Appendix B
Detailed Description of the Main Human Interventions in the Hydrological Cycle


Natural rivers as well as lakes and estuaries including their riparian zones are among the most dynamic, diverse and complex ecosystems of the world and play a major role in the regulation and maintenance of biodiversity in the landscape. Every change in the water regime especially changes of water flow and water level fluctuations and the fragmentation of river systems lead to a destruction of several types of habitats like waterfalls, rapids and floodplain wetlands. As a result of this destruction numerous species of flora and fauna will be endangered and aquatic habitats as well as riparian become fragmented and impoverished (Dynesius and Nilsson, 1994).

The main human interventions in the hydrological cycle in Europe in relation to river, lake and estuary regulation are subdivided for this report into the following topics:

  • Damming, building and management of reservoirs

  • River channelisation

  • Building of weirs

  • Dredging of river channels

  • Lake regulation

  • Estuary regulation

  • Lagoon regulation

B1.1. Damming, Building and Management of Reservoirs

A survey of reservoir characteristics and usage is given in the draft of "MW4/5 Synthesis Report on Importance of Reservoirs, Usage, Environmental Conditions, Trends and Causes". IFEN, (1996).

Reservoirs are often multi-purpose. The most important reasons for reservoir construction are:

  • Public water supply;

  • Irrigation;

  • Hydropower;

  • Flood control;

  • Low flow enhancement;

  • Fish farming.

Information was received from Austria and France for the Alpine region, from Austria, Denmark and France for the Continental region, from Denmark and France for the Atlantic region, from France and Portugal for the Mediterranean region and from Norway.

The usable man-made reservoir capacity in relation to the annual river runoff is about 10 % in Europe (Dynesius and Nilsson, 1994).

Damming of rivers has been identified as one of the most dramatic human impacts on the natural environment. It changes the environmental conditions for all riparian and aquatic organisms in the standing water body as well as in the flowing water. The natural River Continuum (Vannote et al., 1980) is interrupted affecting a number of river ecosystem processes (upstream and downstream migration, stream metabolism (autotrophic/allochtonous C-sources) etc.). Other major effects can be seen in three ways (Dynesius and Nilsson, 1994):

  • Habitats for organisms adapted to the natural discharge of the water-level regimes are impoverished;

  • Ability of a river to serve as a corridor is reduced;

  • Function of the riparian zone as a filter between upland and aquatic systems is greatly modified.

Dams create reservoirs where water has longer residence times than in former river and therefore change the physico-chemical and nutrient balance in the reservoir water and in the downstream river.

B1.1.1. Public Water Supply

Information on damming for public water supply has been received from France for the Atlantic region and from France and Portugal for the Mediterranean region.

Atlantic region:

In the north-western part of the Atlantic region this intervention is made because of the lack of freshwater resources in the receiving area (e.g. crystalline basement in Brittany). In the south-western part damming for public water supply is for the surrounding areas.

Mediterranean region:

France: In the Mediterranean region of France this intervention is made because of:

  • Lack of freshwater resources (surface water and groundwater) in the receiving area;

  • Growing population, particularly coastal and a huge seasonal population.

The main effects in the Mediterranean region of France are:

  • Seasonal draw-down of level, resulting in small stored volume in summer;

  • Stratification, which may lead to eutrophication, with impact on drinking water treatment and quality;

  • Channelisation of rivers;

  • Reduction of river flow (diversion into canals).

Portugal: The following table summarises the reservoirs used for public water supply located north and south of the river Tejo.

Table B.1 Reservoirs in Portugal (Mediterranean region) used for public water supply.

North of river Tejo

South of river Tejo

Number of reservoirs



Reservoirs only used for public water supply



Total gross capacity [106 m3]



Reservoirs used for public water supply + other use



Total gross capacity [106 m3]




This purpose is more significant in the northern part because of lack of freshwater resources (surface and groundwater) in the receiving area. Particularly the coastal zone of this area is densely populated and so the consumption rate is high.

In the south depopulation contributes to a low requirement for water even though it is more of a drought zone.

The effects of this exploitation lead to several problems:

  • Ecological changes in the system, which pass from lotic to lentic system;

  • Seasonal reduced water level, resulting in small volumes in summer;

  • Stratification of the water column and enrichment with nutrients of the reservoirs may lead to eutrophication problems with impact on drinking water treatment and quality;

  • Reduction of river flow upstream the reservoir with impact on its ecological equilibrium, barrier for migratory fish;

  • Climatic changes.

B1.1.2. Irrigation

Information on damming for irrigation purposes has been received from France for the Atlantic region and from France and Portugal for the Mediterranean region.

