alluvial plains. Lacustrine-alluvial plains of the forest and forest-steppe zones Alluvial lowlands

Alluvial plains

plains formed as a result of the accumulative activity of large rivers at the site of extensive subsidence of the earth's crust. Composed from the surface by river deposits, the thickness of which reaches several tens and even hundreds of meters (Hungarian lowland, plains along the valleys of the Ganges and Po rivers).

Great Soviet Encyclopedia. - M.: Soviet Encyclopedia. 1969-1978 .

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Alluvium (lat. Alluvio alluvium, alluvium) deposits formed, moved and deposited by permanent and temporary watercourses in river valleys. Contents 1 Modern rivers with solid bottom runoff 1.1 Longitudinal bars ... Wikipedia

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Republic of India, state in South. Asia. Dr. ind. the name Sindhus from the name of the river Sindhu (modern traditional Indus). From him Avest., Other Persian. Hindu, further other Greek. and Latin. India, where Russian. India and similar names in other Europe. languages: English… … Geographic Encyclopedia

Alluvium- one of the most important genetic types of continental Quaternary deposits. It accumulates in river valleys by temporary or permanent channel flows, the largest volumes are formed during high waters and floods. The accumulative terraces and floodplains above the floodplain are composed of alluvium.

The main influence on the composition and occurrence of alluvium is provided by geological, geomorphological, and climatic factors. There are four genetic subtypes of alluvium: lowland rivers, lowland rivers of periglacial zones, mountain rivers and seasonally drying rivers.

The geological factor is manifested in the close relationship between the lithological and mineral composition of alluvium and the petrographic composition of bedrocks. The intensity of erosion and accumulation depends on the bedrock.

The geomorphological factor determines the hydrodynamic parameters of the river - the intensity of alluvial processes, the structural features of the river valley, the composition and occurrence of sediments, their texture, thickness, etc. depend on them.

The climate influences weathering processes, and consequently, the lithological composition of alluvium. In the circumpolar region, physical weathering dominates - sands predominate in the plain alluvium. In temperate latitudes, the role of chemical weathering is significant - the composition of alluvium is sandy or loamy. In the humid tropics, chemical weathering dominates - clayey and ferruginous alluvium accumulates. Watering, the regime and dynamics of the river, the composition and structure of alluvium depend on the climate. On the plains of the temperate zone, Quaternary alluvium was formed in the interglacial and Holocene periods, as well as during glaciations in the periglacial zone. River sediments here have a two-member structure: the lower horizons are composed of coarse periglacial alluvium, while the upper horizons are composed of finer modern sediments.

The lithological composition and textures of alluvium depend on the hydrodynamics of the river flow. In mountain rivers, with their huge living force of the current, only the largest fragments are deposited. Flat, slowly flowing rivers accumulate sand, sandy loam, and even loam. Coarse clastic material in the composition of alluvium is always rounded, and the shape of the pebbles is very different. The final shape that the pebble tends to take when rolling approaches a triaxial ellipsoid. With a slow flow, pebbles are deposited with a long axis perpendicular to the direction of flow. An increase in the transfer rate leads to the burial of pebbles with a long axis at an angle to the direction of movement. At maximum speeds corresponding to the transfer in suspension, the pebbles settle with their long axis parallel to the flow and the center of gravity down. Flat fragments at the same time lean towards the flow.

12. 1. Alluvial deposits of lowland rivers

Alluvial deposits of the plains, formed under thermogenic conditions, have a thickness of up to several tens of meters. In the areas of sheet glacier development, Holocene alluvium is almost completely confined to river floodplains, and is less common in the composition of the lowest floodplain terraces. Due to the inherited location of the river valleys, the above-floodplain terraces of most of the river valleys of Belarus are filled with periglacial alluvium.

There are three dynamic phases of accumulation of river sediments, and three dynamic type alluvium: instrative, perstrative and constructive.

Instructive(lined) alluvium accumulates at the stage of the youth of the river in the places of the expansion of the channel. It has a small thickness and insignificant sorting of fragments, therefore it is not divided into facies.

Perspective(lined)alluvium formed in the stage of maturity of the river valley. It is a typical plain alluvium, clearly divided into facies (Fig. 36).

