2010. Hydrological–Morphological Processes in the Mouth Area of the Orinoco (Venezuela). M. V. Mikhailova

ISSN 0097�8078, Water Resources, 2010, Vol. 37, No. 2, pp. 160–171. © Pleiades Publishing, Ltd., 2010.Original Russian Text © M.V. Mikhailova, 2010, published in Vodnye Resursy, 2010, Vol. 37, No. 2, pp. 164–175.

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INTRODUCTION

Orinoco R. mouth is among the least studied in theworld. Until the late 20th century, almost no publica�tions on this subject were available in the scientific lit�erature. Data on Orinoco delta in the Russian litera�ture on river mouths are still limited to a small sectionin [7]. However, the delta and the nearshore of theOrinoco have some geomorphological, hydrological,and ecological features associated with the peculiarregime of the river and the oceanic coastal zone. Thesefeatures are of scientific interest.

The interest to studying the hydrological–morpho�logical processes at Orinoco mouth has abruptly rosein recent years in connection with the large�scaleworks involving oil exploration and production. In thelate 20th century, the State Oil�and�Gas Company ofVenezuela (Petróleos de Venezuela) sponsored studiesof the Orinoco delta with the aim to obtain data onphysical processes in the delta, its geomorphology,geology, and ecosystems. The objective of this studywas to assess the present�day state of the delta and todevelop measures to reduce the effect of economicdevelopment of the delta (primarily, the explorationand production of oil) on its natural complex. Someresults of these studies are given in [8, 24, 25].

This paper generalizes data on Orinoco delta pub�lished abroad and poorly known by Russian research�ers. Special attention is focused on the description ofthe river mouth area as a specific geographic object,occupying an intermediate position between the riverbasin and the coastal zone of the Atlantic Ocean, andthe characteristic of the hydrological–morphologicalprocesses taking place in it.

GEOGRAPHIC–HYDROLOGICAL CHARACTERISTIC OF ORINICO BASIN

AND THE COASTAL ZONE OF THE OCEAN

Geography of the Orinoco River

The Orinoco, one of the largest rivers in the World,flows in Venezuela and Columbia (Fig. 1). Accordingto different data, its length is 2740 [3, 22], 2800 [20] km,and the basin area is 0.985 [10], 0.990 [20], 1.0 [3, 22],1.039 [12], and 1.1 [25] million km2. The river origi�nates from the southwestern part of Guayana Shield,on the western slopes of Sierra Parima Mountains atan elevation of 1047 m. Part of Orinoco basin lies inBrazil.

The river is commonly divided into the Upper,Middle, and Lower Orinoco. The Upper Orinoco is areach from the source to the inflow of the left tributaryGuaviare. From its source, the Orinoco flows west�northwestward. Downstream of Los Esmeralda town,the river is divided by a block of rocks into twobranches: the major (Orinoco proper) and a lateral (orthe Casiquiare River) branch, which flows southwest�ward and empties into the Rio Negro River, a large lefttributary of the Amazon. During spring flood in theOrinoco, part of the runoff of its upper reaches entersthe Rio Negro network, while during high�waterperiod in the Rio Negro basin, part of runoff from theupper reaches of this river flows through the Casiqui�are into the Orinoco. This situation exemplifies a rarenatural phenomenon—incomplete interception ofthe runoff of a large river by another large river (theAmazon). Downstream of the separation of theCasiquiare, the Orinoco turns northwestward andreceives the Ventuari tributary from the right. Next theriver flows westward. The Guaviare River empties

Hydrological–Morphological Processes in the Mouth Area of the Orinoco (Venezuela)

M. V. MikhailovaWater Problems Institute, Russian Academy of Sciences, ul. Gubkina 3, Moscow, 119333 Russia

Received February 17, 2009

Abstract—The peculiarities of the hydrological regime of the Orinoco River and the coastal zone of theAtlantic Ocean that affect the hydrological–morphological processes in the mouth area of the Orinoco Riverare considered. The major features of the delta water regime, including its inundation, runoff distribution overthe delta branches, water and sediment balance, and the processes of river and sea water mixing are described.Special attention is paid to the morphological processes at the Orinoco mouth (delta evolution and modernprocesses at its coastline).

Key words: river, ocean, mouth area, delta, water and sediment runoff, delta branches, estuary widenings.

DOI: 10.1134/S0097807810020041

WATER RESOURCES AND THE REGIME OF WATER BODIES

WATER RESOURCES Vol. 37 No. 2 2010

HYDROLOGICAL–MORPHOLOGICAL PROCESSES IN THE MOUTH AREA 161

from the right into the Orinoco near San Fernando deAtabapo town. The Middle Orinoco is the reach fromthe Guaviare mouth to the Maipures rapids. Down�stream of San Fernando de Atabapo town, the riverflows northward and becomes part of the boundarybetween Venezuela and Columbia. Here the riverforms numerous rapids and waterfalls. The major trib�utaries of the Orinoco in this reach are the Guayapo,Sipapo, Autana, and Cuao (right), Vichada, and Tomo(left). The Lower Orinoco is the reach from Maipuresrapids to the ocean. Here the river receives large lefttributaries the Meta, Sinaruco, Capanaparo, Arauca,and Apure. After the inflow of the Apure River, theOrinoco turns eastward, flows along the right marginof the Llanos Orinoco plains and receives the tributar�ies Cuchivero, Caura (right), Manapire, Suat?, Pao,and Caris (left). The tributary Caroni empties into theOrinoco from the right downstream of Ciudad Bolivar.

The Orinoco forms a large delta at its emptying intothe Atlantic Ocean.

Climate of the Basin

Orinoco basin lies in the tropical climate zone. Twodistinct seasons can be identified: the rainy season(May–November) and the dry season (December–April). Air temperature varies only slightly within theyear. The seasons differ mostly by the amount of pre�cipitation. Rainy season begins in the summer underthe effect of equatorial air masses, while dry tradewinds blow in winter. Mean monthly air temperaturesvary within the year from 27 to 30°С [21]. In coastalplains and the plains of Llanos Orinoco, the climate ishot and air humidity is high. Mean daily air tempera�ture in the delta (~26°C) is close to the mean annualvalue [8]. The mean annual precipitation in Orinoco

South

AmericaCARIBBEAN SEA

Venezuela

Orinoco

Columbua

Columbua Brazilia

Guy

ana

N

S

1

2

3

4

5

5

67

8

9

1011

12

1314

15

16

12

0 400 km

Fig. 1. Schematic map of the Orinoco basin according to [25]. Tributaries: (1) Ventuari, (2) Guaviare, (3) Vichada, (4) Tomo,(5) Sineruco, (6) Capanaparo, (7) Meta, (8) Arauca, (9) Apure, (10) Cuchivero, (11) Caura, (12) Caroni; hydrometric stationsin the lower reaches of the river: (13) Musinacio, (14) Ciudad Bolivar (Puente Angostura hydrometric section), (15) CiudadGuayana, (16) Guri Dam. (1) Basin boundary, (2) state boundary.