Atlantic region:

The following list shows the reasons, why this intervention is of main significance for the south-western part of this region in France (Agence de l’Eau Adour Garonne, 1993):

  • Dramatic increase in irrigation over the last 10 years (increases observed in irrigated surface area and in specific requirements per hectare).

  • Agriculture is an important part of regional economy.

  • Recent years have seen very low river levels (in some cases below authorised minimum flow) due to dry weather conditions and an irrigation demand increasing faster than the supply available. This has led to controversial water use restrictions (irrigation and domestic).

  • A major reservoir building programme (PDRE 1988-1998) is underway in the region. A protocol with the national electricity company has also been signed to enable the release of water stored for hydropower use (80 % of total stored water in the Adour-Garonne basin) in some areas and under certain circumstances. When complete, the storage capacity available aims to meet supply and river flow objectives.

Table B.2 illustrates the extent of this intervention on the entire Adour Garonne basin (includes western Pyrénées) in the south-west Atlantic region of France.

The benefits in the south-western Atlantic region of France are:

  • High importance for the agricultural economy (high water demand crops, e.g. fruit).

  • Security for public water supply.

  • Satisfy minimum flow requirements for rivers, protecting river habitats.

The effects in the south-western Atlantic region of France are:

  • Regulation of flows in many small rivers with possible effects on river ecosystem (winter storage and summer releases).

  • Creation of artificial lakes in areas with few large standing water bodies.

Table B.2 Characteristics of the entire Adour Garonne basin - includes western Pyrénées.

Average annual precipitation volume

Total capacity of large reservoirs (>2.106 m2 approx. 35 in number) dedicated to low flow enhancement during summer months (situation in 1992)

Total capacity of small reservoirs (<2.106 m2) dedicated to low flow enhancement during summer months (situation in 1992)

Target total basin reservoir capacity (year 2000, end of PDRE)

Total abstractions for irrigation in 1990 (Adour Garonne)

Surface water abstractions for irrigation compared to groundwater abstractions

Total irrigated surface area in 1990

90,000 hm3

305 hm3

265 hm3

1,000 hm3

870 hm3

approx. 75 %

560,000 ha

The Mas-Charban reservoir in river Charente catchment, for example is in the planning stage. It is being designed to assure the irrigation of 7,500 hectares and public water supply of a small town. Currently irrigation is limited or prohibited nearly every summer in order to ensure a minimal flow. The future dams will reduce these restrictions to one in 7 years.

Mediterranean region

France: The following list shows the French reasons and the main importance of damming for irrigation in this region.

  • Large quantities of water regulated to guarantee agricultural output in areas that would not otherwise have sufficient water supply, despite difficult climatic conditions: long drought periods, violent autumn rains, torrential floods.

  • Lack of surface water and groundwater resources.

  • Characterised by large reservoirs for river regulation, diversion to aqueducts, pumping stations, tunnels through mountains in order to deliver water under pressure for irrigation.

This intervention is also important for flood protection in the French Mediterranean region.

Because of the multi-purpose usage of the reservoirs in the French Mediterranean region for public water supply and irrigation, the effects of damming have been described in chapter 1.1

Example: In the Provence region (PACA region covers 3.106 ha), a large number of interventions have been constructed (many large dams, 250 km of canals/aqueducts/channels, including 135 km of underground galleries) in order to guarantee water for agriculture, industry and domestic consumption. Three important reservoirs have a total capacity of 250 hm3 (Castillon, St Cassien and Bimont), which are used for regulating the rivers and canals. Larger reservoirs, such as Serre-Ponçon (5,000 hm3), are located in the Alpine area and provide flexibility in managing the Mediterranean area water, although they are primarily used for hydropower production (Agence de l’Eau Rhône-Méditerranée-Corse, 1991).

Irrigation water is provided for 55,000 ha (previously, most of this area did not have a reliable supply), benefiting the regional economy and permitting diversification into new crop types. Water delivered under pressure means that more efficient irrigation techniques can be used.

Portugal: As the Southern part of Portugal is characterised by long drought periods, intense autumn rains and torrential floods, reservoirs for irrigation purposes play an important role and guarantee water supply especially in the dry season.

H hydroelectricity generation
S public water supply
I industrial water supply
R irrigation water supply.

Figure B.1 Distribution of the different uses of the reservoirs located south of Tejo.


Table B.3 Reservoirs in Portugal (Mediterranean region) used for irrigation purposes.


North of river Tejo

South of river Tejo

Number of reservoirs



Reservoirs only used for irrigation



Total gross capacity [106 m3]



Reservoirs used for irrigation + other use



Total gross capacity [106 m3]








The intensive agriculture and the large amounts of fertilisers and pesticides used are responsible for the quality problems in the water resources, especially of reservoirs.