Rice. 36. Scheme of the perstrative phase of river accumulation

(according to E. V. Shantser):

A- channel ( A 1 - channel shallow); IN- floodplain ( IN 1 - river bank); H– the level of hollow waters; h– low water level; M is the normal thickness of alluvium; I– zone of alluvial sedimentation, drawn by transverse circulation currents; II- zone of sedimentation of suspended sediments; 1 –3 – channel alluvium ( 1 – coarse-grained sands, gravel and pebbles; 2 – fine and fine-grained sands; 3 – layer of silting); 4 – floodplain alluvium; 5 – currents of transverse circulation in the channel; 6 – the direction of displacement of the channel trough during the accumulation of alluvium

constructive(spreadable)alluvium arises in the tectonic mode of subsidence of the earth's crust, when a thick alluvium layer compensating for this subsidence is accumulated (Fig. 37).

On the territory of Belarus, the thickness of the Holocene alluvium reaches 18 m; it is represented by three groups of facies: channel, floodplain, and oxbow (Fig. 38). There is a relationship between the age of river valleys and the proportion of these facies in the total volume of alluvium: the older the age, the higher the role of oxbow facies, the younger the age, the greater the proportion of floodplain deposits. In the young river valleys of the Baltic basin, the proportion of floodplain facies is increased, while that of oxbow facies is low. In the relatively ancient river valleys of Polesye, the role of oxbow facies is significant, occupying up to 25% of the volume of alluvium. In all valleys, channel facies predominate, making up from 1/2 to 2/3 of the volume of river sediments.

Rice. 37. Scheme of the structure of the stratum of constructive alluvium

(according to E. V. Shantser):

H– the level of hollow waters; h– low water level in the current riverbed; h 1 , h 2 – low-water level of oxbow lakes and secondary reservoirs of the floodplain; M is the normal thickness of alluvium during perstrative accumulation; M S is the total thickness of the constructive alluvial stratum; 1 – channel alluvium; 2 – old alluvium; 3 – floodplain alluvium; 4 – deposits of secondary reservoirs of the floodplain; 5 – the direction of displacement of the channel trough during the accumulation of construc- tively layered alluvial strata.

Rice. 38. Scheme of the structure of the floodplain of a flat river:

1 – channel facies of alluvium, 2 – floodplain alluvium facies, 3 – old alluvium facies, 4 - scree, 5 - deluvium, 6 - bedrocks 7 - the level of hollow waters.

The material composition of flat thermogenic alluvium is dominated by inequigranular sands. They are enriched in fine-grained sand and silty-clay particles. In the mineral composition of the alluvium of Belarusian rivers, according to E.A. Levkova and others, quartz dominates (80–95%), feldspars are represented in a much smaller volume (3–15%), and only fractions of a percent are accounted for by other minerals: carbonates, micas. The dependence of the mineral composition of alluvium on the composition of eroded rocks is manifested. In the north of Belarus, where rivers erode glacial rocks enriched in carbonates and fragments of gneisses and granites, the proportion of carbonates (up to 0.5%) and micas (up to 0.2%) is increased in alluvium. In the alluvium of the Polesye rivers, an admixture of glauconite (0.1–0.3%) is significant due to the redeposition of glauconite sands of the Paleogene age.

Channel alluvium composes the bottom of the river, islands, spits, shallows. It accumulates at the highest flow velocities, is characterized by a relatively coarse composition, oblique or oblique wavy bedding. Oblique (diagonal) layering is typical with a constant slope of the layers and with rectilinear boundaries of the series. The composition of alluvium is closely related to the eroded rocks. Within the development of marginal glacial forms in the channel alluvium, lenses of pebble-gravel-sand material are observed, and in the areas of distribution of loess deposits, the alluvium is enriched in the silt fraction. Within each layer, vertically and horizontally, there are differences in the diameter of the fragments due to changes in hydrodynamics. Up the section, the particle size decreases, reflecting the weakening of the eroding and transport work of the river. The diameter of the grains decreases from the upper reaches of the river to the lower reaches. The composition of alluvium in the north and center of Belarus is dominated by uneven-grained gritty sands, the proportion of silty and clay particles is small. The alluvium of the Belarusian Polissya is dominated by fine sand, which occupies 55–80%, the proportion of silty and clay particles is up to 35% or more. Changes in dynamics are also imprinted in the textures: the slanting layering of the lower part of the layer, accumulated at high flow velocities, can be replaced at the top by wavy layering corresponding to lower velocities.

Due to differences in hydrodynamic conditions, channel alluvium is represented by a number of facies .

