Suspended sediment yield on the Earth:
basic regularities

 

A. P. DEDKOV, V.I. Mozzherin, A. V. GUSAROV

Department of Geography and Geoecology, Faculty of Geography and Ecology,
Kazan State University, Kreml’evskaya Str., 18, 420008 Kazan, Russia,
phone:  7 (843) 2315417 or 2315487

avgusarov@mail.ru

 

 

INTRODUCTION

In the last three decades of the 20th century a database on suspended sediment yield (SSY) and its controlling factors, which contains information for 4140 river basins of the Earth, was created by geomorphologists of Kazan State University (Russia). Factors determining the values of SSY are runoff, river basin area, relief height, rock and soil composition, the density and structure of vegetable cover and degree of anthropogenic (mainly agricultural) mastering of basin natural landscapes. According to N.I. Makkaveev (1955) and a number of the other researchers, the analysis of river sediment yield is the most objective and accurate method of estimation of erosion intensity.

It’s obvious, however, that the mass of sediments transported by river cannot be a measure for all erosion products in its basin. A considerable part of these products is accumulated on valley’s slopes, flood plains and river channel beds, and cannot carried outside the river basin. The portion of such accumulated products is not uniform depending on landscape-climatic, geological, geomorphic and anthropogenic conditions in river basins, and cannot be accurately determined. Therefore, the measurement of the mass of river sediments, mainly the suspended one, can be used only for the comparative estimation of erosion processes and, on the whole, mechanical denudation on various territories. Mainly specific SSY expressed in t km-2 year-1 is used to assess the erosion intensity in our investigations. Its value is very changeable because of changeability of river basin area, and this circumstance reduces the accuracy of the comparative estimation of erosion intensity. In order to some reduction of influence of this factor all the basic calculations have been made on two principal intervals of the basin areas with a boundary in 5000 km2. It’s conventionally accepted that this boundary divides the small and large river basins [Dedkov, Mozzherin, 1984].

THE MAIN THESISES OF RESEARCH RESULTS

Suspended sediment yield as a function of landscape zonality, relief and lithology

The first information on the river SSY on the Earth’s plains and mountains was published by authors a more 20 years ago [Dedkov, Mozzherin, 1984]. The increase of database allowed us to define more precisely the estimation of erosion intensity in different landscape zones. The zonal erosion depends on two main factors such as climatically stipulated runoff and human activity. On all continents, except for Asia, the distinct correlation between runoff and SSY is established. In Asia this correlation is broken by sharp differences in the degree of agricultural land use intensity of northern and southern parts of subcontinent. It’s known the climate and human activity determine also the character of vegetable cover which is natural protection of soils and grounds from erosion.

 

 

Figure 1. The average specific SSY (r) in various landscape zones of the plains of the Earth for present time (1) and pre-agricultural period (2): A – in large river basins, B – in small river basins; landscape zones:  1 – tundra, 2 – taiga, 3 – broad-leaved forests, 4 – forest-steppes,
5 – steppes, 6 – semi-deserts of temperate climatic zone, 6a – semi-desert of subtropical climatic zone, 7 – coastal area of Mediterranean Sea, 8 – subtropical steppes, 9 – subtropical forests,
10 – droughty savanna, 11 – typical savanna, 12 – tropical forest, 13 – equatorial forests

Analysis of SSY shows that the most intensive erosion on the plains of the Earth is characteristic for equatorial, tropical and subtropical climatic zones (excluding arid and semi-arid landscape zones). The values of specific SSY vary here mainly within a range of 100–300 t km-2 year-1 (fig. 1). The middle values of specific SSY (up to 100 t km-2 year-1) are determined for the temperate climatic zone, where runoff is almost by approx. 3.0 times less than in tropics. The minimum values of specific SSY (up to 30–40 t km-2 year-1) are in arid and semi-arid zones of different climatic zones and in subarctic climatic zone. In mountain regions the most intensive erosion and, on the whole, mechanical denudation are in the glacial zone (the average specific SSY is 1800 t km-2 year-1) and also in the subnival zone and the coastal area of the Mediterranean Sea (up to 1200 t km-2 year-1). Our results agree, on the whole, with those that obtained earlier by N.M. Strakhov [Strakhov, 1959] and F.Fournier [Fournier, 1960].