Disposals of waste from piggeries, cattle farms, olive-oil extraction, etc., in soil or direct drainage to river systems affect water quality. Torrential floods of the rivers of this area will carry out all the nutrients to the rivers and reservoirs contributed to enrichment of these systems.

B1.1.3. Hydropower

Information on damming for generating hydroelectricity have been received from Austria and France for the Alpine and Continental region, from Portugal for the Mediterranean region and from Norway (Scandinavia).

In many countries like Austria, Finland, France, Greece, Iceland, Ireland, Italy, Norway, Portugal and Sweden the major usage of reservoirs is for the production of hydropower. Italy and France own over 250 hydroelectricity-producing dams each, Norway 300, Austria 130, Spain 97, Scotland 51 and Portugal 45. Over 1,450 reservoirs in Europe are used for hydroelectricity production. (Only dams stated with a minimum height of 15 m, according to the ICOLDŽŽŽŽS "World Register of Dams") (IFEN, 1996).

Hydropower is considered as one of the cleanest sources of energy available, being renewable and non-polluting. Hydroelectric power plants operate at 85-90 per cent efficiency, about twice that of fossil fuel power stations and almost three times that of nuclear power stations (Veltrop, 1991).

There are different types of power plants due to the topographic situation and different modes of production like peak-, seasonal- and base-production.

Austria: A recent survey of the larger rivers reported that only 4 % of the 2,813 river kilometres investigated are uninfluenced by impoundments, intermittent power generation, water diversion and channelisation. One third of these reaches are influenced by multiple superimposed impacts. The following figures show the significance of hydropower in Austria in general, available data are not collected for the regions.

Table B.4 Key figures on hydropower in Austria.

1. Total production of energy operated by hydropower (1993)

2. Portion of the total electric power generation operated by hydropower

3. Exploitation of the usable hydropower potential

4. Portion of the hydroelectricity produced by:

runoff power plants
day- and week-storage plants
annual-storage plants

39 %

~ 70 %

65 %

73 %
6 %
21 %

1. Bundeslastverteiler, 1993 in: Lassnig, (1996)
4. Bundeslastverteiler, 1989 in: Lassnig, (1996)

Norway: Norway is ideally suited for hydropower, with steep river gradients in the areas with the highest precipitation (west Norway has got up to 4000 mm/a), and good possibilities for reservoirs in natural lakes in uninhabited upland areas. The main drawback is the adverse seasonal runoff distribution, with low winter runoff and concentrated melt-floods in spring/early summer. This gives demand for high seasonal regulation and large reservoirs. As Norway has 100 % hydropower, there is also need for multi-year carryover reservoirs. Most reservoirs are dammed and/or lowered natural lakes.

On the continent, Norwegian electricity has the highest value of peak power - this tends to change the operation strategy of Norwegian plants, with more emphasis on peaking, where made possible by the installations and the operation restrictions. Peaking operations usually have more adverse environmental effects than base load operation.

The average hydropower production in Norway is 110 TWh/year. Approximately one third is used for electro-metallurgic industry and other energy-intensive industry, one third for industry supply and one third for general consumption. Approximately two thirds of the economic and technically feasible hydropower potential of the country is developed, most of the rest is protected against development.

Hydropower has been an important factor in the industrialisation of Norway and the Norwegian electricity supply is close to 100 % hydropower. About 63 % of the potential is developed while 20 % is protected against development. Import/export with Sweden and the continent is increasing, but still limited by transmission capacity.

The most significant impact on the hydrological cycle in Norway is through regulation - mainly for hydropower production. As mentioned above, the need for seasonal and multi-year regulation capacity is large, so the impact on runoff distribution is strong, even on river reaches that carry the full volume of water.

  • In such reaches winter flow is usually strongly increased - this often causes ice jam problems.

  • Over long stretches, the water is taken out of the river bed, which then only carries compensatory releases and spilled water.

  • As the rivers in western Norway generally have small catchments, and a high relief, interbasin transfers and "gutter" schemes, cutting off creeks and small rivers at intake level through long tunnels with intakes, are common. In such schemes many small rivers are completely dried out downstream of the intake. Larger rivers usually have some compensatory releases.


Alpine region:

Austria: In the Austrian Alpine region there are primarily operating intermittent, runoff-river, pumped-storage and diversion type power plants. The reasons for the significance of hydropower are shown in the general part of this chapter.