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basin varies from 1000 mm in its northern part to4000 mm in the southern part [21], the potential evap�oration is 1700 mm [13].

Hydrological Regime of the Orinoco

Rivers of Orinoco basin are mostly rain�fed. Theleft tributaries also receive glacier�derived nourish�ment, though its contribution to the water runoff is notlarge. Groundwater plays an appreciable role in rivers'runoff formation. Water reserves accumulated in aqui�fers during wet years become an important source ofriver nourishment during drier years. The high degreeof natural runoff regulation in Orinoco basin is due tothe presence of vast lowland areas, which are oftenwaterlogged and inundated by flood.

Water balance components for Orinoco basin are asfollows: precipitation is 1990 mm, runoff is 914 mm,evaporation is 1076 mm, and runoff coefficient is 0.46[3].

The literary data on the water runoff in the lowerreaches of the Orinoco are very contradictory. Themean Orinoco water flow at Ciudad Guayanatown upstream of the delta, according to [8, 21], is~36000 m3/s (1136 km3/year). According to [10], themean water flow at the mouth is 28857 m3/s(911 km3/year). According to [20], water runoff at themouth is 1100, and according to [17], it is1200 km3/year. Russian experts estimated water flowat Orinoco mouth at 914 [3] and 1010 [22] km3/year.

The longest series of runoff observations is availablefor Puente Angostura hydrometric section at CiudadBolivar gauging station (the basin area is 836000 km2)in the lower reaches of the Orinoco, 390 km from thesea [16]. The author used these data for evaluating themean annual water flow over 1925–1989 to obtain31100 m3/s (980 km3/year) (table). Some cyclic com�ponent was revealed in the long�term variations in theannual runoff. Phases of higher water abundance wererecorded in 1942–1956 and 1966–1989. Water runoffwas below the mean in 1925–1941 and 1957–1966.The largest mean annual water flow over the period1925–1989 was recorded in 1954 (37620 m3/s), 1981(37610), and 1943 (36900 m3/s), while its least valueswere recorded in 1926 (21600), 1974 (24800) and1965 (26500 m3/s). The ratio of the maximal meanannual water flow to the minimal one was as little as1.74, suggesting the low variability of annual runoff.According to [22], the variation coefficient of Orinocoannual runoff is low (0.15). No trends were found to

exist in long�term variations of the annual runoff.According to [12], the mean annual water runoff at thePuente Angostura hydrometric section is 984 and thatat Orinoco mouth is 1129 km3/year. If we assume thatriver runoff between Puente Angostura and the mouthincreases in proportiom to the increase in basin area(from 836 to 1000 thous. km2, i.e., by 19.6%), Orinocorunoff at the mouth, according to the author’s calcu�lations, will be ~1170 km3/year. By water runoff, theOrinoco ranks third after the Amazon and the Congo.

The largest tributaries of the Orinoco in terms ofrunoff are the Guaviare (mean water flow is 8200 m3/sor 259 km3/year), Caura (~3000 m3/s or94.7 km3/year), and Caroni (~4850 m3/s or153 km3/year). A chain of HPPs was constructed onthe Caroni: the Guri, Caruachi, Macagua�I,Macagua�II, and Macagua�III with a total capacity of15.9 million kW. These HPPs account for ~70% of theelectric power produced in Venezuela.

The water regime of the Orinoco is typical of sub�equatorial rivers: water flow increases during rainfloods and rapidly decreases during dry periods. Thewet season lasts from May to November; and the dryperiod, from December to April. The most water�abundant months in the lower reaches of the river areJuly, August, and September (a flood peak is com�monly recorded in late August–early September),while February, March, and April are the driestmonths (the minimal flow falls on late March–earlyApril). Mean monthly water flows in the lower reachesof the Orinoco at Musinacio hydrometric section(downstream of the mouth of the Caura with the basinarea of 787000 km2) varied from 1330 to 81100 m3/sover period 1970–1981, depending on the season ofthe year [24, 25].

According to long�term observational data atPuente Angostura hydrometric section (Table), July–October account for ~61% and the rest of the year(November–June), for 39% of the annual runoff. Thewater abundance is the largest in August (17.5) and theleast in March (1.9% of the annual runoff). The ratioof the mean runoff of these months is >9, suggestingthe strong nonuniform distribution of water flow overthe year. The mean monthly water flow commonlyattains its maximum in August (86000 m3/s in 1976),while its minimums are attained most often in Marchor April (3500–3900 m3 in 1926 and 1959, respec�tively) [16].

Variations in water levels during the year are alsovery wide. Large areas in the river basin are inundated

Annual distribution of the Orinoco water runoff at Puente Angostura hydrometric section over 1923–1989, according to [16]

Characteris�tic Jan. Feb. Mar. Apr. May June July Aug. Sep. Oct. Nov. Dec. July–

Oct.Nov.–June Year

Q,m3/s 13100 8830 7110 9130 19300 35800 53100 65300 62100 45900 31500 21200 22600 14600 31100

% 3.5 2.4 1.9 2.4 5.2 9.6 14.3 17.5 16.7 12.3 8.5 5.7 60.8 39.2 100.0



during the rain season. Measurements at Mucinaciogauging station show that the difference between waterlevels in the dry and flood periods reaches 17 m [21].

The mean value of seasonal variations in water levelat Ciudad Bolivar gauging station 190 km upstream ofthe delta head is 14 m (Fig. 2). The lowest level isrecorded in March, and the highest level, in August.