Table 1. The specific suspended sediment yield (t km-2 year-1) and degree of anthropogenic mastering of river basin landscapes in various altitude zones of the Earth

Altitude zone

Small river basins (<5000 km2)

Large river basins (>5000 km2)

A

N

r

R

r/R

N

r

R

r/R

Low plains

412

60

9.5

6.4

215

40

13

3.1

1.9

High plains

370

140

15

9.3

227

80

1.9

4.2

2.2

Plain basins of rivers with sources in mountains

298

148

24

6.2

223

156

40

3.9

2.1

Low mountains (up to 1500 m)

926

505

233

2.2

551

325

101

3.2

1.7

Middle mountains (1500 – 2500 m)

377

396

295

1.3

268

280

252

1.1

1.5

High mountains (more 2500 m)

181

423

411

1.03

144

541

403

1.3

1.1

N – a quantity of considered river basins, r – contemporary (natural + anthropogenic) specific SSY, R – natural (pre-agricultural) specific SSY, r/R – coefficient of anthropogenic increase of  specific SSY, A – the average mark of agricultural land use on a three-mark scale (1 – poor agricultural mastering of river basin landscapes (cultivated area < 30 %) or undisturbed landscapes here, 3 ‑ very significant agricultural mastering of river basin landscapes (cultivated area > 70 %))

 

The humid and semi-humid subtropical-tropical-equatorial maximum of erosion intensity and SSY has been expressed during the pre-agricultural period on the plains, but the average specific SSY did not exceed 100 t km-2 year-1 in any landscape zone (fig. 1). To it testifies contemporary SSY in river basins with undisturbed natural landscapes. Comparing it with the total SSY of all rivers we can estimate the degree of anthropogenic intensification of erosion in this or that landscape zone or region. According to our data the human activity intensified SSY on the plains of the Earth by 7.5 times in small river basins and by 3.5 times in large river basins. The basin area there is less it is more role of the anthropogenic factor in value of SSY. These data are rather underestimated than overestimated because the SSY in basins with slightly changed natural (but not undisturbed) landscapes was used in a number of cases to determine the pre-agricultural level of erosion intensity. The anthropogenic component of the contemporary erosion and SSY on the plains of most landscape zones (except for tundra, deserts and semi-deserts) exceeds significantly the natural component (fig. 1), particularly on the high plains (tab. 1).

In mountain regions the human activity increased the erosion intensity only by 1.6 times, and the natural component keeps, on the whole, the prevailing role. However, in the low mountains the anthropogenic factor increased erosion intensity more than 2.0 times. In the global scale the direct dependence of specific SSY on the relief height under both contemporary and natural pre-agricultural conditions is distinctly expressed (tab. 1).

On the plains as well as in mountains erosion is strongest in sedimentary rocks, than in crystal ones. On the whole, on the dry land of the Earth the specific SSY in basins composed by sedimentary rocks is by 2.4 times lager than that in the basins composed by crystal rocks, and by 1.4 times lager than that in the basins with mixed rocks. In the mountains these values increase, that testifies to strengthening selectivity of erosion intensity defined by coefficient of petrographic selectivity introduced by us [Dedkov, Mozzherin, 1984].

The tendencies of erosion intensity and suspended sediment yield on the Earth during the second half of the 20th century

The analysis of the long term time series of SSY observations shown that during the second half of the 20th century the dry land of the Earth was dominated by areas with mainly rising tendencies of erosion intensity and SSY changes (tab. 2).

 

Table 2. The areas (×106 km2) with different dominating tendencies of erosion intensity and suspended sediment yield changes in hemispheres of the Earth during the second half of the 20th century

Dominating tendency

Hemispheres of the Earth

northern

southern

Rising

34.02 (  34.7)

24.94 (  72.0)

Descending

26.83 (  27.4)

1.69 (    4.9)

Relatively permanent

21.30 (  21.8)

5.42 (  15.6)

No data area

15.79 (  16.1)

2.60 (    7.5)

Total area

97.97 (100.0)

34.65 (100.0)

In parentheses, %

 

The ratio of areas with different tendencies of erosion intensity and SSY changes in various regions and within different climatic zones of the Earth was uneven during this period. The majority of areas dominated by mainly rising tendencies of erosion intensity are located in the equatorial, subequatorial and tropical climatic zones. A lower number of such areas is located in climatic zones of the middle and high latitudes. The decrease of erosion intensity and SSY (descending tendencies) in river basins of arctic, subarctic and temperate climatic zones of the Northern Hemisphere is more considerable (fig. 2).

