Study of erosion, river-bed deformation and sediment transport in river basins as related to natural and man-made changes

Study of erosion,

river-bed deformation and sediment transport in river basins

as related to natural

and man-made changes

INTERNATIONAL HYDROLOGICAL PROGRAMME

Study of erosion, river bed

deformation and sediment transport in river bakins as related to natural and man-made changes

IHP-IV Project H-l -2

IHP-V 1 Technical Documents in Hydrology 1 No. 10 UNESCO, Paris, 1997

The designations employed and the presentation of material throughout the publication do not imply the expression of any opinion whatsoever on the part of UNESCO concerning the legal status

of any country, territory, city or of its authorities, or concerning the delimitation of its frontiers or boundaries.

CONTENTS

INTRODUCTION ^V

1. EROSION IN RIVER BASINS

Erosion and sediment yield in a changing environment

D. E. Walling 1

Watershed Management in China: Concepts and Techniques

Ding Lianzhen 54

The application of geographical information systems to soil conservation strategies

W. Summer and E. Klaghofer 77

Erosion and sediment yield on plains in the temperate zone A. P. Dedcov and V.I. Mozzherin

Improved methodology for the computation of normal annual yield of suspended sediments from rivers

N.N. Bobrovitskaya and KM. zubkova Erosion of cohesive materials

T. E. Mirtskhoulava

84

92 104

2. SEDIMENT TRANSPORT

Sediment non-uniformity effects on entrainment and transport K. G. Ranga Raju and M. K. Mittal

Sediment transport patterns observed in Southern African rivers A. Rooseboom

Theoretical premises for determining of bed scour and accretion areas A. N. Butakov

Sediment associated transport of Chernobyl radionuclides in the Pripyat river, Ukraine 0. Voitsekhovitch, V. Kanivets and V. Vishnevsky

Fluvial sediment transport in the arid regions of Central Asia H.A. Ismagilov

3. RIVER CHANNEL DYNAMICS

River response to natural and man-made change S. Bruk

The relation between river channel dimensions and discharge of water K. V. Grishanin

On the fractal sinuous&y of rivers

V. N. Nikora, D. M. Hicks and G.M. Smart Channel processes and their role in river ecosystems

R.S. Chalov and A.M. Alabayan

Impact of gravel excavation on channel processes in the Laba river, North Caucasus A.B. Shvidchenko and Z. D. Kopaliani

Changes of channel morphology of the Rioni river Central Caucasus 0. G. Khmaladze and O.D. Shautidze

Airborne data for river bed deformations study D. V. Snishchenko

116 135 143 155 165

174 195 205 216 227 239 245

4. RIVER CHANNEL DESIGN CONCEPTS AND APPLICATIONS Theory and practice of river channel processes

B.F. Snishchenko and Z.D. Kopaliani

Methodology for the inventory of channel processes for water projects B. F. Snishchenko

Changes in sediment transport and river engineering concepts, case study of the river Drau in Austria

H.M. Habersack and H. P. Nachtnebel

Improvement of the navigable width in river bends by periodic dredging, case study of the river Waal, The Netherlands

M. Tml, H. Barneveld and T. Swanenberg

Program for computation of channel deformations downstream of dams:

case study of the Votkinsk Hydropower Plant on the Kama river, Russia A. B. Veksler, V M. Donenberg, Y. L. Manuilov and R. S. Fried

Design of bank revetment based on reliability concept M. Bozinovic

5. MODELING STRATEGIES AND APPROACHES River modeling

G. Di Silvio

On methods for the prediction of river planform changes E. Mosselman

Prediction of long-term evolution of lowland river channels V. A. Bazilevich, V. V. Kozitsky and J. A. Gaiduchenko Hydromorphological aspect of channel process modeling

A. B. Klaven

Flume investigations into the influence of river training structures on sediment transport

S. Wieprecht, W. Bechteler A perspective of mobile bed river models

G. Glazik

251 268

277

287

298

313

322 345 354 361

368

380

. . .

111

INTRODUCTION

Erosion and sedimentation were represented in the Fourth Phase (1990-1995) of the International Hydrological Programme by IHP-IV Project H-l-2, entitled “Study of erosion, river bed deformation and sediment transport in river basins as related to natural and man made changes “.