The mean sediment runoff of the Orinoco variesfrom 150 to 212 million t/year according to differentsources [8, 18, 25]. The major portion of sediments(up to 90%) forms in the basins of the left tributaries ofthe river. According to [18], the regulation of sometributaries has resulted in a decrease of Orinoco sedi�ment runoff from 210 [19] to 150 million t/year. Themean water turbidity in the lower reaches has droppedfrom 170 to 130 g/m3. The runoff of dissolved solids inthe Orinoco is 28.0 [20] or 28.6 [8] million t/year.

Seasonal variations in suspended sediment runoffand water turbidity in the lower reaches of the Orinocodiffer from such variations in most major rain�fed riv�ers of the world. While the Amazon, Ganges, Brah�maputra, Mekong, etc. feature one peak in sedimentdischarge per year, the Orinoco has two such peaks(Fig. 2). The first peak (as in many other rivers) isrecorded at the rise of flood (in April–May in thiscase), and the second peak (not typical of most otherrivers) is attained during flood recession (October–November). During flood peak (August–September),sediment runoff attains its minimum. The secondminimum is recorded in the winter dry period (Janu�ary–March). According to [25], the unusual changesin the sediment content of Orinoco water are due tothe large masses of sediments that are deposited in thechannel and on the floodplain in the end of the previ�ous flood and enter the river during the followingflood. These are supplemented by the sediments thathave accumulated on river banks under the effect ofwinds during dry winter season. Water level rise in themain river during flood peak creates backwater levelrise in numerous tributaries, resulting in the inunda�tion of vast areas. This causes large�scale accumula�tion of sediments and reduces their input into the mainriver. The backwater effect declines during flood reces�sion, and the previously accumulated deposits areagain involved in the motion, thus forming a secondpeak in sediment discharge. During the dry season, 30or 40% of bottom sediments may dry out and startmoving under the effect of strong northeastern winds.Such deposits will be washed out into rivers during therise of the next flood.

Hydrological Regime of the Coastal Oceanic Zone

The long�term rise in the Atlantic level is similar tothe level rise of the entire World Ocean. According to[9], they averaged 1.5–2.0 mm/year in the 20th cen�tury, while in the late 20th–early 21st century, theyrose to 3.0–3.5 mm/year. According to forecasts [9],the ocean level can rise by more than 1 m by the end of

the 21st century. Variations in the mean water levelover the year in the ocean near Orinoco mouth do notexceed 4 cm [2].

The tides in Orinoco nearshore are semidiurnal.According to [11], the mean range of tides is 1.8 m.According to [8, 23], the ranges of tides in the near�shore vary from 1.30–1.40 m at Pedernales town to1.60–1.87 m at the Boca Grande mouth. The largestrange of tides in the southern part of the coastal zoneis 2.9 m [2].

Eastern trade winds predominate in Orinoco near�shore during the year [8]. The wide and flat nearshorealong with individual storms and hurricanes facilitatethe formation of weak and moderate waves (meanwave height is 1.2 m [10]). According to data of [2], themean wave height and period are <1 m and 6 s, respec�tively, and their maximum values are 5.0 m in Februaryand 7.5 m in August and 12 s, respectively. Northeast�ern winds with high recurrence create permanentstorm surges in delta branches.

The permanent warm northwestern alongshoreGuayana Current with a speed of 50–75 cm/s in thespring and 25–40 cm/s in the autumn dominates onthe Orinoco shelf [14, 25]. The Guayana Currentannually transports ~200 million t of Amazon sedi�ments from southeast. This amount is comparablewith the sediment runoff of the Orinoco. Therefore,more than half the sediments accumulating on theOrinoco shelf have Amazon origin [8].

Air and water temperature in the Orinoco coastalzone are almost equal and vary only slightly within theyear (from 26 to 28°С on the average [2]). Water salin�ity at the outer boundary of the coastal zone variesfrom 34.5 in February (during the low�water period inthe Orinoco) to 32‰ in August (during flood) [2].

300 18

12

6

200

100

00Jan. Feb. Mar. Apr.May June July Aug. Sep. . Nov. Dec.

Months

s, g/m3 Н, m

1

2

Fig. 2. Annual variations in (1) mean monthly water levelH, m, above the mean sea level and (2) water turbidity s,g/m3, at Ciudad Bolivar gauging station according to [25].

Oct.

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HYDROGRAPHY AND LANDSCAPES OF ORINOCO MOUTH AREA

Hydrography

The Orinoco mouth area is of the estuarine–deltaictype. The delta occupies part of a vast coastal plain. Itis bounded by Guayana Shield from the south, theCoastal Range from the west, the Gulf of Paria andBoca de Serpientes from the north, and an opencoastal zone of the Atlantic Ocean from the east(Fig. 3). The delta has almost classic triangular shape.According to different data, the area of the Orinocodelta is ~20000 [8], ~22000 [24], 29640 [11] km2. Thedelta head (DH) lies near Barrancas town (~200 kmfrom the ocean), where the river starts separating intodelta branches. The sources of dozens of deltabranches and many small distributaries are located inthe upper part of the delta. The southernmost andlargest branch is the Rio Grande. The branches of Ara�guao, Guiniquina, and Mariusa are located north ofthe Rio Grande (counterclockwise). Distributary sys�tems Cocuina, Pedernales, and Manamo are locatedin the northern part of the delta.

Many branches emptying into the Atlantic Oceanand the Gulf of Paria form mouths (estuarine widen�ings) with the same names. The Rio Grande branchforms the mouth Boca Grande, and other branchesform the mouths of Araguao, Guiniquina, Mariusa,Macareo, Cocuina, etc. (in Spanish maps, they arecalled Boca de Araguao, Boca de Guiniquina, Baca de

Mariusa, Boca de Macareo, Boca de Cocuina, etc.).The hydrographic network also includes the lowerreaches of lateral watercourses flowing from theCoastal Range: the Amana, Guanipa, Tigre, and Mor�ichal Largo. The Tigre, and Morichal Largo water�courses merge with the northernmost major branchand empty into the Gulf of Paria.

The Orinoco delta coastline (DC) >200 km inlength extends from Pedernales town on the shore ofthe Gulf of Paria in the northwest to the Boca Grandemouth in the southeast. The left part of the Orinoconearshore is the semiclosed Gulf of Paria, and its rightpart is the open coastal zone of the Atlantic Ocean orthe Orinoco shelf. The Gulf of Paria (the maximaldepth is 40 m) is separated from the ocean by TrinidadIsland and is connected with the ocean by two narrowstraits: Boca de Serpientes in the south and Bocas delDragon in the north [24]. The Orinoco shelf is up to30 km in width, and its slope varies from 0.02 to 0.05%[25]. A navigation channel, starting in the BocaGrande mouth and connecting the Orinoco with theocean, cuts the shelf. The channel is 120 m in widthand more than 10 m in depth [14].