The temporal dynamics of anthropogenic (i.e. reforestation and deforestation, cultivation and grassing etc.) and hydro-climatic conditions are the main reasons of trend variations of erosion intensity and SSY changes on the Earth during the second half of the 20th century. The ratio of areas in which specific factors predominate is also different on the continents and in climatic zones [Gusarov, 2004; Dedkov, Gusarov, 2006].

 

The estimation of global suspended sediment yield from continents into the World Ocean

In the last years the sphere of our researches covers a problem of estimation of SSY from the whole dry land of the Earth into the World Ocean [Dedkov, Mozzherin, 2000; Dedkov, Gusarov, 2006]. The works of almost 30 researchers, published during the second half of the 20th century, arrived at very different estimates of the sediment mass carried by rivers into the World Ocean. Especially large differences in estimates (from 5.2×109 t year-1 [Corbel, 1964] to 51.1×109 t year-1 [Fournier, 1960]) is characteristic for works of the period 1950–1970 based on comparatively small numbers of observations. In the last two decades the range of estimates is narrowed considerably (tab. 3).

 

Figure 2. The relative structure of dry land areas with different dominating tendencies of erosion intensity and suspended sediment yield changes during the second half of the 20th century in various regions (upper graph) and climatic zones (lower graph) of the Earth (without no data areas); hemispheres: N – northern, S ‑ southern

 

The area of the Earth having a water flow into the World Ocean (area of external water flow) is 97.6×106 km2 (65.5% of total area of dry land of the Earth). The other 51.4×106 km2 (34.5%) is deprived of water flow into the World Ocean (area of internal water flow). From the total area of dry land with water flow into the World Ocean, only 52.6×106 km2 (53.9%) are provided with direct data on SSY. These data are for 330 rivers from our database with hydrological stations at river mouths or near to them. The total SSY into the World Ocean from all 330 rivers is approx. 10 248.7×106 t year-1, or 195 t km-2 year-1. It is noteworthy that this specific SSY is near to that obtained for 4140 hydrological stations of the Earth including the area of internal water flow (202 t km-2 year-1, including 70 t km-2 year-1 for the plains and 374 t km-2 year-1 for mountain regions). For the vast area of external water flow for which no direct data on SSY are available (45.0×106 km2, or 46.1% of all regions with water flow into the World Ocean), SSY was determined by data extrapolation from neighbouring river basins with hydrological stations, with corrections for relief, lithology, runoff, vegetable cover and land use intensity. The values of such corrections depended on earlier estab­lished dependences of SSY on the various natural and anthropogenic factors [Dedkov, Mozzherin, 1984; Lvovitch et al., 1991 and others]. The estimated SSY from areas where no data are available is 5219.9×106 t year-1, or 116 t km-2 year-1. Consequently, the SSY from such areas is considerably (by 1.7 times) below that from areas with direct SSY data. This may be explained by the fact that hydrological stations are located mainly in territories with large anthropogenic influences on SSY [Dedkov, 2004]. The total global suspended sediment yield from dry land into the World Ocean is estimated at approx. 15 469×106 t year-1. Similar results were obtained by Alekseev, Lisitcina (1974), 15.7×109 t year-1; Milliman (1991), 16.0×109 t year-1, Lvovitch et al. (1991); 14.9×109 t year-1; Walling and Webb (1996), 15.0×109 t year-1 and others.

 

Table 3. The characteristic of average estimates of global SSY on decades of the second half of 20th century

Decade, years

N

Average value of global SSY for a decade, × 109 t year-1

Coefficient of variation of estimates of global SSY for a decade, %

1951 – 1960

3

25.6

43.8

1961 – 1970

6

21.2

77.0

1971 – 1980

7

22.2

50.9

1981 – 1990

7

17.0

14.1

1991 – 2000

9

17.3

17.9

N – quantity of the considered works where estimates of global SSY are given

 