Core facies accumulates on the inner (rod) side of the reaches, in the zone of maximum speeds. The mechanical composition is rough and unstable - due to the rapid erosion and abrasion of the fragments.

Perluvial facies(residual alluvium) gravitates towards the midstream of the river. It is composed of coarse-grained sands with grains of gravel and small pebbles, from which smaller particles have been washed out by the current.

River bank facies in the section it overlies the deposits of the core facies and is distinguished by the homogeneity of cross-bedded sands. In the lower part of the shoal, silt sometimes accumulates on its surface.

rift facies formed in the straightened sections of the channel. Composed of cross-bedded sorted sands, sometimes with inclusions of gravel and small pebbles.

floodplain alluvium lies on top of the channel. It is deposited during river floods on the floodplain surface, where the depths and flow velocities are small, the lightest organic and fine mineral particles are transported and deposited, forming a wide range of floodplain facies . The latter are represented by organic-filled silty sands, sandy loams and loams, painted in different shades gray color, and forming horizontal layers And lenses. The surface of the floodplains of lowland rivers is distinguished by the complexity of the relief. This is due to channel migrations and uneven distribution of water flow velocities.

In the floodplain relief, three geomorphological parts are distinguished, replacing each other in the direction from the channel to the bedrock bank: the near-channel, central, and near-terrace floodplains. Riverside floodplain closely adjacent to the riverbed, composed of the alluvial channel facies, differs in the highest heights - it is represented by a riverside rampart, and in the floodplains of large rivers by a system of parallel riverbed ramparts (manes). Central floodplain occupies the largest area. Its surface, predominantly composed of a floodplain alluvial facies, is complicated by oxbow lake depressions and various kinds of elevations: ancient riverside ramparts, eolian forms, etc. Terraced floodplain differs in the smallest heights. Its surface is composed of the thinnest sediments: sorted silty sandy loams and loams, as well as peat in wetlands. Often there are small, heavily overgrown oxbow lakes and streams that have arisen in places where groundwater exits.

old alluvium accumulates in meanders cut off from the main channel - in its structure it bears signs of alluvial, lacustrine and, often, swamp deposits. The lower part of the section is formed by cross-bedded alluvial sands accumulated before the branching of the bend. Higher in the section, organic-rich silts and clays occur horizontally, which arose during the lacustrine stage of the development of the oxbow lake. In the uppermost part, one can find layers and lenses of peat that arose during the overgrowth and swamping of the reservoir.

Delta deposits occur in sea and lake estuaries. Deltas form when the bottom of a pool does not submerge; no tidal waves; there are no strong sea currents parallel to the shore; when the rate of accumulation at the mouth is higher than the rate of tectonic subsidence of the reservoir bottom. At the mouth, the speed of the river flow decreases, the brought material settles to the bottom, forming first an underwater fan, then, with continued accumulation, surface. The river clutters up the mouth section of the channel with debris, the water breaks through the dam, forming a new channel - a branching system of branches (channel) may appear, dividing the surface of the delta into islands. The branches develop in the same way as the main channel - all groups of plain alluvium facies participate in the structure of the deltas. The delta section includes layers and lenses of lacustrine or marine sediments. Under the influence of salty sea waters, mineral and organic colloids brought by the river coagulate and precipitate. This process is active during floods, when river waters are saturated with suspensions.

The texture of the delta deposits is cross-bedded. In contrast to typical channel alluvium, in the deltas oblique layers form series of considerable thickness. Due to the low flow velocity, the oblique puffs are bent in the shape of the letter S, and in the lower part of the layer they lie at a very gentle angle and smoothly articulate with the sole. Near the top, oblique layers are often either eroded and cut off by the base of the overlying layer, or replaced by relatively homogeneous coarse-grained material.

Arising as a result of the accumulative activity of large rivers. Particularly extensive alluvial plains arise when rivers wander in areas of tectonic subsidence. From the surface they are formed by river deposits (most often sands of various sizes), the thickness of which can reach up to several hundred meters (Indo-Gangetic plain, Congo depression, Hungarian lowland, and so on).

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An excerpt characterizing the Alluvial Plain

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ALLUVIAL PLAINS

plains, plains formed as a result of the accumulative activity of large rivers at the site of extensive subsidence of the earth's crust. Composed from the surface by river deposits, the thickness of which reaches several tens and even hundreds of meters (Hungarian lowland, plains along the valleys of the Ganges and Po rivers).

Great Soviet Encyclopedia, TSB. 2012