For the execution of the Project, the IHP Intergovernmental Council set up a Working Group. Based on the proposals of IHP National Committees and NGOs, the following seven Working Group members were nominated (in alphabetical order): S. Bruk, G. Di Silvio, Ding Lianzhen, KG. Ranga Raju, A. Rooseboom, B.F. Snishchenko and D.E. Walling.

Coordination was provided by S. Bruk.

The Working Group met on 7 April 1992 during the International Symposium on River Sedimentation in Karlsruhe, in cooperation with the Fluvial Hydraulics Committee of the IAHR. The main principles of the project were then discussed and it was concluded that the main objective of the Project was to synthesize existing information on various aspects of erosion and sedimentation in different regions of the world, emphasizing those points in which differences in opinions and methods are particularly evident. The synthesis was expected to be achieved through exchange of views and discussions at international symposia.

In order to incite discussions and responses of the scientific community, it was requested that the Working Group members provide thought-provoking personal perspectives on the subject, rather than text-book type contributions or state-of-the-art reports.

The first opportunity for an exchange of views was offered by the international symposium, convened in the framework of IHP-IV Project H-l-2, entitled “East-West, North-South Encounter on the State-of-the-art in River Engineering Methods and Design Philosophies”. The Symposium took place on 16-20 May 1994 in the State Hydrological Institute, St.Petersburg, Russia, organized by the Russian IHP National Committee and UNESCO, and sponsored by IAHR, IAHS and UNEP. The Working Group members presented their contributions as special lectures at the Symposium.

The Proceedings of the St.Petersburg Symposium were published by the State Hydrological Institute, and comprise 75 papers, in three chapters:

A.

B.

C.

Physical processes and their elements in watersheds and rivers - 24 papers Modelling of alluvial processes and sediment transportation - 26 papers River response to hydraulic structures and case studies related to projects affecting sediment transport - 25 papers.

V

The Working Group met the second time during the Symposium. It decided then to include into the Technical Report a representative selection of the papers presented at the Symposium, along with the contributions of the Working Group members, bearing in mind that the Symposium Proceedings are practically accessible only to the participants, whereas the Technical Report will easily reach all interested researchers, through the regular channels of UNESCO.

The present publication is result of the above endeavours. The authors alone are responsible for any statements and opinions, contained in their contributions. The writers trust that their report will invite responses from researchers from various fields of activity in different regions of the World, in accordance with the objectives of the project.

Acknowledgement

Most authors took personal care to get their contributions into camera-ready form. Some of the papers, nevertheless, needed language editing and had to be re-typed. This task was accomplished through the efforts and competence of the Department of Fluvial Processes of the State Hydrological Institute, St.Petersburg, under the direction of Professor B.F.

Snishchenko.

Dr. Habib Zebidi, Programme Specialist, Division of Water Sciences, UNESCO, organized the activities of the Working Group, and took part in all its meetings. His contribution to the project was essential and highly valuable.

Dr. Stevan Bruk

vi

1. EROSION IN RIVER BASINS

EROSION AND SEDlHENTYIELD IN A CHANGING E3TVlRONMENT

D.E. Walling

Department of Geography, University of Exeter, Exeter, UK.

In a book published in 1976, Eckholm contended that " excess sediment is the major form of human-induced water pollution in the world today and exacts a heavier cost . . . . possibly more than all other pollutants combined."

Similar sentiments have been used to emphasise the importance of problems of loss of reservoir storage due to sedimentation (cf. Stevens, 1936; Dendy, 1968; Mahmood, 1987), the off-farm impact of eroded sediment (cf. Clark - et al., 1985>, the role of sediment in the transport of contaminants (cf. Allan, 1986; Novotny and Chesters, 1982) and various other environmental and

operational problems associated with enhanced suspended sediment transport by water courses (cf. Table 1). These problems have a very significant economic dimension, since, for example, Clark et al. (1985) estimated that the annual economic cost of off-farm sediment problems in the United States was of the order of US $6.1 billion at 1980 prices and similar calculations undertaken for South Africa by Braune and Looser (1989) have estimated the cost of the off-site damage caused by soil erosion to be of the order of US $36 million.

