Geografia d'Europa: textos de suport

Introduction

This section focuses on European inland surface waters, that is, rivers, lakes and reservoirs. Considerable environmental information on rivers and lakes is currently collected and reported by various regional or national authorities. However, as the European continent covers about 10 million km² and there are several million kilometres of flowing waters and more than a million lakes, this information is very heterogeneous, and therefore difficult to collate on a pan-European basis. Nevertheless, a first attempt at doing so has been made in this section. The primary focus is on frequently measured water quality parameters (eg, organic matter in rivers, nutrients in rivers and lakes, and acidification of rivers and lakes) since the wide geographical coverage makes these variables well suited to illustrate the general environmental state of European inland surface waters.

Characteristics of European rivers, reservoirs and lakes

A river is a system comprising both the main course and all the tributaries that feed into it; the area that the river system drains is known as the catchment. The main characteristic of rivers is their continuous one-wayflow in response to gravity. In addition, because of changes in physical conditions such as slope and bedrock geology, rivers are dynamic and may change nature several times during their course (eg, from a fast-flowing mountain stream to a wide, deep, slowly flowing lowland river).

When assessing river characteristics and water quality it is important to bear in mind that a river comprises not only the main course, but also a vast number of tributaries. Thus although the main course of Europe's largest river, the Volga, is 3500 km long, it receives water from ten tributaries each longer than 500 km, and more than 151 000 tributaries each longer than 10 km (Fortunatov, 1979).

Rivers are greatly influenced by the characteristics of the catchment area. The climatic conditions influence the water flow, as does bedrock geology and soil type. The latter also affects the mineral content of the river water. Human activity affects river systems in numerous ways, for example, through afforestation or deforestation, urbanisation, agricultural development, land drainage, pollutant discharge, and flow regulation (dams, channelisation, etc). The lakes, reservoirs and wetlands in a river system attenuate the natural fluctuation in discharge and serve as settling tanks for material transported by the rivers. For example, whereas the water of the Rhine is very muddy and turbid when entering the Bodensee, it is clear and transparent when leaving. Water flow and water quality are therefore the net result of the various characteristics of the catchment.

Lakes are bodies of standing water that is usually fresh, but which may also be brackish. Although lakes may be characterised by physical features of the lake basin, such as lake area and water depth, the characteristics of the catchment are important when describing the lake environment. Nutrient loading of a lake is determined not only by the bedrock geology and soil type in the catchment, but also by the human activity.

Reservoirs are human-made lakes created to serve one or more purposes. As their water residence time is generally relatively short, and as the water level fluctuates much more widely and frequently than in natural lakes, they can be regarded as hybrids between rivers and lakes.

European rivers

Major European river catchments

In proportion to its land area, Europe has the longest coastline of all continents. As it is a relatively young and structured continent, geologically, river catchments are numerous but relatively small and rivers are short. About 70 European rivers have a catchment area exceeding 10 000 km², and only rivers arising deep inside the continent are relatively large. The three largest rivers in Europe, the Volga, the Danube and the Dnepr, drain one quarter of the continent, but are only small by world standards, their catchments ranking 14th, 29th and 48th, respectively (Showers, 1989).

The 31 largest European rivers, all of which have catchments exceeding 50 000 km², drain approximately two thirds of the continent. More than half of these rivers have their catchment area in the European part of the former USSR. The major rivers flowing north into the Barents Sea and the White Sea are the Severnaya (Northern) Dvina and the Pechora. The Volga and the Ural which flow south and the Kura which flows east drain into the Caspian Sea while the Dnepr and the Don drain south into the Black Sea. The largest river to discharge into the Black Sea is the Danube, which has its catchments in 16 countries of Central Europe and the Balkans. The main rivers to discharge into the Baltic Sea are the Neva, the Wisla, the Oder and the Neman. Ten rivers with catchments larger than 50 000 km² drain into the Atlantic and the North Sea, with the Rhine, the Elbe, the Loire and the Douro being the largest. The European rivers that drain into the Mediterranean are relatively small, the Rhone, the Ebro and the Po being the largest. Nevertheless, since the damming of the Nile, the Rhone has become the Mediterranean's most important freshwater source (Kempe et al, 1991).
 

Major rivers in European Countries

Countries whose coastline is long in relation to their area, for example Iceland, the UK, Ireland, Norway, Sweden, Denmark, Italy and Greece, are usually characterised by having a large number of relatively small river catchments and short rivers, the three to four largest of which drain only 15 to 35 per cent of their area (Table 1). The population tends to congregate in towns along the coastline and wastewater is consequently discharged directly into coastal areas rather than into the river systems. Many European countries are drained by only a few river catchments; thus the Wisla and Oder drain more than 95 per cent of Poland, and the Danube drains most of Hungary, Romania and Slovenia (Table 1).
 

Table 1 - The major European rivers, their catchment areas, and the percentage of the country area drained

Source: Compiled by NERI from multiple sources. Same rivers with different names: Strimon/Struma; Daugava/Zapadnaya Dvina.
 

Note: Symbols refer to tributaries to: #, Danube; Ý, Rhine; ×, Neva; ý, Kura; ¶, Neman.
 
 

European lakes and reservoirs

In contrast toglaciation, processes such as tectonic and volcanic activity have played only a minor role in the formation of European lakes. Numerous lakes have been created by natural damming of rivers and coastal areas, however.

Countries that were little affected by the glaciation period, such as Portugal, Spain, France, Belgium, southern England, central Germany, the Czech Republic, the Slovak Republic and the Central European part of the Russian Federation, have few natural lakes. In these areas human-made lakes such as reservoirs and ponds are often more frequent than natural lakes. Many river valleys have been dammed to create reservoirs, and a large number have been built in mountain ranges for use by the hydroelectric industry. In several countries, for example The Netherlands, Germany, France and the former Czechoslovakia, numerous artificial small lakes have been created by other human activities such as peat and sand quarrying, and for use as fish ponds.