The researchers of the Orinoco identify two majorelements of the delta channel network: first, the majordelta branches or major distributaries [8] (or river dis�tributaries [23], or muddy distributaries [24]) and,second, caños [8] or small distributaries. The cañosare mostly confined to the left part of the delta; whilethe major branches, to its right part. The majorbranches are directly connected with the river, transferthe major portion of river water and sediments into theocean, and flow in well defined sand channels. In [8]the major branches are referred to as fluvial�domi�nated distributaries. These include the Araguao, Mar�iusa, Macareo, and Manamo (before regulation). In[8] the Rio Grande is also identified as a major branch(distributary). The major branches are 0.5 to 1.0 km inwidth and 10–20 m in depth. The channel bed is cov�ered by deposits from silty sand to medium sand. Thechannel is bordered by natural levees 3–5 m in heightand 100–200 m in width, composed of silty sand andsandy silt. The major branches are often referred to asbrown�water rivers [23]. Small breakthroughs withfan�like channel network branch off from the Man�amo and Macareo branches in their upper reaches.Each “fan” covers an area of up to 5 km2 and consistsof brown and grey silty sand (the layer thickness is upto 5 m). The caños (small distributaries) include thewatercourses of Tucupita, Pedernales, Cocuina,Caiguara, etc. They receive small amounts of riverwater and sediments, but they drain interdistributaryplain basins, which are rich in organic matter. Distrib�utary water is rich in humic acids and low in mineralmatter. The small distributaries are referred to asblack�water caños or black�water streams [8, 23, 24].The caños are 100–200 m in width and 5 to 10 m indepth. Their banks are covered by vegetation.

Gulf of Paria

Atlantic OceanTrinidad Isl.

Tucupita T.

Barrancas T.

Rio Grande Branch

0 100 km

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2

34

56

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310

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Fig. 3. Schematic map of the Orinoco delta according to[25]. Delta branches: (1) Araguao, (2) Guiniquina,(3) Mariusa, (4) Macareo, (5) Cocuina, (6) Capure,(7) Pedernales, (8) Manamo; lateral tributaries: (9) Mor�ichal Largo, (10) Tigre, (11) Guanipa; capes (12) PuntaPedernales, (13) Punta Pescadores; straits: (14) Boca deSerpientes, (15) Bocas del Dragon; DC zones: (16) fluvial�tidal�dominated zones, (17) littoral�current�dominatedzones, (18) navigational canal at the Boca Grande mouth;(19) Volcàn dam.



Near DC, the major branches and caños featurelesser meandering than in the upper part of the delta,they become wider, divide up, and deviate northwest�ward in the northwestern part of the delta. In somecases, watercourses near the DC more often join thandivide up (e.g., the Capure and Cocuina caños). Over�bank flooding becomes more frequent toward the sea�shore. This is due to a decrease in the height and widthof the natural levees. At the same time, the sand con�tent of the natural levees decreases, while that of siltand peat increases. The banks of the major branchesnear the DC are covered by red mangroves (Rhizo�phora) [8].

Delta Landscapes

The delta landscapes identified in [8] include flu�vial�dominated interdistributary flood basins, fluvial�marine transitional environments, and marine�influ�enced coastal environments.

The interdistributary flood basins, covered by for�ests and bogs, occupy the major portion of the deltaplain. These areas are seasonally or permanently inun�dated because of overbank flooding and the effect oftides and local precipitation. The upper part of thedelta plain is mostly covered by bogs that have formedas the result of forest cutting in the delta and its sea�sonal inundation. The substrate of the interdistribu�tary flood basins is grey silt, dark grey organic silt, andsmall amounts of peat and peaty clay. The interdistrib�utary flood basins in the near�sea part of the delta plainare permanently moistened because of large rainfall;they are covered by forest and bogs [8, 24].

The fluvial�marine transitional environments (10–20 km in width) in the lower part of the delta includethe zone of bogs and swamps extending northwest.These areas are often inundated during rains and, to alesser extent, during tides. Some bogs, underlain by a10�m�thick bed of peat and humus clay, cover areas of>200 km2.

The marine influenced coastal environments areparallel to the marine coastline and extend into theland over distances of up to 20 km. These areas includetidal channels and silty awash areas, thick mangroves,and forested and grassy bogs. Except for narrow sandbeaches, these areas are permanently either inundatedor waterlogged by tides and rains.

HYDROLOGICAL PROCESSES IN THE ORINOCO MOUTH AREA

Water Regime of the Delta

As shown above, the Orinoco DH receives waterrunoff averaging 1170 km3/year. The year�to�yearvariations in river runoff are small; therefore, the deltawater regime, depending mostly on seasonal variationsin river water runoff, generally varies only slightly fromyear to year. The delta water regime experiences the

influence of not only the river water runoff, but also oftides, local rainfall, and land subsidence.

At the Ciudad Bolivar gauging station, about190 km upstream of the DH and 390 km from theocean, the range of seasonal variations in water levelaverages 14 m: the level falls to the mark of 3 m abovethe mean sea level (m.s.l.), and in flood peak itincreases to 17 m above m.s.l. (Fig. 2).

The extrapolation of these variations further down�stream yields the range of seasonal level variations inDH of 8–10 m. The seasonal variations within thedelta decrease toward the DC to 1.8–2.0 m (the rangeof tidal variations [25]).

The relatively wide variations in water levels withinthe delta during the year, the low relief of its surface,the dense channel network, and the lack of artificiallevees determine the permanent waterlogging of thedelta. According to [25], >80% of delta area is alwaysinundated, thus significantly limiting the economicdevelopment of the delta.

The effect of the permanently inundated delta onriver water discharge into the ocean can be evaluatedfrom delta water balance. According to [5], under theconditions of sufficient, and even more so, excessivemoistening in the deltas of large rivers (the Amazon,Mekong, Niger, and others), the precipitation beinglarger than evaporation results in additional supply ofwater, because of which the ocean receives more waterthan that transported by the river into the delta.According to [5], runoff losses due to evaporation inmoistened delta are near the potential evaporation.