Both estimates exclude the less tangible environmental damage, which is extremely difficult to quantify, and are therefore likely to significantly underestimate the true cost. Mahmood (1987) has also estimated that, as a result of sedimentation, the major reservoirs of the world are currently losing storage at the rate of 1 % of gross capacity, or 50 km3, per year.

Viewed in terms of replacement costs, this loss is equivalent to an annual cost of $6 billion.

The problems associated with increased sediment loads in rivers consequent upon increased erosion within their drainage basins are now widely recognised and this recognition has been paralleled by growing evidence of greatly

increased rates of soil loss and sediment yield in many areas of the world as a result of human activity and particularly land use change. Table 2, for example, contrasts rates of soil erosion documented under natural undisturbed conditions with those occurring in cultivated areas. In all cases there is an order of magnitude increase, and in several instances the increases are even greater. The global implications of the data presented in Table 2 may be highlighted by recognising that over the past 200 years the area of the earth's surface given over to crop production and livestock grazing has increased by more than five-fold (Buringh and Dudal, 1987) and that the recent ISRIC / UNEP global survey of human-induced soil degradation (Oldeman et al., 1991) has shown that nearly 10% of the total land surface of the globe is currently adversely affected by water erosion. Table 3 provides further information on the degree of soil degradation associated with these areas and Table 4 indicates the relative importance of the major causative factors of soil degradation (including also wind erosion and chemical and physical damage) in different areas of the world. According to Table 4, deforestation, overgrazing and agricultural mismanagement represent the primary cause of >90% of the current global soil degradation.

A substantial proportion of the eroded material generated by this increase in both the incidence and intensity of water erosion finds its way into rivers and there are reports of greatly increased sediment yields in many parts of the world and particularly in developing countries. Figure 1, based on the work of Abernethy (1990), provides an example of the magnitude of the increase documented for several reservoir catchments in Southeast Asia. Data obtained from reservoir surveys undertaken at different times in the past have been used to indicate that sediment yields in these catchments which

2

Table 1 Potential physical impacts of increased suspended sediment loads in rivers.

A IN-STREAM EFFECTS

Biological impacts e.g. turbidity, sedimentaton, reduced productivity and species diversity.

Recreational impacts e.g. restriction of swimming, boating and fishing and reduction of overall aesthetics.

Sedimentation of chauels and water storage bodies e.g. reservoir

sedimentation, impairment of navigation, siltation of training structures.

Increased abrasion of hydraulic equipment e.g. REP turbines.

B OFF-STREAM EFFECTS

Flood damage e.g. aggradation, increased damage from muddy water.

Sedimentation of conveyance systems e.g. irrigation and drainage channels.

Increased cost of water treatment e.g. increased sedimentation times, clogging of filters.

Impairment of industrial water use e.g. reduced cooling efficiency, abrasion of pumps and turbines.

Sealing of irrigated soils Based on Walling (1989a)

Table 2 A comparison of soil erosion rates under natural undisturbed conditions and under cultivation in selected areas of the world.

Country Na ural

(kg m -5 year-l)

Cul$vated-1 (kg m year )

China < 0.20 15.00 - 20.00

USA 0.003 - 0.30 0.50 - 17.00

Ivory Coast 0.003 - 0.02 0.01 - 9.00

Nigeria 0.05 - 0.10 0.01 - 3.50

India 0.05 - 0.10 0.03 - 2.00

Belgium 0.01 - 0.05 0.30 - 3.00

UK 0.01 - 0.05 0.01 - 0.30

Based on Morgan (1979)

Table 3 Global human-induced soil degradation by water erosion (10 ha of terrain affected)

Tn= Light

Degree Total

Moderate Strong Extreme

Loss of topsoil 301 454 161 3.8 920

Terrain deformation 42 72 56 2.8 173

TOTAL 343 526 217 6.6 1093

Based on Oldeman et al. (1991)

Table 4 Factors cgntrolling global soil degradation.