The main effects of these measures of hydraulic engineering on the ecological integrity of running waters in Austria include - beside others:

  • Disturbances of the;

  • flow regime (flood peaks and frequency);

  • physico-chemical properties of the water;

  • sediment transport;

  • flow velocities;

  • lateral (floodplains), longitudinal (river continuum) and vertical (groundwater) interactions.

  • Changes of the autochthonous species composition and biodiversity;

  • Changes of variable habitat structures.

France: In the Alpine region there are a lot of reservoirs primarily, or only, used for generating hydropower, in the Alps as well as in the Pyrénées. The main types of power stations in the French Alpine region are intermittent and diversion (esp. Pyrénées) power plants.

The following reasons explain the significance of this intervention in the French Alpine region:

  • Large numbers of large and small hydropower dams in French Alps and Pyrénées. Characterised by long and relatively deep valley-filling reservoirs.

  • Provide significant proportion of national electricity production and export to Italy and Switzerland.

  • Represent major interventions, requiring high levels of investment.

  • Dam management controls the temporal and spatial discharge of large quantities of water, and therefor affects long lengths of river reaches.

  • Transfers of large volumes of water between catchments.

  • Often located in nature protection areas.

  • Dams commissioned throughout this century. Large numbers constructed in the 1950’s and 1960’s.

Table B.5 illustrates the extent of river regulations for hydropower dams in the Alpine region of France.

Table B.5 Key figures on damming for hydropower in the Alpine region of France (approximate figures).

1. Number of large dams

2. Total gross capacity of reservoirs of large dams

3. Total area of reservoirs of large dams

4. Potential electricity production 1988

5. Total national potential hydraulic electricity production (1988)

6. Total national actual hydraulic electricity production (1989)

7. Total national electricity production (1989)


11500 hm3

3791 ha

35 TWh

68 TWh

50 TWh

387 TWh

1, 2, 3. Estimation from ETC MW4 programme reservoirs database - dams fulfilling International Commission on Large Dams criteria (generally over 15 m), located in the départements of 64, 65, 31, 09, 11, 74, 73, 05, 04 and 06 (approximating to Alps and Pyrénées region).

4,5. Centrales hydrauliques et réservoirs en France, Situation au 1er janvier 1988, published by the Ministére de l’Industrie et de l’Aménagement de Territoire. Note that the potential hydraulic electricity production for the Alps and Pyrénées bio-geographic region has been estimated using figures for the Rhône-Alpes, PACA and Midi-Pyrénées region (49 TWh) and subtracting those power stations located outside the bio-geographic region - essentially the hydropower dams on the Rhône downstream of Lyon and the power stations on the Saône, Doubs, Ardèche and Eyrieux (estimated to represent approximately 14 TWh).

6,7. Perspectives enérgetiques de la France à l’horizon 2000. Ministère de l’Industrie et du Commerce Extérieur. Xème plan 1989-1992.

An additional benefit is seen as the flexible power generation to meet peak supplies (especially in intermittent power plant).

For the Alpine region in France the major effects of river regulation by hydropower dams are:

  • Perturbation of downstream flow regime - temporally (variations in releases) and volumetrically (as much as 50 to 90 % can be diverted into another catchment). Effects on breeding and growing areas for young fish;

  • Reduction in downstream floods - river bed tends to close, because of plant colonisation, fauna adapts to regulated flows (conditions more favourable for white fish to the detriment of salmonids);

  • Thermal shock from cold water releases in summer, low oxygen levels;

  • Migration barrier;

  • Impacts on downstream water quality - gas super-saturation, iron/manganese deposits, effects of periodic emptying of reservoirs;

  • Sediment accumulation (large particles) - effect on downstream river bed, erosion problems, effect on reduction conditions in reservoir;

  • Visual impact of dam in small valleys.

Continental region:

Damming and river channelisation in the Continental region for generating hydropower is a main intervention in the hydrological cycle - except in Denmark.

Austria: In the Continental region of Austria there are primarily operating runoff-river and diversion type power plants. The reasons for the importance of hydropower are shown in the general part of this chapter. The main effects in the Continental region are the same as in the Alpine region.

France: The main types of power stations in the Continental region of France are intermittent and runoff-river power plants (e.g. long chain of runoff-river dams with no permanent water bodies down the Rhône and Rhine).

The significance of this intervention in the Continental region of France is illustrated by the following reasons:

  • Important economic national asset: hydropower and navigation. Construction from 1940’s-1980’s (including four major hydropower development upstream of Lyon in the 1980’s).

  • Large number of interventions on the river Rhône (river with highest annual average flow discharge in France). Also provides flood protection for urban development and agriculture. Possibility of providing irrigation water to surrounding area, by diversion.

