although rather discontinuous because repeated cut-and-fill processes Calderoni et al.

A: Close to the present river mouth. B: km upstream from river mouth. C: km upstream from river mouth. Numbers refer to radiocarbon ages, expressed in ka BP Elmi et al. Vertical and horizontal scales are only indicative.

It simply states that major episodes of alluvial fan development in the northern Marche are cyclical fig. A progressive confinement of fans, locally so strong that no alluvial fan could develop in the youngest cycles Savelli et al. This accounts for enhancing rates of stream entrenchment, likely related to increased uplift rate Nesci et al. Alluvial fans lithology is greatly heterogeneous, depending on the geology of the drained area Nesci and Savelli, b according to what already recognised in different contexts e.

Specifically, O. Savelli b recognised two different fan-types, e. As a result, where along a trunk-valley a given major fill terrace consists of both alluvial fan and braided-river sediments fig. In this frame, also the remark by M. Coltorti , p. Moreover, where a complete alluvial suite occurs the undissected braided-fill is systematically topped by alluvial fan deposits e.

A: Upper Pleistocene terraced fan confined by topographic constraints arrows on the NE side of the Catria massif Acquaviva fan, after Savelli and Ballerini, B: Middle Pleistocene piedmont terrace-alluvium derived from the dissection of a previous glacis on the NE side of the Catria massif Serra S.

C: Stratigraphic superposition of alluvial-fan alluvium af on braided alluvium f of a trunk-valley; latest middle Pleistocene, Metauro valley downstream Urbania.

Such arrangement clearly indicates a progressive backfilling into the tributary catchment, thus highlighting an overall upstream shifting of the depositional loci. This behaviour, according to O. Significantly, in several upstream reaches of trunk-valleys alluvium supplied from lateral tributaries also occurs i. In this area deposits of the main stream are lacking or reduced to a very thin layer upon rock-cut straths Nesci et al.

Here, the strath terraces have also to be regarded as an equivalent of the cut-and-fill episodes characterising the downstream aggradation during full glacial stages. Pazzaglia , remains hitherto quite univestigated. Similarly to trunk-valley fills glacis and related deposits have been re-incised and terraced fig. As a result, remnants of ancient glacis at present rest high above the valley floors, either on the divides or along the hillslopes Savelli et al.

Further information in the text. Pour de plus amples explications, voir le texte. However, it is proved to be frequently stepped by unpaired both rock-cut and fill-strath terraces, commonly resulting in nearby terrace-patches at different heights without any tectonic displacement could be advocated.

Actually, an important downcutting stage occurred ever since Lateglacial-early Holocene Cilla et al. Savelli b, p. Minor straths lying at about the same height of the fluvial-fan transition have been thus generated, also fostered by meander shifting and highlighting a rapid re-adjustment of trunk-stream long-profiles fig. In this frame, according to O. Such interpretation accounts for fan dissection, which has been proved to be very effective in producing local altimetrical differentiation of terrace-heights fig.

A principal geomorphologic result was fig. Explanation in the text. After Nesci and Savelli, b, modified. The series of strath terraces generated in downcutting regime, realistically associates with fill-units better and better developed and thick approaching the present river mouths.

However, also in this case, no specific study or correlations is available yet for highlight chronologic-functional relations between alluvial bodies and straths. Conversely, some data exist about channel-pattern metamorphoses in recent times, also driven by anthropogenic controls Coltorti, In the near-coast offshore incision took most likely advantage of preceding highstand depositional topography i.

Talling , alone can explain an incision as deep as m. Incised valleys thus formed, deepening up to 30 m below present sealevel into pre-Quaternary bedrock Calderoni et al. Although possible mechanisms of incision have not been clearly stated yet, headward erosion e. This hypothesis can be also substantiated by a knick-zone likely one of the several expected along trunk-streams in pre-aggradation stages always noticeable close to the present shoreline at the base of the alluvial fill Nesci et al.

Furthermore, despite the uncertainty of any correlation, the deepening stage of incised-valleys sensu Dalrymple at al. The incised valleys were filled-up by an over m thick pile of upper Pleistocene-Holocene mostly alluvial deposits unconformably resting on bedrock Elmi et al. Accordingly, generalised climate cooling was the major forcing for aggradation of incised valleys Nesci et al. At least until the end of Upper Pleistocene-early Holocene the aggradation style was the same outlined for trunk-valleys: as for inland areas, mechanic drilling and electric geotechnical tests actually demonstrated that late Pleistocene-early Holocene deposition in downstream zones is mostly related with braided rivers Nesci et al.

Although interrupted by prolonged stages of stream downcutting and terrace formation, a thick aggradation of the lowermost sectors of trunk-valleys could then continue throughout the Holocene: up to over m thick alluvial suites were thus deposited Nesci et al. First T. However, for a long time this report remained the only citation of coastal-fans in the Marche region, until D. Di Bucci et al. Later on, O. The more or less pronounced fan-relief reflects primarily the geology of the river basin: G.

Some questions arise for the Esino valley, where despite relatively high amounts of resistant rocks in the catchment, no fan is detectable by surface surveying. It is indeed possible that no coastal fan formed in this area; it is likewise possible that huge amounts of later Holocene sediments Coltorti, , buried such landforms, perhaps controlled by local subsidence Elmi et al.

Coltorti Postglacial rise of sea level progressively destroyed the outer sectors of the fans, thus originating landward migrating wave-cut scarps that with the maximum Holocene ingression attained their overall present position fig.