(10 ha of terrain affected)

Continent Deforestation Overgrazing Agricultural Over- Bio-industrial Mismanagement exploitation

Africa 67 243 121 63 +

Asia 298 197 204 46 1

S America 100 68 64 12

N & C America 18 38 91 11 +

Europe 84 50 64 1 21

Australasia 12 83 8 +

WORLD 579 679 552 133 23

Based on Oldeman et al. (1991)

Table 5 Lake sediment-based evidence of increases in sediment flux due to catchment disturbance by human activity from tropical environments.

Lake Location Documented increase

in sediment flux

Source

Lake Patzcuaro Mexico x7 O'Hara et al. (1980)

Lake Sacnao Mexico x35 Deevey et al. (1979)

Lake Ipea Papua New Guinea x10 Oldfield et al. (1980)

Lac Azigza Morocco x5 Flower et al. (1989)

4

have experienced substantial land clearance and intensification of land use have evidenced annual rates of increase of between 2.5 and 6.0 percent.

Abernethy (1990) suggested that these increases closely paralleled the rates of population growth in the catchments concerned (cf. Figure 1, inset),

although the ratio of the rate of increase in sediment yield to that for the population was greater than unity. Based on this evidence he suggested that in many developing countries annual sediment yields are currently increasing at a rate equivalent to 1.6 times the rate of population increase and that sediment yields could be expected to double in about 20 years. Contrasting situations will, however, clearly exist in some other areas of the world where reservoirs now trap a major proportion of the sediment formerly transported by the rivers. In the classic case of the lower Nile,

construction of the Aswan High Dam has caused the annual sediment load at the mouth of this major river to decrease from c. 100 million tonnes to near zero.

With the current concern for global change and the impact of both climate change and human activity on the global system promoted by international

scientific programmes such as the IGBP, there is clearly a need to consider changes in erosion and sediment transport as a key component of such change.

These changes are important from a scientific and environmental viewpoint related, for example, to land degradation, terrestrial inputs to the oceans and global element budgets, but, as noted above, they also have important economic and management implications relating to river management, loss of valuable reservoir storage and water quality degradation. It has, for

example, frequently been suggested that the major flood disasters which have ravaged Bangladesh in recent years are, at least in part, the result of excessive sedimentation within the lower reaches of its rivers consequent upon land use change and increased erosion in the foothill and mountain regions upstream. This contribution attempts to review available evidence concerning the sensitivity of sediment yields to global change and the magnitude of the associated changes.

200.

100'

1910 1920 1930 1940 1950 1960 1970 1980 1990

Year

Figure 1 Trends of increasing sediment yields in selected reservoir catchments in Southeast Asia (based on Abernethy, 1990).

6

CHANGING SKDIMENT YIELDS

In comparison with other hydrological and hydrometeorological parameters, such as river floods and annual precipitation, information on the sensitivity of river basin sediment yields to environmental change is difficult to assemble, due to the general absence of reliable long-term records of

sediment yield in most areas of the world. Few, if any, records extend back beyond the present century and, in view of the many problems associated with obtaining accurate estimates of annual loads (cf. Walling and Webb, 1981), the reliability of early records is frequently open to question. Other sources of information must therefore also be exploited. These sources reflect a variety of timescales ranging from a long-term geological

perspective, through the evidence afforded by lake sediments which extends back over lo2 to lo3 years, to recent catchment experiments and attempts to use current data to provide a longer-term perspective by means of space-time substitution. The information furnished by these various sources of

information, including available long-term records, will be reviewed in turn.

RECONSTRUCTING PAST SEDIMENT YIEDS The long-term geological perspective

Information on the present-day distribution of sedimentary rocks of different ages on the earth's surface and their associated mass can provide a basis for estimating the.sedimentation rate at different periods in the past (cf.