  • Typical configuration is an upstream dam, raising upstream river levels (over 10-20 km) and controlling flow discharge to the short-circuited river and flow discharge to a channel with a runoff-river power station and a fluvial transport lock (see following figure).

As an example, Table B.6 illustrates the extent of hydropower generation in the Rhône corridor - Continental region.

Table B.6 Hydropower dams in the Rhône corridor (approximate figures).

1. Number of major dam developments (Lyon to Mediterranean)

2. Length of river under consideration (Lyon to Mediterranean)

3. Number of locks (Lyon to Med., boats up to 190 m length)

4. Freight traffic (on Rhône in 1986), 31% related to oil industry

5. Length of side channels (control alluvial groundwater levels)

6. Hydroelectricity production (total Rhône including upstream of Lyon, 18 power stations, 1988)

7. Total national hydropower production (1989)

8. Total national electricity production (1989)


330 km


400.106 t/km

300 km

17.9 TWh

50 TWh

387 TWh

1,2,3,4,5,6. Agence de l’Eau Rhône-Mediterranée-Corse, (1991)

7,8. Perspectives enérgetiques de la France à l’horizon 2000. Ministère de l’Industrie et du Commerce Extérieur. Xème plan 1989-1992.

Figure B.2 Typical schematic layout of a hydroelectric development on the river Rhône.

The following additional benefits are seen in France (especially for the Rhône corridor) (Agence de l’Eau Rhône-Méditerranée-Corse, 1991):

  • Fluvial transport - significant growth 1945-1970 to reach 3.5 106 tonnes in 1970, general stagnation since this date due to problems with connections to Fos-Marseille, competition with other transport forms (fluvial transport represents only 30 % of total traffic in Rhône port zones) and lack of connection to Rhine system;

  • Agricultural development - flood protection, stabilise alluvial groundwater levels, water supply;

  • Urban development - flood protection, town expansion (e.g. Lyon), development of industrial port areas, bridge and road construction;

  • Wastewater management - drains and treatment system often upgraded during dam development because of dike construction (formerly wastewater discharged directly to river);

  • Tourism zone development;

  • Reservoirs and reeded river banks provide good bird migration habitats; hunting is generally prohibited in these areas.

For the Continental region of France the major effects of river regulation due to hydropower dams on, for example, the Rhône river are (Bravard, 1996):

  • Migration obstacle (although partially compensated by numerous fish ladders);

  • Ecological impacts on short-circuited rivers (often long reaches) - small reserve flows, slow morphological and ecological evolution, possibly resulting in channelisation effects and eventual reduction in habitat value;

  • Drop in groundwater level, effect on groundwater exchange between river and alluvial aquifer, which results in the liberation of iron and manganese from sediments, which can affect drinking water quality;

  • Drying out of bank vegetation, drying out of lateral arteries from the river.

For example, Pierre-Bénite development, located just downstream of Lyon and commissioned in 1966, is a typical multi-purpose development: energy, navigation, wastewater system improvement, flood protection, industrial platform construction, industrial rail freight station construction, motorway construction.

Mediterranean region:

Because of the physiographic characteristics of Portugal, damming for hydropower plays a major role north of the Tejo river. Hydropower generation represents about 40 % of the total production of electricity in an average year.

Figure B.3 shows the importance of reservoir use for hydropower north of the Tejo (compared with Figure B.1 which shows the distribution south of the Tejo).

H hydroelectricity generation
S public water supply
I industrial water supply
R irrigation water supply.

Figure B.3 Distribution of the different uses of the reservoirs located north of Tejo.

Table B.7 Reservoirs in Portugal (Mediterranean region) used for hydropower generation.


North of river Tejo

South of river Tejo

Number of reservoirs



Reservoirs only used for hydropower



Total gross capacity [106 m3]



Reservoirs used for hydropower + other use



Total gross capacity [106 m3]



Table B.8 Hydropower generation in Portugal.



Number of reservoirs


Potential electricity production 1994 [GWh]


Portion of the total electric power generation operated by hydropower

40 %


North of river Tejo

South of river Tejo

Number of reservoirs



Potential electricity production 1994 [GWh]



Portion of the total electric power generation operated by hydropower

99.67 %

0.33 %

Portion of hydroelectricity [GWh] produced by:

Runoff power plants

Day- and week-storage plants

Annual-storage plants









The benefits and major effects are similar to France.

B1.1.4. Flood Control and Low flow Enhancement

Information on damming for flood control and low flow enhancement have been received from France for the Atlantic and Continental regions.