These peculiar fans are common to all northern Marche valley-mouths: their origin, however, have not been proven yet. Seemingly, any tectonic control is to be excluded because of fan occurrence in basins of different lithology and physiography Bull ; nonetheless it remains unclear what the role climate and related parametres may have had.

Delimitating seaward previous fan remnants i. Noteworthy, in such regard also K. Pazzaglia , p. Later on, trunk streams generated new, rather narrow, alluvial plains largely related to the sedimentary construction of a coastal plain and consequent re-advance of the shoreline Coltorti, ; Elmi et al. The treads of already terraced-fans have been thus separated by more recent alluvial sediments that, where flattened fans occur Foglia, Misa , also partially buried some treads: however, such recent alluvial plains always merge into the coastal plain fig.

Moreover, immediately upslope the wave-cut scarps the outer fan margins also make transition to an older coastal plain fig. Since interglacial alluvial deposition dramatically increases seaward fig. Different conclusions arise from the northernmost sector unpublished , where such chains are missing, likely confirming for this sector a marked erosional coast retreat Veggiani, ; Elmi et al. The reported staircases consist of transitions from wide flat erosional surfaces to strath terraces and lastly to fill terraces.

Accordingly, also the onset of thick aggradational suites fig. However, since the definition MPR indicates a marked intensification and prolongation of glacial-interglacial cycles initiated between and ka e. In this concern, the upper Pleistocene-Holocene cycle provides suitable tools for deciphering stages and modes of older fill terrace formation. The principal morphologic elements reported in the text are shown. The authors would like to thank Paola Fredi and two other anonymous reviewers for their helpful comments and suggestions that enhanced the results reported in this manuscript and improved readability of the paper.

The authors are also grateful to the Editor-in-Chief G. A special thank to A. Alessio M. Geografia Fisica e Dinamica Quaternaria 2, Geografia Fisica e Dinamica Quaternaria 10, Alvarez W. Basin Research 11, Argnani A. Il Quaternario 10, Baldelli G. Bartolini C. In Smith B. Geological Society of London, Special Publication , Bernini M. Bigi G. Studi Geologici Camerti , Special Publication 1, Bisci C. In Centamore E. Blair T. Journal of Sedimentary Research A64, Blum M.

Sedimentology 47, Supplement 1, Boccaletti M. Bridgland D. Streams with a steep gradient slope have a faster velocity and greater competence. Streams channel water downhill under the influence of gravity. Stream capacity is a measure of the total sediment material other than water a stream can carry.

Stream competence reflects the ability of a stream to transport a particular size of particle e. The velocity of a river is determined by many factors, including the shape of its channel, the gradient of the slope that the river moves along, the volume of water that the river carries and the amount of friction caused by rough edges within the riverbed.

What three factors affect how fast a river flows and how much sediment it can erode? Both natural and human- caused factors affect the amount of erosion a stream may experience. Natural factors include the gradient or steepness of the streambed since that affects the speed of the flow of water.

Rainfall and snowmelt affect the amount of water in a stream as well as the speed of the flow. There are several factors that affect stream discharge. The velocity of the water affects it; faster water means more passes per second so more discharge. The width and depth of the river also affects it; a larger river at the same speed will have higher discharge. Most of the sediment in a stream is carried as suspended load. It does not contribute greatly to stream erosion, since it is not in frictional contact with the stream bed.

Bed load. Drainage Basins and Divides — Drainage systems develop in such a way as to efficiently move water off the land. Each stream in a drainage system drains a certain area, called a drainage basin.

In a single drainage basin, all water falling in the basin drains into the same stream. When a stream channel can no longer accommodate increased discharge it overflows its banks and a flood occurs. Anything smaller or larger requires a higher water velocity to be eroded and entrained in the flow.

The main reason for this is that small particles, and especially the tiny grains of clay, have a strong tendency to stick together, and so are difficult to erode from the stream bed.

As the particle size gets larger, the minimum flow velocity needed to erode the particle decreases, with the lowest flow velocity being 30 centimetres per second to erode a 0. To erode particles larger than 0. Slope gradient of the land — as slope increases, the water velocity increases, the particle size that the water can carry also increases, therefore the amount of erosion increases.

Gradient looks at how far the water drops over the distance the water actually flows. The gradient influences the velocity of the stream. The steeper the gradient, the higher will be the velocity if all other factors are held constant. Flood Erosion and Deposition: As flood waters rise, the slope of the stream as it flows to its base level e. Stream terraces form when streams carve downward into their floodplains, leaving discontinuous remnants of older floodplain surfaces as step-like benches along the sides of the valley.

Stream terraces are common throughout the Western United States. In the context of this discussion on the Mojave region, older surfaces represent flattened areas plateaus, mesa, uplands areas, hillside benches that are stable or isolated, neither experiencing significant rates of sediment buildup aggradation or down cutting by erosion.

These older surfaces may have no clear or obvious connection to a more modern drainage system in a particular area. Terraces and older surfaces preserve or display unique characteristic soil profiles or weathering characteristics because of their long-standing isolation from stream erosion. Many factors influence why streams episodically carve into their floodplains, forming stream terraces. Because stream terraces are typically widely distributed along steams throughout a region, changing climatic conditions are likely a most important contributing factor to their formation.

Streams broadened their floodplains when sediment supplies are high and down cutting by stream erosion is abated. In cool, wet periods, plants typically cover the landscape, and hence sediment supply is low; enhanced moisture increases stream flows, and streams draining mountainous regions will cut downward. During dry periods, plants don't provide enough cover to prevent intense erosion during infrequent storms.

As a result, high sediment yields may result in the backfilling of stream channels.