The first attempt to evaluate the Orinoco deltawater balance was made in [5]. According to [1], theannual rainfall in the Orinoco delta varies from1200 mm in the west to 1600 mm in the east with themean of 1400 mm. The potential evaporation wastaken equal to 1300 mm/year according to [1]. Thus,the difference between precipitation and water lossesthrough evaporation is 100 mm/year, and the addi�tional water discharge at the delta area assumed to be24500 km2 is 2.45 km3/year, or 0.22% of the annualriver runoff (1130 km3/year).

New data on rainfall within the Orinoco deltaallowed the author to more accurately estimate theadditional runoff that forms within the delta. Accord�ing to [8], the mean annual rainfall in the deltaincreases from 1500 mm in its western part to2000 mm in the eastern part. If we take the meanannual rainfall in the delta to be 1750 mm, and thepotential evaporation, according to [1], to vary from1250 to 1500 mm (with the mean of 1375 mm), theimproved estimate of the rainfall–evaporation differ�ence becomes 1750 – 1375 = 375 mm. With the deltaarea of 22000 km2 [25], the additional runoff will be8.25 km3/year or 0.71% of the total river runoff(1170 km3/year). The obtained value is not large, lyingwithin the accuracy of the river runoff estimate. Thus,we can conclude that the amount of water discharge by

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the Orinoco into the ocean is about the same as thatentering the delta. The near�zero contribution of theOrinoco delta to the river runoff is due to the moisten�ing balancing at the edge between sufficient and insuf�ficient, when the ratio of potential evaporation z0 torainfall x is equal to 1. The value of z0/x ratio for theOrinoco delta is 0.78.

The maximum of seasonal inundation of theOrinoco delta falls on August–September. The maxi�mum of local rainfall, which also affects delta inunda�tion, is attained from May to October [8, 25].

While the effect of water level variations due to riverwater runoff decreases downstream toward the DC,the effect of tides increases in this direction anddecreases toward the river. According to [8, 23, 25],the range of tidal variations in water level decreasestoward the middle part of the delta from 1.8–2.0 to0.9–1.2 m. In the DH, this range averages 0.6 m andmanifests itself only during dry season. The extrapola�tion of these data upstream the river allows us toapproximately evaluate the maximal distance of tidepropagation into the river, which amounts to ~300 kmfrom the DC (100 km upstream of the DH).

Soil subsidence in the Orinoco delta is due to thecontinuing tectonic processes in the East VenezuelaBasin, as well as to the slow compaction of the bed ofHolocene deltaic deposits in the middle and lowerparts of the delta plain with a thickness of 30 and 70 m,respectively. Land subsidence in the delta leads to aslow rise in water level. This level rise adds to theeustatic rise in ocean level and can be large in deltas.Such effect was called the relative sea level rise.

The rate of land subsidence is estimated in [24] bythe formula

S = [SedTh – (SL + WD + El)]/A,where S is the rate of land subsidence, SedTh is sedi�ment thickness to dated horizon, SL is sea level (belowpresent mean sea level) at time of deposition, WD iswater depth at time of deposition, El is the delta�plainelevation where the core was recovered, A is the age ofthe dated horizon.

The estimates of land subsidence rate are com�monly based on radiocarbon dating of peat and largetree fragments in core samples. The land subsidencerates in the Orinoco delta were found to increase fromthe DH to the DC and vary along the latter. Thus, thesubsidence rate is 0.8–1.0 mm/year in the upper partof the delta and 0.8–2.0 in its middle part; it variesfrom 2.8 to >6.0 at the DC near Punta Pescadores andfrom 0 to 3.3 mm/year near the Guanipa mouth. Theland subsidence rate on the shore of the Gulf of Pariavaries from 2.2 to 4.6 mm/year. In [24], this differenceis attributed to a zone of active geological faults thatmay be located near the Guanipa mouth and PuntaPescadores or to the fact that different parts of thedelta feature different extent of dewatering, compac�tion, drying, and deformation of delta deposits. Therate of land subsidence in the Orinoco delta in many

cases is appreciably greater than the eustatic oceanlevel rise, which in the 20th century was ~1.5–2 mm/year [9]. The total effect of the eustatic oceanlevel rise and land subsidence on water level in thedelta may increase in the future.

Estimating the distribution of water runoff overOrinoco delta branches is hampered by the compli�cated structure of delta channel network and the per�manent inundation of its surface. Approximate dataon the character of water runoff distribution over deltabranches is given in [25]. The left branch Manamo(Fig. 3) was closed by the Volcán dam in 1965. Thedam was designed to protect Tucupita town and agri�cultural fields in the delta from floods and to raise thelevel in the adjacent Rio Grande branch to make itsdepth large enough for navigation. Two large leftbranches—the Manamo and the Macareo (Fig. 3)—before 1965 accounted for ~20% of total river waterrunoff in the DH, while the runoff was dividedbetween these branches in about the same parts. Theentire right�hand part of the delta received ~80% ofriver runoff in the DH. With the branching of numer�ous left distributaries from the main Rio Grandebranch, its runoff decreased from its beginning to themouth from 80 to ~15% of river runoff. The distribu�tion of runoff over delta watercourse has changed afterdamming the Manamo branch. The runoff in the headof the Rio Grande branch increased to ~85% and thatin the mouth, to 20%. The share of runoff in the Mac�areo branch, adjacent to the Manamo branch alsosomewhat increased.

No data are available on the seasonal changes inwater runoff distribution over delta branches; however,by analogy with hydrological processes in other largedeltas of the world [4], we can suppose that in dry peri�ods at low water level in the delta, the runoff concen�trates in the deepest branches, which have clear entriesand relatively free (moreover, artificially deepened)exits into the ocean. The opposite process takes placeduring floods—the runoff distributes over the deltaarea. The runoff share of the Rio Grande—the deep�est branch, artificially deepened at the exit to theocean—seems to radically increase during dry periodin the Orinoco delta.

Sediment balance in the Orinoco delta signifi�cantly differs from its water balance. According to[25], out of the mean annual amount of 150 million tof river sediments, entering the delta, ~50% is depos�ited within its boundaries, both in the branches and onthe surface, which is annually inundated by river water.At first sight, the 50% share of river sediments depos�ited in the delta appears to be an overestimation. How�ever, the comparison of these data with estimates ofsediment balance in other large deltas existing undernatural conditions (i.e., without artificial levees),shows that in such deltas, especially when they havelow relief, dense channel network, and many bogs andlakes, the share of retained sediments can be 50% ofriver sediment runoff and even more. Thus, the shares



of sediments deposited in the deltas of the Indigirka,Selenga, Ob, and Lena is 50, 58, 63, and 75%, respec�tively [6].