The first priority of these dams in France is to defend urban and agricultural areas against floods and to guarantee sufficient flow (low flow enhancement) to ensure water supply for many different uses. These reservoirs are mostly multi-purpose also used for hydropower and tourism.

Flood control benefits population, industry and agriculture. The reservoirs also serve variously for hydropower, fisheries and tourism (e.g. Natural Park of Forêt d’Orient). Low flow enhancement ensures minimum flows - benefiting water supply and avoiding ecological impacts due to drying out of the river.

The major problems are seen in the artificial regulation of flows and problems related to periodic emptying of large reservoirs which is required by law every 10 years.

The second usage of these dams in the Atlantic region of France is to ensure sufficient water primarily for cooling purposes far downstream on the Loire in Massif Central (nuclear power stations).

The significance of this intervention in the Continental region of France is shown by the Seine reservoirs.

  • It ensures water supply for many different uses in the whole Seine basin, in the Parisian region especially for public and industrial water supply in terms of quantity.

  • Lack of sufficient freshwater resources (seasonal surface water variations) - large demand for public water supply, industrial supply and cooling water in Parisian region (over 10 million people).

  • Lack of water quality in supply area (surface and groundwater) - high domestic water demand in Parisian region.

  • Major investment programme in 1950’s to 1980’s to create four large capacity reserves in the upper Seine basin, regulating the flow of water in the rivers Seine, Marne, Aube and Yonne (refer to following figure).

Table B.9 illustrates the extent of this intervention in the upper Seine basin.

Table B.9 Reservoir capacities at the Upper Seine (Courants, 1994).

Total reservoir capacity (4 reservoirs)

Reservoir capacity (2 largest reservoirs Seine, Marne)

Reservoir surface areas (reservoirs Seine and Marne)

805 hm3

555 hm3

7100 ha

Table B.10 Capacities of four major Seine reservoirs.


Date of commissioning

Total capacity (hm3)













Figure B.4 Situation of the large reservoirs at the Seine.

B1.1.5. Fish Farming

Information on damming for fish farming has been received from Denmark for the Atlantic and Continental region.

Fish farming as a major reason for damming of rivers is only found in Denmark. With a few exceptions all were situated in Jutland (in both ecoregions). Because of their obviously low dam heights respectively their small water capacities the "reservoirs" did not get listed in the "World Register of Dams". Nevertheless it represents the main intervention in the continuity of Danish rivers, hydrologically as well as ecologically.

The typical fish farm in Denmark has a water inlet upstream of a dam and the inlet may, especially in the (dry) summers, be equal to the total discharge.

There exist about 450 dams in about 25 watersheds. In the watershed of the River Gudenå (2,638 km2) there are about 38 fish farms and associated dams. The total river length of suited for fish production and located upstream dams is estimated to 2,600 km, about 1,900 km being affected by the dams (Jensen, 1991).

In 1994, 485 inland fish farms were in operation (Miljøstyrelsen, 1995). They had an annual production of 35,000 t (value approx. 500 mill. DKK) , the main part is being exported.

The special effects of damming in Denmark are seen as following (Iversen, 1995):

  • No or reduced upstream migration of migratory fish as sea trout and eel (fish ladders or eel passes do not secure upstream migration but may help). No or reduced spawning of sea trout.

  • The natural drift and upstream migration of invertebrates are not possible.

  • The river sections upstream dams function as depositing areas, the transport of organic matter from upstream to downstream areas is interrupted and hence ecosystem processes are significantly impacted.


B1.2. River Channelisation

Physical deterioration of rivers due to channelisation and river maintenance significantly intervene in the hydrological cycle.

The primary reasons for river channelisation are for:

  • Flood control;

  • Land drainage;

  • Navigation

Information according to river channelisation has been received from Austria and France for the Alpine region, from Denmark and France for the Atlantic region, from Austria and Denmark for the Continental region and from Norway


B1.2.1. Flood Control

Information about river channelisation due to flood control has been received from Austria and France for the Alpine region, from Austria for the Continental region and from Norway.

The extent and the main effects of these measures in Austria are the same as already listed in chapter 1.3 (damming for hydropower)


B1.2.2. Land Drainage

Information according to river channelisation due to land drainage has been received from Denmark for the Atlantic and Continental region and from Norway.

On the Scandinavian Peninsula and in Finland, drainage of wetlands in forested areas influence the hydrological regime.

Denmark: This intervention covers all Denmark and is among the most important cause why two thirds of all Danish rivers do not fulfil the politically decided quality objectives.