Gregor, 1985). Since in broad terms the sedimentation rate can be equated with the global erosion rate, it is possible to derive estimates of global denudation rates in the past. These values will include both mechamical and chemical denudation, but because the two are closely related (cf. Walling and Webb, 1983), and the former dominates the total denudation rate, they can provide a useful indication of gross variations in global erosion rates over

the past 500 million years. Coupled with estimates of the area1 extent of the land area at different times in the geological past, these values can in turn provide estimates of changing rates of specific sediment yield over this period. This approach has been used by Tardy et al., (1989) to reconstruct the temporal pattern of global specific sediment yield over the past 500 million years depicted in Figure 2A. Figure 2A indicates that global specific

sediment yields have ranged between about 30 t km -2 -1 and 70 t km -2 year

year -1 during the geological past. These variations reflect fluctuations in the global climate, and more particularly in runoff amounts, as well as in vegetation cover, in the relief of the land masses and in tectonic activity.

The Cambrian, Devonian, Cretaceous and Tertiary periods were thus

characterised by relatively high sediment yields, whereas values in the Carboniferous, Permian and Triassic were substantially lower. The Devonian period ( c. 350-400 x lo6 years BP.) which is marked by relatively high sediment yields is known to have been a particularly wet period with high runoff rates.

Also of significance in Figure 2A is the trend of increasing sediment yields towards the present, which has been represented by an estimate of contemporary transport of sediment from the land surface of the globe to the oceans and which is characterised by values in excess of 100 t km -2 year -1 . Whilst these higher values are, at least in part, a reflection of the

increasing rates of tectonic activity that have prevailed since the end of the Jurassic period (c. 130 x lo6 years BP.) they must also reflect the influence of human activity and more particularly forest clearance and land use change in increasing sediment yields. The importance of these latter effects have been emphasised by Ye et al. (1983) in their study of rates of sedimentation on the Huang-Hai-Huai Plain in China by the Lower Yellow River. In this case annual sediment yields from the Loess Plateau were estimated to have more than doubled between the Middle Holocene and the

OROGENIC CYCLES

ALPINE ) HERCYNIAN 1 CALEDONIAN

Global sediment yields

Time ( lo6 Years B.P.)

600

5 lb 115 ^r ‘

Time (lo3 Years B.P.) The Umberumberka Catchment,

NS W, Australia

3000 6000

Time (Years B.P.)

0

Figure 2 Variations of sediment yield over geological time. Based (A) on Tardy et al. (1989), (B) on Degens et al. (1991), and (C) on Wasson and Galloway (1986).

present as a result of human activity within the region. Similarly, in their study of rates of Holocene sedimentation in the Yellow and East China Seas, which receive sediment from the Yellow River, Milliman et al. (1987)

estimated that the recent river input was approaching an order of magnitude greater than that existing in the early and Middle Holocene.

The estimates of past global sediment yields presented in Figure 2A serve to emphasise the 'natural' variability of sediment yields in response to long-term environmental change and more particularly changes in climate and tectonic activity. However, they also highlight the important role of human activity in causing the the total sediment yield from the land surface of the earth to increase approximately threefold relative to the geological background. More detailed information regarding fluctuations in sediment yields over the recent geological past may be usefully introduced by

considering two further case studies which have analysed long-term changes in deposition rates in sedimentary basins. The first relates to the Black Sea where detailed analysis of sediment cores (cf. Ross and Degens, 1974) have been used by Degens et al. (1976,1991) to reconstruct the record of sediment input from its ca. 2.3 x lo6 catchment area over the past 20000 years (cf.

Figure 2B). This record demonstrates that the sediment input to the Black Sea, and therefore sediment yields from its catchment area, were relatively low during the Weichselian glaciation. The sediment input increased

dramatically during the subsequent period of deglaciation in response to the increased runoff, the abundant sediment supplies exposed by the retreating ice and the lack of vegetation cover, but slowly declined towards the

Atlantic climatic optimum when a relatively dense vegetation cover would have existed. The significant increase in sediment inputs during the past 2000 years has been directly related to the impact of human activity, and more particularly deforestation and development of agriculture within the area, which has caused sediment yields to increase by a factor of about 3. The data

presented in Figure 2B thus afford a useful means of demonstrating the impact of human activity on sediment yields in this region, but they also emphasise that natural variations associated with periods of glaciation and

deglaciation may cause even greater changes in sediment yield.