According to [25], sediment runoff distributionover delta branches (at least in their sources) is approx�imately proportional to water runoff distribution andhas changed in the appropriate manner after dammingthe Manamo branch. Nowadays, 85% of river sedi�ment runoff enters the Rio Grande branch [8]. Thesediment runoff in the Manamo branch decreasedfrom 25 million t/year to zero due to the constructionof the dam [24]. The sediment runoff in the lower partof this branch is partially compensated for by the sed�iments delivered by the Tigre and Morichal Largowatercourses, emptying into it.

Data on the distribution of sediments in the near�shore and the character of Orinoco bottom sedimentsare given in [14]. Bed soil samples were taken duringoceanographic studies in July 1971 in the BocaGrande mouth, in the navigation canal, and in theupper part of the shelf slope at depths of 1–20 m fromLa Salle research vessel. The results of sample analyseswere as follows:

—The Orinoco nearshore deposits are mostly afine mixture of silt and clay material with a small por�tion of sand (<10%). The sand in the upper outerboundary of the estuarine widening of the BocaGrande is composed of quartz particles mixed withsmall amounts of other minerals (feldspar and mica).

—Two types of deposits were identified: east of thenavigation canal, at depths of >3 m, where fine clayswith shell fragments predominate, and west of the nav�igation canal, in shallower areas with the alternation offine and coarse deposits, containing woody detritus,seeds, and other plant remains.

River and Sea Water Mixing Processes

The tidal estuaries at the mouths of Orinocobranches have a feature, which is typical of classicestuaries, i.e., seasonal and tidal upstream and down�stream migration of zones with the predominance ofdirect (on the surface) and reverse (at the bottom) cur�rents, averaged over the tidal cycle; the zones of riverand sea water mixing and salinity field; and the zone ofmaximal turbidity.

Measurements carried out during floods in 1971and 1974 at the mouth of the Rio Grande branch andalong the navigation canal, issuing from this branchand ending at the shelf edge, were used to study theprocesses of river and sea water interaction [14]. Thedynamic interaction and mixing of river and sea waterduring floods were found to follow the type of “saltwedge.” Such type is characteristic of the conditionswhere the effect of river runoff on the mouth processespredominates over the effect of tides [4, 15].

A distinct stratification was found to exist at themouth of the Rio Grande and in the navigation canal,

including three layers: the upper freshened layer withlow�turbidity water; the medium layer representing a“salt wedge”; and the bottom water layer, consisting offluid mud (Fig. 4).

Water salinity S on the surface was 0.2–0.5‰,reaching ~1‰ only near the shelf edge. The value ofsalinity S was 1–25‰ within the “wedge” and 30–35‰ at the bottom near the seaward end of the canal.The length of the “wedge” was 50–60 km.

The transition from the fluid mud to the “saltwedge” is rather sharp, but boundary between theupper freshened layer and “salt wedge” is more diffuse,though the halocline is well pronounced (Fig. 4a).Within this layer, the value of S varies from 5 to 30‰over a vertical distance of ~2 m. Thus, the vertical gra�dients of S in the halocline are large (~12.5‰ per1 m).

According to [4, 15], the”salt wedge” can be asso�ciated with a value of the so�called stratificationparameter n > 1, where n = ΔS/Smean,, ΔS = Sbott –Ssurf, Smean = 0.5(Sbott + Ssurf), Ssurf and Sbott are watersalinity values on the surface and at the bottom,respectively. The above�mentioned observational datashow the value of n for the major part of the “saltwedge”at the mouth of the Rio Grande to vary within1.8–2.0.

The difference between water temperature in thesurface and near�bottom layers is not large—from27.7 to 27.5°С (Fig. 4b). Therefore, the vertical distri�bution of water temperature has almost no effect onthe density stratification in the zone of the “wedge”.

The difference between water turbidity at differentdepths is greater (Fig. 4c). Water turbidity in the sur�face layer is 0.3–0.5, while at the bottom it abruptlyrises to 100–200 kg/m3. In the major portion of thewater body in the mixing zone of river and sea water,water turbidity is much greater than in the river or inthe ocean.

A layer with higher water turbidity (up to500 kg/m3) lies at the bottom of the navigation canal.This zone is up to 50 km in length, and the layer thick�ness is up to 6 m. Such layer is referred to as fluid mud.This layer is limited only to the navigation canal, whilewater turbidity in other parts of both the branch andthe shelf does not exceed 0.7 kg/m3. A layer with lowerturbidity is separated from the fluid mud layer by a dis�tinct interface, which can be clearly seen in soundinggraphs and is referred to as a double�bottom pattern. Azone of maximal water turbidity is located at a distanceof 40–50 km from the seaward end of the canal.

The authors of [14] have come to the conclusionthat the maximal�turbidity zone in the Boca Grandepractically coincides with the apex of the “salt wedge”near the so�called nodal point (or null point), wherethe flow velocity at the bottom, averaged over the tidalcycle, is nearly zero.

The sources of sediments in the Boca Grandewhich form fluid mud on the bottom are as follows: the

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4

8

12

16

10203040506070 0

0s, kg/m3

L, km

4

8

12

16

0T, °C

4

8

12

16

0

S, ‰

0.2

0.2 0.2 0.

5

5

10

1015

20

25 3035

20

15

1 5

27.7

27.5

27.5

27.5

27.5

27.5

27.5

27.3

27.027.7

28.0

27.5

27.727.5

27.527.5

27.5

27.0

27.5

27.7

0.3

0.3

0.5 0.

3

0.5 0.5

0.1 0.3

0.5 0.3

1.0100

400100200

300500

0.1

1.0

(a)

(b)

(c)

Fig. 4. The distribution of (a) water salinity S, (b) temperature T, and (c) turbidity s at Rio Grande branch mouth and in the nav�igation canal according to [14]. The dashed line is the upper surface of the fluid mud bed (double�bottom). L is the distance fromthe seaward end of the canal (mouth shelf edge) at a depth of 14 m.

deposition of river sediments, retained near the nodalpoint; sediments subject to secondary roiling seawardfrom the nodal point and transferred by the resultinglandward currents at the bottomd; marine sedimentsalso transferred landward.