About 35,000 km of the 65,000 km Danish rivers are natural (Iversen et al., 1993). Markmann (1990) estimated that only 15 % of the natural rivers have retained their natural properties. An assessment by Brookes (1987) based on map analyses suggested that only 2 % of natural rivers had retained their natural physical properties. The conclusion is that the great majority of Denmark’s natural rivers have lost their natural physical properties due to channelisation.

Environmental objectives were not included in Danish legislation on rivers until 1982. Since then increasing activities on restoration of rivers and associated riparian areas have taken place. Re-establishment of the natural physical properties of channelised rivers in Denmark through re-meandering (and reopening of culverted brooks) has included 100-200 km rivers (Iversen et al., 1993), but the activity is increasing.

The density of channelised rivers in Denmark may be one order of magnitude higher than in United Kingdom (Brookes, 1988) mainly because of differences in land use (agriculture).

To secure a high and stabile agricultural production even in marginal areas, activities as drainage of wetlands, channelisation of rivers and maintenance of rivers have been subsidised. In Denmark agriculture is an important sector, 62 % of the land areas are cultivated. The Danish agricultural export amounted to 49 bill. DKK in 1994 - direct and indirect employment was approx. 237,000 individuals.

Most important effects of channelisation due to land drainage in Denmark include:

  • Increased slope, water velocity and lowered water level, which may impact riparian areas

  • Increased erosion and transport of sandy sediments

  • The macroinvertebrate and fish communities are significantly affected and impoverished

  • It is estimated that in 25 % of the Danish rivers not fulfilling the quality objectives, the main reason is reduced physical variability.

Norway: Many river reaches have been straightened and former wetlands and river bends turned into agricultural areas. The straightened rivers are often lined with rockfill. In addition to serving as flood protection, the rockfills are often used for roads.

The main impacts of river straightening in Norway are:

  • The natural bank vegetation is usually removed or impaired and the habitat diversity is strongly reduced.

  • In addition the hydraulic properties of the channel is changed and as the inundated areas are reduced, the flood alleviation properties of the watercourse are reduced.

  • The same applies to flood embankments in general. In particular the Glomma river is extensively controlled by flood protection works.

B1.2.3. Navigation

Information about river channelisation for navigation has been received from France for the Atlantic region (e.g. weirs and canalisation on Seine, Rhône,…).


B1.3. Building of Weirs to Improve Fish Habitats

Information about building of weirs to improve fish habitats has been received from Norway.

In Norway weirs have been used to increase the water covered area and to improve fish habitat in rivers with strongly reduced discharge. The rivers can have strongly reduced discharge because many Norwegian hydropower plants are high head plants (up to 1100 m head), taking water directly from highland reservoirs to sea or lowland lakes, bypassing long river reaches. In Norwegian salmon rivers, there is a long tradition for constructing weirs to improve fishing.

Building of weirs is an obstruction to fish (and invertebrate) migration and therefore has a negative ecological impact. Significant efforts have been used in Denmark to remove weirs for ecological reasons (Iversen et al., 1993).

B1.4. Dredging of River Channels

The primary reasons for river dredging are due to

  • Land drainage;

  • Mining of river bed gravel.

Information about dredging of river channels has been received from France for the Alpine region, from Denmark for the Atlantic and Continental region and from Norway.

B1.4.1. Land Drainage

Information about dredging of river channels due to land drainage has been received from Denmark for the Atlantic and Continental region.

Dredging and weed cutting (stream maintenance) are often associated with channelisation and intervene in the hydrological cycle in the same way by improving the river discharge capacity.

During the last 10-15 years environmentally appropriate methods have been developed and gradually introduced (Iversen et al., 1993).

Denmark: The majority of Danish rivers in both regions are being maintained (see the following table). In recent years more environmentally appropriate methods have been used as a compromise between agricultural and environmental interests.

Table B.11 Stream maintenance in Denmark 1990 (Iversen et al., 1993).

Stream maintenance practice

small (municipal) rivers

larger (county) rivers


63 %

38 %

Environmentally appropriate

37 %

56 %

None at all


6 %

Dredging and weed cutting in rivers secures a faster drainage of agricultural areas and reduces the risk of floods in the area channelised.

The ecological benefits of environmentally appropriate maintenance are more habitats for plants, macroinvertebrates and fish.

For both regions in Denmark, the Atlantic and the Continental, the major effects are:

  • It may increase the risk of floods at downstream areas due to higher peak flows;

  • Lowered water level which may impact riparian areas;

  • Less variable physical river environment;

  • Fewer habitats;

  • Macroinvertebrates and fish communities are significant affected and impoverished.

B1.4.2. Mining of River Bed Gravel

Information has been received from France for the Alpine region and from Norway.