It’s known the contemporary total global SSY has two components: natural and anthropogenic. The natural component is formed by erosion in natural conditions not influenced by human activity. The anthropogenic component reflects greater erosion intensity and increase of SSY under the influence of human activity, mainly agricultural. To estimate the influence of the anthropogenic factors on SSY, all (4140) river basins of the dry land of the Earth in our database were divided into three principal categories of land use intensity. Unmastered or poorly mastered (cultivated area less than 30%) river basins were classed as Category I, the SSY of which is conventionally accepted as a natural (i.e. pre-agricultural) component. River basins with an intermediate degree of mastering (cultivated area from 30% to 70%) were classed as Category II, and with strong mastering as Category III (cultivated area more than 70%). The indices of reduction of specific SSY from Categories II and III to Category I (i.e. to almost the natural level) are adopted as the coefficients of anthropogenic transformation (CAT); CAT(II/I) and CAT(III/I), respectively. With the aid of these coefficients all basins of Categories II and III were compared with basins of Category I, i.e. with almost natural (pre-agricultural) levels of erosion intensity and SSY. Using this method the natural component of SSY from the whole dry land into the World Ocean is 5978×106 t year-1. The difference between the total global SSY into the World Ocean and its natural component is the value of the anthropogenic component, 9491×106 t year-1. Consequently, the anthropogenic component of SSY into the World Ocean exceeds the natural component by approx. 1.6 times, and the total global SSY into the oceans as a result of human activity increased by approx. 2.6 times. A number of other researchers, using other methods, came to the conclusion that anthropo­genic factors doubled the total global SSY (tab. 4).

Table 4. The some estimates of natural and anthropogenic components of contemporary global suspended sediment yield

Authors of estimates

Suspended sediment yield

natural component

anthropogenic component

total

× 109 t year-1

%

× 109 t year-1

%

× 109 t year-1

%

Bondarev, 1974

7.9

54

6.7

46

14.6

100

Milliman, Sivitsky, 1992

10.0

50

10.0

50

20.0

100

McLennan, 1993

12.6

60

8.4

40

21.0

100

Harrison, 1994

6.0

33

12.0

67

18.0

100

Dedkov, Mozzherin, 2000

6.0

39

9.5

61

15.5

100

 

Table 5. The contemporary SSY from various region of dry land into the World Ocean

 

Region of the Earth

Total SSY into the World Ocean

Natural component

AI

Specific SSY
(t km-2 year-1)

×106 t year-1

%

×106 t year-1

%

total

natural

Europe

579.6

3.8*

171.6

29.6**

3.4

69

20

Asia

9 132.7

59.0

3 525.2

38.6

2.6

337

130

Africa

1 043.1

6.7

314.0

30.1

3.3

57

17

North America

1 080.0

7.0

322.9

29.9

3.3

50

15

South America

1 238.4

8.0

993.7

80.2

1.2

72

58

Australia

164.6

1.1

54.8

33.3

3.0

44

15

Pacific islands

2 230.2

14.4

595.5

26.7

3.7

1770

458

Whole Earth

15 468.5

100.0

5 977.7

38.6

2.6

158

61

AI – coefficient of anthropogenic increase of SSY (times)

* portion of total global SSY into the World Ocean

** portion of total SSY into the World Ocean from this region

 

The largest suppliers of suspended sediments into the World Ocean are Asia and islands of west and southwest parts of the Pacific (tab. 5).

Direct and close dependence between runoff and SSY from continents is established on the whole. However, northern and southern parts of Asia and islands of the Pacific are exceptions here. The area of external water flow of the southeast part of Asia exceeds that in the northern part by 1.4 times, but its rivers transport by 95.6 times more suspended sediments into the World (7742×106 t year-1 and 81×106 t year-1, respectively). The specific SSY in the river basins of the southeast part exceed those in the north part by 110 times (approx. 990 t km-2 year-1 and 9 t km-2 year-1, respectively). Rivers with record masses of suspended sediments for the whole dry land of the Earth fall into the Pacific and Indian Oceans: the Yellow River and the Ganges with the Brakhmaputra – approx. 1×109 t year-1; while each of the large rivers of Siberia – the Ob, Enisei and Lena – less than 1×107 t year-1 into the Arctic Ocean. Such significant distinctions in SSY between the two parts of Asia are predictable. Favorable conditions for erosion intensity both natural (high relief, heavy atmospheric precipitation and large runoff, widespread easily eroded rocks) and anthropogenic (intensive and long established agricultural mastering of plains, the densest rural population on the Earth) combine in the southeast part of subcontinent. In the northern part of Asia the very small SSY into the World Ocean is related to the vast areas of low relief and comparatively poor agricultural mastering of natural landscapes of the tundra and taiga zones.