According to [25], the flow of freshened and mod�erately turbid river water in the surface layer propa�gates far into the ocean above the seaward slope of theOrinoco shelf. This surface layer was called “buoyantsuspension.” This layer is only slightly affected by the



underlying alongshore northwestward current [25]. Asthe buoyant layer spreads westward, particulate matter(first coarser and next finer) deposits from it onto thebottom.

MORPHOLOGICAL PROCESSES IN THE ORINOCO DELTA

Evolution of the Orinoco Delta

The modern Orinoco delta is the latest formationamong the Tertiary and Quaternary deltas that haveprograded into the Eastern Venezuela Basin. The for�mation of the wide shelf in the Pleistocene had astrong effect on the structure of the modern shelf, thehydrodynamics of the coastal zone, and, accordingly,the formation of the modern delta. The modern sedi�mentation began when the postglacial rise in the oceanlevel slowed in the Early Holocene (~9500⎯6000 yearsago) [24]. In this period, river sediment inputexceeded the ability of the coastal and near�shore pro�cesses to remove river sediments from the mouth area.The river sediment runoff in the Early Holocene waslarge; a vast fluvial�dominated delta plain existed nearthe shore. The accumulation of sediments in the near�shore was considerable in that time.

In the Middle Holocene (~6000–3000 years ago),the delta plain continued extending, the slopes of itssurface decreased, and the effect of marine factors,such as tides, on it grew stronger. The seaward progra�dation of the delta narrowed the Boca de SerpientesStrait, thus dividing the East Venezuela Basin into twosubmarine accumulation zones. The input of Amazonsediments increased the input components of sedi�ment balance near the shore and on the shelf of theOrinoco.

The Orinoco delta plain continued extending inthe Late Holocene (~3000–1500 years ago). Thisreduced the alluvial sediment input into some parts ofthe delta plain, thus facilitating the formation of a peatlayer. The extension of the delta plain also changeddeltaic ecosystems subject to the joint effect of riverrunoff, land subsidence, local precipitation, tides, andmarine currents. The continuing filling of the EastVenezuela Basin by sediments and the narrowing ofthe Boca de Seprientes Strait progressively enhancedthe effect of the near�shore currents on the shelf on thedelta. This, in turn, caused the formation of mudcapesalong the shore and restricted sediment accumulationon the shelf bottom [24].

The Holocene climate in northeastern SouthAmerica was mostly tropical with distinct dry season.During the Holocene, the air temperature, rainfall,and wind regime featured considerable variations.Such climate changes had an appreciable effect on theriver and coastal processes, as well as on biota compo�sition and the structure and distribution of landscapeson the delta plain [24].

The wave and current regime along the Guyanacoast and Orinoco delta and the development of thedelta during Holocene were also subject to the effect ofthe alongshore Guayana Current, Amazon sedimentinput to the Orinoco shelf, and climate changes. Theresult was the formation of numerous specific reliefforms of the Orinoco delta plain.

The Holocene evolution of the Orinoco delta hassome features in common with the development of theNile and Danube deltas [24]. These features are as fol�lows: the delta consisted of several large brancheswithin a single cone�like accumulation center; therunoff of the large branches and their positions variedover time, but their progradation into the sea was gen�erally uniform without considerable regression cycles;sediment deposition zones were generally associatedwith tectonic�setting areas; alongshore current playedimportant delta�forming role along the shelf and theseashore; considerable progressing changes werefound to take place in the character of all deltasbecause of changes in the effect of river sediment run�off. As the result, the effect of marine processesbecame stronger.

Present�Day Morphological Processes

Present�day delta formation processes in theOrinoco mouth area include, first, large�scale deposi�tion of alluvial sediments on delta surface and in itsbranches; second, the slow progradation of mouthbars into the ocean at the mouths of largest deltabranches; third, coastal processes along the DC, asso�ciated with the effect of alongshore currents andwaves. Alluvial sediment accumulation processes,channel processes in the branches, and the formationof mouth bars at the Orinoco mouth have received verylittle attention.

The specific features of manifestation of coastalprocesses were considered in [8, 25]. Two major zoneswere identified in the Orinoco DC by the character ofcoastal processes (Fig. 3): the southern zone, wherethe influence of the river and tides predominates and apowerful freshwater flow, propagating from the deltaonto the shelf, has a strong effect on shore formation;and the northeastern, littoral�current�dominatedzone. The northwestward littoral currents play a signif�icant role in sediment transport and deposition alongthe shelf, in the formation of tidal silty awash areas andthe generally smooth, arc�like shape of the northerncoast of the delta [8]. Alongshore currents, along withwaves and tides govern the protrusion of the moderndelta into the ocean in some zones (e.g., the formationof silty protrusions of Punta Pescadores and PuntaMariusa in the northern part of the DC). The silty pro�trusions are accumulative formations extended alongthe DC in the Orinoco nearshore (mostly in the mid�dle and northwestern parts of the DC). These protru�sions are similar to the spits and barrier bars, but theformer are composed of silt, rather than sand. Silty

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protrusions are commonly located north or northwestof the mouths of delta branches. The sitly protrusionsare 5–10 km in width and extend along the shore overup to 100 km; they are often covered by mangroves[24]. In the Orinoco delta, they are similar to such for�mation on the shores of Guyana and Surinam.

CONCLUSIONS

The Orinoco mouth area is of the estuarine–deltaictype. The delta occupies part of the vast coastal plainand has almost classic triangular shape. The delta headlies at Barrancas town, where the river starts dividingup into branches. Researchers of the Orinoco identifytwo major elements of the delta channel network:major branches or major distributaries and ca?os orsmall distributaries. Many branches, emptying intothe Atlantic Ocean and the Gulf of Paria, form mouths(estuarine widenings) with the same names.

Water runoff through the Orinoco delta head aver�ages 1170 km3/year. In terms of runoff, the Orinocoranks third after the Amazon and Congo. The meansediment runoff of the river is now 150 million t/year.