The second case study relates to an investigation of sediment yields over the past 6000 years from the 420 km2 Umberumberka catchment near Broken Hill in western New South Wales, Australia, reported by Wasson and Galloway

(1986). This was based on an analysis of the development of the alluvial fan deposited where the stream flowed onto the Mundi Mundi plain. Lack of organic remains precluded high resolution dating, but the results presented in Figure 2C emphasise the importance of human activity in increasing sediment yields during the period following the first European settlement in about 1850. In addition, the data also evidence substantial shifts in sediment yields during the period prior to European settlement. These shifts were probably

climatically modulated, but they again emphasise the need to recognise that the natural system may be characterized by significant variations in sediment yield in response to climatic variability.

Although it is inevitably fraught with uncertainties, particularly as the timescale involved increases, reconstruction of temporal patterns of sediment yield during the geological past provides a useful mean of demonstrating both the long-term 'natural' variability of sediment yields and the importance of human activity in perturbing the system and causing increased sediment

yields. In the latter case, increases in sediment yield of the order of 2-3 times have been documented by several studies. In many situations such

increases will exceed the long-term 'natural' variability of the system, but in the example of the Black Sea cited above, the natural variability can be seen to be as high as an order of magnitude and thus to be substantially

greater than the shift caused by man-induced perturbations.

Evidence from lake sediments

Where lakes occur at the outlet of a drainage basin and trap a large

proportion of the sediment output, analysis of the sedimentary record can provide valuable evidence concerning past fluctuations in sediment loads. The Black Sea example cited above demonstrates the value of this approach for reconstructing long-term trends, but it is also capable of providing more detailed information on the magnitude and timing of changes in sediment inputs from the lake catchment over periods of lo2 and lo3 years.

Furthermore, where the catchment draining to the lake is relatively small in size it may be possible to relate the reconstructed record of sediment yield to documentary evidence regarding land use changes and other human impacts within the catchment. The reliability of the reconstructed record will depend on the accuracy of the core dating techniques employed, the number of cores and the accuracy of the core correlation procedures used to estimate the total volumes of sediment deposited during particular periods. Evidence of changing rates of deposition obtained from a single core can provide a basis for evaluating changes in the relative magnitude of sediment inputs through time, but multiple cores and core correlation techniques are an essential prerequisite for estimating the volumes of sediment involved and therefore the absolute values of sediment yield (cf. Dearing and Foster, 1986). Because of the increased temporal resolution associated with dating techniques applicable to the recent past (eg. Lead-210 dating) this approach to reconstructing the sediment yield record has been most widely applied to the historical period and particularly to periods of extending back several hundreds of years. Several examples of the evidence provided by this approach are presented in Figure 3 and these will be considered in turn.

The classic example of the Frains Lake catchment in Michigan, USA,

illustrated in Figure 3A is based on the work of Davis (1976). This lake is located at the outlet of a small 0.18 km2 drainage basin and a detailed

n

‘2 500 -

K

Frains Lake Catchment,

? 400 -

E

M ichigan, USA

24

LAND USE

a Forest

3 200- I

Crops

v3

n

1800 1850 1900 1950 2000

Year

a 20

z 2 5 10 .3 E a 0 I

G 4

Year

n

‘2 300

6, 1

Lake Havgtidssjb, S. Skane, Sweden

Estimates based on single core

Estimates based on core grid

2000

BC 1000 0 1000 2000

Year

F igure 3 Lake sediment-based evidence of historical trends in sediment

yields. Based (A) o n Davis (1975), (B) o n Dearing et al. (1981) a n d (C) o n Dearing et a1.(1987).

programme of sediment coring, core analysis and dating enabled the record of sediment yield for the past 200 years to be reconstructed. The

reconstructed sediment yield record shows low rates of sediment yield in pre-settlement times, rising by a factor of up to 70 with the onset of

settlement and agricultural clearance after 1830, and stabilizing after 1900 at a rate about 10 times the pre-settlement rate.

The reconstructed record of sediment yields from the 38 km2 catchment of Lyn Peris in North Wales, UK, illustrated in Figure 3B was generated by Dearing et al. (1981). Again substantial changes in sediment inputs to the

lake are apparent, with sediment yields increasing about 8-fold, from about 5 t km -2 -1

in the earliest period to about 40 t km -2 -1

year year in recent

years. These increases have been ascribed to the erosional impact of mining, quarrying, overgrazing and recent constructional activity in this upland catchment. Maximum and minimum estimates of sediment yield have been provided for the individual periods in order to take account of some of the

uncertainties involved in the calculations.