The large SSY in the island part of the Pacific is related to the heavy rains, high relief, small areas of river basins and considerable agricultural transformation of natural landscapes.

The variation of SSY on oceanic drain areas is presented in the tab. 6.

 

Table 6. SSY into various oceans of the Earth

Ocean

Drain area of ocean

SSY

×106 km2

%

× 106 t year-1

%

t km-2 year-1

Pacific

18.71

19.2

6250.9

40.4

334

Atlantic

45.26

46.3

3212.2

20.8

71

Indian

14.42

14.8

5706.4

36.9

396

Arctic

19.21

19.7

299.0

1.9

16

World Ocean

97.60

100.0

15468.5

100.0

158

 

Suspended sediment yield as function of river basin area

New dependences of the specific SSY on river basin area are determined. Until recently the inverse (negative) dependence of specific SSY on river basin area was considered for all basins as general. Our study shows that this dependence characterizes only river basins with prevailing basin erosion (soil and gully erosion) [Dedkov, Mozzherin, 1992; Dedkov, 2004]. The soil and gully erosion supply the large sediment mass into the river channel, and rivers cannot manage its transportation and intensively its accumulation, increasing of heights of flood plains in the valleys. The sediment mass downstream increases, but this increase occurs more slowly than the increase of river basin area. Therefore, the specific SSY decreases downstream in these rivers.

Other dependence characterizes rivers of different climatic zones with undisturbed or slightly disturbed natural vegetable cover in basins. The leading role in sediment formation in these basins belongs to the river channel erosion. The water discharges increase, as a rule, downstream these rivers more or less proportionally with the increase of basin areas, but SSY increases more considerably downstream the rivers. Therefore, in such river a typical direct dependence of SSY on basin area is forming.

The inverse dependence is not only the result of deforestation and cultivation in river basins. It’s also characteristic for river basins with natural landscapes without dense vegetable cover and prevalence of soil and gully erosion (droughty savanna and steppes, for example).

Together with the usual (integral) specific SSY a differential specific SSY (rd), characterizing the changes of specific SSY in basin between two hydrological (monitoring) stations on rivers is introduced into analysis (1):

rd = (SSY1 – SSY2)/(S1 – S2) t km-2 year-1 ,                            (1)

where index 1 and index 2 are refer to hydrological stations (with 1 downstream from 2), S – river basin area.

Differential specific SSY can have either positive or negative values. The latter indicates the prevalence of accumulation in such river basin. For successful interpretation of research results of changes of erosion and accumulation intensity the analysis of differential specific SSY is necessary for spending together with analysis of differential specific runoff in the same river basin (fig. 3).

 

 

Figure 3. Dependence of types of specific SSY (t km-2 year-1) and runoff (l s-1 km-2) on river basin area (km2): upper graphs for basin of  River Vym’ (north of the East-European plain) with cultivated area less than 30 %,  lower graphs for basin of River Tersa (south of the East-European plain) with cultivated area more than 70 %

 

Two principal types of erosion systems

Two principal types of erosion systems in river basins of plain rivers are distinguished: river channel and basin erosion systems. The difference between these systems is expressed in characteristic features of erosion processes, sediment transport and accumulation, the degree of transitivity of sediments and their amounts, the character of the dependence of specific SSY on river basin areas. The consequences are also the differences in structure, thickness and composition of alluvium, relationship of latter with slope sediments, the ratio between suspended and bed load sediments, some characteristic features of river valley morphology. Thus, steady establishment of a basin system results in increase of slope levelling, accumulation of river channel and slope sediments, heights of valley bottoms, in destruction of relief relict forms on slopes and so on. Both systems are linked by gradual transitions (a mixed (transitive) system) where the role of basin and river channel erosion is more or less balanced in formation of sediment yield.

During the Quaternary period the repeated changes of humid, periglacial and semi-arid climates promoted the formation of a complex of river terraces and changes of valley morphology on the whole.

 

Acknowledgements

This study was supported by Russian Foundation for Basic Research, RFBR (projects no.03-05-64896 and no.05-05-65001).

 

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