Seasonal variations in river runoff, tides, and landsubsidence have the strongest effect on the delta waterregime. The Orinoco delta, which has a low relief,dense channel network, and many bogs, is subject toannual inundation, hampering its economic develop�ment. The maximal propagation length of tides intothe river reaches 300 km from the DC (100 kmupstream of the DH). The rate of land subsidence is0.8–1.0 mm/year in the upper part of the delta and0.8–2.0 mm/year in the middle part of the delta; alongthe DC, this characteristic varies from 0 to>6.0 mm/year. The land subsidence in the Orinocodelta notably exceeds the eustatic rise in the oceanlevel, which in the 20th century was ~1.5–2 mm/year.The tidal estuaries at the mouths of Orinoco branchesand the navigation canal have a feature, which is typi�cal of classic estuaries, i.e., seasonal and tidalupstream and downstream migration of zones with thepredominance of direct (on the surface) and reverse(at the bottom) currents, averaged over the tidal cycle;the zones of river and sea water mixing and salinityfield; and the zone of maximal turbidity.

The present�day delta�formation processes in theOrinoco mouth area include the large�scale deposi�tion of alluvial sediments on delta surface and in itsbranches and the slow progradation of mouth bars intothe ocean at the mouths of largest delta branches.Moreover, the alongshore currents in combinationwith waves and tides determine the progradation of themodern delta into the ocean in some zones (e.g., theformation of silty protrusions Punta Pescadores andPunta Mariusa in the northern part of the DC).

ACKNOWLEDGMENTS

This study was supported by the Russian Founda�tion for Basic Research, projects nos. 07�05�00406,08�05�00305.

REFERENCES

1. Atlas mirovogo vodnogo balansa. M.; (Atlas of WorldWater Balance), Leningrad: Gidrometeoizdat, 1974.

2. Atlas okeanov. Atlanticheskii i Indiiskii okeany (Atlas ofOceans: Atlantic and Indian Oceans), Leningrad: Glav.upr. navigatsii i okeanografii MO SSSR, 1977.

3. Mirovoi vodnyi balans i vodnye resursy Zemli (WorldWater Balance and Water Resources of the Earth), Len�ingrad: Gidrometeoizdat, 1974.

4. Mikhailov, V.N., Gidrologiya ust’ev rek (River MouthHydrology), Moscow: Mosk. Gos. Univ., 1998.

5. Mikhailov, V.N., The Impact of Deltas on the MeanLong�Term Water River Runoff, Vodn. Resur., 2004,vol. 31, no. 4, pp. 389⎯394 [Water Resour. (Engl.Transl.), vol. 31, no. 4, pp. 351⎯356].

6. Mikhailov, V.N., Magritskii, D.V., and Mikhailova, M.V.,Water and Sediment Balance at River Mouths, inMakkaveevskie chteniya�2005 (Makkaveev’s Read�ings), Moscow: Mosk. Gos. Univ., 2006, pp. 47⎯64.

7. Samoilov, I.V., Ust’ya rek (River Mouths), Moscow:Geografgiz, 1952.

8. Aslan, A., White, W.A., Warne, A.G., and Guevara, E.H.,Holocene Evolution of the Western Orinoco Delta,Venezuela, GSA Bull, 2003, vol. 115, no. 4,pp. 479⎯498.

9. Climate Change 2007. Synthesis Report. Summary forPolicymakers. http://www.ipcc.ch

10. Coleman J.M., Huh O.K. Major World Deltas. A Per�spective from Space. http://www.geol.lsu.edu

11. Coleman, J.M. and Wright, L.D., Modern River Del�tas: Variability of Processes and Sand Bodies, Deltas �models for exploration, Houston: Houston GeologicalSociety, 1975, pp. 99⎯149.

12. Dai, A. and Trenberth, K.E., Estimates of FreshwaterDischarge from Continents: Latitudinal and SeasonalVariations, J. Hydrometeorology, 2002, vol. 3,pp. 661⎯687.

13. Echezuria, H., Córdova, J., Gonzalez, M., et al.,Assessment of Environmental Changes in the OrinocoRiver Delta, Reg. Environ. Change, 2002, no. 3,pp. 20⎯35.

14. Eisma, D., van der Gaast, S.J., Martin, J.M., and Tho�mas, A.J., Suspended Matter and Bottom Deposits of theOrinoco Delta: Turbidity, Mineralogy and ElementaryComposition, Netherlands: J. Sea Res., 1978, vol. 12,pp. 224⎯251.

15. Guidelines on the Study of Seawater Intrusion IntoRivers, Studies and Reports in Hydrology, Paris:UNESCO, 1991.

16. http://grdc.bafg.de

17. Meade, R.H., River�Sediment Inputs to Major Deltas,Sea�Level Rise and Coastal Subsidence, Dordrecht:Kluwer Acad. Publ, 1996, pp. 63⎯85.



18. Meade, R.H., Suspended Sediments of the ModernAmazon and Orinoco Rivers, Quatern. Int., 1994,vol. 21, pp. 29⎯39.

19. Milliman, J.D. and Meade, R.H., World�Wide Deliv�ery of River Sediment to the Oceans, J. Geol., vol. 91,no. 1, pp. 1⎯21.

20. Milliman, J.D., Rutkowski, Ch., and Meybeck, M.,River Discharge to the Sea. A Global River Index(GLORI), LOICZ Reports and Studies, 1995.

21. Schot, P.P., Poot, A., Vonk, G.A., and Peeters, W.H.M.,A Surface Water Model for the Orinoco River Basin.Technical report // www.mk.geo.uu.nl.homepage/Paul/Orinoco_rap_ 9.pdf

22. Shiklomanov, I.A., Water Resources as a Challenge of theTwenty�First Century. Tenth IMO Lecture. WMO, 2003,no. 959.

23. Van Andel, Tj.H., The Orinoco Delta, J. SedimentaryPetrol., 1967, vol. 37, no. 2, pp. 297⎯310.

24. Warne, A.G. and Guevara, E.H., Aslan, A., et al., LateQuaternary Evolution of the Orinoco Delta, Venezuela,J. Coast. Res, 2002, vol. 18, no. 2, pp. 225⎯253.

25. Warne, A.G., Meade, R.H., White, W.A., et al.,Regional Controls on Geomorphology, Hydrology, andEcosystem Integrity in the Orinoco Delta, Venezuela,Geomorphology, 2002, vol. 44, pp. 273⎯307.

Documents

2010. Hydrological–Morphological Processes in the Mouth Area of the Orinoco (Venezuela). M. V. Mikhailova