An example of a reconstructed record of sediment yield extending back over a longer period of several thousand years is provided by Figure 3C which is based on the work of Dearing et al. (1987) on Lake Havgardssjon, a small lake set in the hummocky moraine landscape of southern Skane, Sweden. In this study a grid of 47 cores was used to reconstruct the record of sediment yield for the period post-1550, and evidence from a single 4m core was used to extend the record on a more tentative basis back to 3050 BC. Estimates based on the single core have been ascribed precision limits in order to take account of uncertainties in sediment dating. Based on this record, it can be seen that sediment yields during the period 3OOO:SO BC were of the order of 25 t kmm2 year-', a level which is consistent with an area of essentially

undisturbed woodland. From about 50 BC sediment yields increased, rising to a peak of 86-250 t km -2 year -1 during the period AD 950-1300. This is again

consistent with the known history of the area, since this was a period of forest clearance, village establishment, and agricultural expansion,

following the introduction of the heavy-wheeled plough. The decrease in sediment yields in the subsequent period from AD 1300-1550 also coincides with the agrarian depression documented for many areas of N.W. Europe and with the climatic deterioration of the early part of the 'Little Ice Age'.

More recent increases in sediment yield in the period post-1550 again

correspond closely with the historical records which point to an expansion in the area under cultivation and therefore susceptible to higher rates of

surface runoff and erosion.

The examples of lake sediment-based reconstructions of sediment yield cited above relate to temperate environments in Europe and North America.

Similar investigations have been undertaken in other areas of the world where it has again proved possible to document changes in sediment yields

reflecting anthropogenic disturbance. For example, O'Hara et al. (1993) report results obtained from Lake Patzcuaro in the severely degraded landscape of the volcanic highlands of central Mexico which permitted

reconstruction of sediment inputs to the lake over the past 4000 years. The findings indicate that sediment yields increased more than five-fold as a result of extensive land clearance and were at least as high under the land management of the indigenous population as after the Spanish conquest of the region. These findings and those from several other studies in tropical

environments are summarised in Table 5. The trends evidenced in Figure 3 and Table 5 again demonstrate the sensitivity of sediment yield to land use change. This sensitivity is reflected in both the substantial gross increases evident from a comparison of sediment yields before and after human-induced land disturbance, and the variations reflecting particular phases of human activity.

15

THE EVIDENCE FRON UING-TRRM RECORDS

AS previously indicated, there is a general lack of reliable long-term records of suspended sediment load which can be used to analyse long-term trends in sediment yields and their controlling factors. Changes in sampling equipment, in sampling frequency and sampling protocols and in load

calculation procedures can all introduce uncertainties into the consistency and continuity of log-term records. In addition, major changes in river behaviour, such as produced by reservoirs or river regulation, may introduce further discontinuities into the records such that it becomes increasingly difficult to interpret measured loads in terms of the sediment yields from the upstream catchment. More work is required to collate available long-term records and to evaluate their potential for documenting long-term trends.

Several examples based on available analyses may, however, be usefully introduced.

Long-term records of suspended sediment load stretching back to the 1940s and 1950s are available for a number of rivers in the former Soviet Union and a general assessment of the trends exhibited by these data undertaken by Bobrovitskaya (1994) indicated that of the order of 70% of the rivers were characterised by non-stationary sediment load series. About 40% of the rivers evidenced decreases in sediment load which were in most cases the result of dam and reservoir construction and diversion of water for irrigation schemes.

The construction of the Krasnoyarsk power generation plant on the Yenesei river in 1967, for example, caused a decrease in sediment loads for a

distance of more than 900 km downstream. Similar decreases were recorded on the Ob, Don and Dneiper Rivers. Abstraction of water from the Kuban River for

irrigation schemes by the Nevinnomyssk and Great Stavropol Canals has

likewise caused the sediment loads of this river to decrease by 2-4 times. In contrast, ca. 30% of the rivers evidenced significant increases in sediment