Natural lakes in Europe

There are more than 500 000 natural lakes larger than 0.01 km² (1 ha) in Europe; of these about 80 to 90 per cent are small, with a surface area between 0.01 and 0.1 km², and only about 16 000 have a surface area exceeding 1 km² (Table 2). Three quarters of the lakes are located in Norway, Sweden, Finland and the Karelo-Kola part of the Russian Federation.
 

Table 2 - Estimated number of natural lakes in different surface area classes
 

Source: Compiled by NERI from multiple sources
 

Table 3 - Number of lakes in different European countries

Source: See table
Note: Country estimates may include some reservoirs
 

Notes:
­ Missing information.
There are no or only a few lakes in Belgium, the Czech and Slovak republics, Luxembourg and Portugal. There is no information from Belarus, Romania and Lithuania.

Information obtained from:
1) Questionnaires;
2) Ministère de la Culture et de l'Environnement, 1978;
3) Estimated by NERI;
4) Biró, 1984;
5) Gaggino et al, 1987;
6) Dill, 1990.
 

The approximate number and size distribution of natural lakes is shown for each country in Table 3; however, the number of small lakes is somewhat uncertain, and the figures given are generally minimum estimates.
 

Human-made lakes

Reservoirs are the most important human-made lakes in Europe, there being more than 10 000 major reservoirs covering a total surface area of more than 100 000 km². The numbers of relatively large reservoirs are greatest in the Russian Federation (ca 1250), Spain (ca 1000), Norway (ca 810) and the UK (ca 570). Other countries with a large number of reservoirs are Hungary (ca 300), Italy (ca 270), France (ca 240) and Sweden (ca 225). Many European countries have numerous smaller human-made lakes, for example Latvia, Bulgaria and Estonia, which have about 800, 500 and 60, respectively.
 

Large lakes and reservoirs in Europe

There are 24 natural lakes in Europe that have a surface area larger than 400 km², the largest being Lake Ladoga, which covers an area of 17 670 km². The latter is located in the northwestern part of the Russian Federation, together with Lake Onega, the second largest lake in Europe. Both are considerably larger than other European lakes and reservoirs, but nevertheless rank only 18th and 22nd in world order (Herdendorf, 1982). The third largest European freshwater body is the 6450 km² Kuybyshevskoye reservoir on the Volga. Another 19 natural lakes larger than 400 km² are found in Sweden, Finland, Estonia and the northwestern part of the Russian Federation, and three in Central Europe ­ Lake Balaton, Lac Léman and Bodensee, the surface areas of which are 596, 584, and 540 km², respectively.

The six largest reservoirs are located in the Volga river system in the Russian Federation, the two largest being the 6450 km² Kuybyshevskoye and the 4450 km² Rybinskoye reservoirs. Of the 13 European reservoirs with an area exceeding 1000 km², only the Dutch reservoir Ijsselmeer lies outside the Russian Federation and Ukraine.
 

Deep and shallow lakes and reservoirs

Lake water depth is an important parameter with which to characterise the lake environment. It is determined largely by the surrounding topography, lakes in mountainous regions generally being deeper than those in lowland areas. In two lowland countries, Finland and Poland, most lakes have a mean depth of 3 to 10 m; lakes with a mean depth greater than 10 m are rare. In Austria and Switzerland, in contrast, large shallow lakes are virtually absent, and most lakes have a mean depth greater than 25 m. As with natural lakes, the deepest reservoirs are located in mountainous regions of countries such as Norway, Spain, France, Scotland and Greece. Examples are the 190 m deep Spanish reservoir Almendra, the 132 m deep Greek reservoir Kremasta, and the 125 m deep Norwegian reservoir Blåsjø (these are maximum depths).
 

REFERENCES

• Biró, P (1984) Lake Balaton: a shallow Pannonian water in the Carpathian Basin. In: Taub, F B (Ed) Ecosystems of the world ­ lakes and reservoirs, pp 231­ 45. Elsevier, Amsterdam, Oxford, Tokyo.
• Dill, W A (1990) Inland fisheries of Europe. EIFAC technical paper No 52. Food and Agricultural Organization of the United Nations, Rome.
• Fortunatov, M A (1979) Physical geography of the Volga basin. In: Mordukhai-Boltovskoi, E D (Ed) The River Volga and its life, Junk Publishers, The Hague, 1­29 Monographiae Biologicae 33.
• Gaggino, G F, Cappelletti, E, Marchetti, R and Calcagnini, T (1987) The water quality of Italian lakes. European Water Pollution Control Association. International Congress, Lake Pollution and Recovery, Rome 15­18 April, 1985.
• Herdendorf, C E (1982) Large lakes of the world. Journal of Great Lakes Research 8, 379­412.
• Kempe, S, Pettine, M and Cauwet, G (1991) Biogeochemistry of European rivers. In: Degens, E T, Kempe, S and Richey, J E (Eds) Biochemistry of major world rivers, pp 169­211. SCOPE 42, Wiley, New York.
• Meybeck, M and Helmer, R (1989) The quality of rivers: from pristine stage to global pollution. Palaeogeography, Palaeoclimatology, Palaeoecology 75, 283­309.
• Ministère de la Culture et de l'Environnement (1978) Environnement et cadre de vie ­ dossier statistique ­ Tome 2. La Documentation Française, Paris.
• Showers, V (1989) World facts and figures (3rd edn). Wiley, New York.