France: This intervention is related to chapter 5.3 (activities in the catchment - wet cuts)

Norway: The straightened rivers are often lined with rockfill. In addition to serving as flood protection the rockfills are often used for roads. Hydropower development works is strictly controlled in Norway but control and supervision is more lax when it comes to other interventions. Road-fills encroaching on the river channel, and mining of river bed gravel are examples of interventions that can be done without licence and which often give adverse effects both on river hydraulics, habitat and erosion processes.

B1.5. Lake Regulation

Primary kinds/reasons for lake regulation are:

  • Lake shore modification at natural lakes;

  • Hydropower production.

Information about lake regulation has been received from Austria and France for the Alpine region and from Norway.

B1.5.1. Lake Shore Modification at Natural Lakes

Information about lake shore modification at natural lakes has been received from Austria and France for the Alpine region.

Most of the lakes play an essential role for the Austrian and French tourism.

Austria: The number of lakes in Austria is about 9,000; 62 % are natural lakes. About 30 natural lakes have an area > 1 km2. The first great success in water pollution control was achieved by the restoration of the eutrophic lakes starting in the 1960ŽŽŽŽs and lasting about two decades.

However, the littoral zones (with reed stands, terrestrialisation areas etc.) of a large number of lakes has been modified by the creation of hotels, second homes and guest-houses with bathing grounds, promenades, public bathing grounds or roads.

In Austria the main effects of these measures are seen in the negative effects on the biodiversity of these areas. The occurrence of many species typical for standing water ecosystems like birds, amphibians, fish, macroinvertebrates or many plant species depend on littoral areas with highly diversified structural components. These areas are also important for nutrient binding processes.

B1.5.2. Hydropower Production at Natural Lakes

Information about lake regulation at natural lakes for generating hydropower has been received from Norway.

Wetland degradation is an important environmental problem and in addition to land cultivation, land fills for industrial sites, harbours etc., and road-fills encroaches on the remaining areas - both in watercourses and estuaries.

Most reservoirs are dammed and/or lowered natural lakes. This reduces the submerged area, but of course strongly influences the natural conditions of the lake. The regulating interval can be large, up to hundred meters.

It is a general requirement that spillways and flood release systems are designed and operated in such a manner that natural floods are not increased. As a general rule, flood flows are reduced by hydropower development, but flood damage might easily increase, as former flood plains are developed without proper consideration of the reduced but existing flood risk. The stage/discharge relationship may also be impaired due to reduced conveyance of the river channel, caused by sedimentation and in-channel vegetation.


B1.6. Estuary Regulation for Flood Defence

Information about estuary regulation for flood defence has been received from France for the Atlantic region. This is a significant intervention in the north-western part of the French Atlantic region and multi-purpose.

The benefits are not only flood defence for towns on estuary and upstream, but also hydropower, water storage, land reclamation and upstream tidal control (e.g. Vilaine estuary).

Several important estuary modifications are located in this region - possibly the most significant are the Rance dam (tidal power dam / navigation) and the Arzal dam. Following the Arzal dam is briefly described (Elie and Rigaud, 1986).

The Arzal dam, located in the estuarine zone of the river, was commissioned in 1970. The objectives were to block off saline water at 8 km from the estuary mouth (saline influence was previously 40 km) and to manage freshwater discharge from the catchment into the estuary. The dam formed the first stage of a major intervention programme in the catchment (river dredging and regarding, dikes and other dams further upstream), aimed at improving flood and low flow control, which had previously caused major disasters.

Table B.12 Arzal dam - key figures.


Length of crest of dam

Maximum height of dam

Total reservoir capacity

Catchment area

584 m

21 m

35 hm3

10100 km2





Some benefits of this estuary regulation in France are:

  • Elimination of risk of flooding in inhabited and agricultural areas

  • Guarantees year-round upstream fluvial transport (commercial, tourism)

  • Water supply for several towns and villages

  • Leisure port immediately upstream of dam - increase in tourism

  • Drying out of marshland (Redon marsh), safe from floods

For the Atlantic region in France and especially for the Arzal dam the major effects are:

  • Silting up of estuary requiring dredging to clear navigable channels;

  • Transformation of 30 km of river from marine-influenced to freshwater river;

  • Continuous use of navigation locks during summer results in saline water in the town’s pumping stations;

  • Contribution to anoxic episodes in the Vilaine Bay leading to fish kills;

  • Water quality problems have affected fishing, mussel and shellfish production in the estuary;

  • Technical and economic difficulties in exploiting reclaimed marshland;

  • Effects on fish, ducks and geese populations;

  • Impact on fish and agnathe migration (e.g. eels, lampreys).

B1.7. Lagoon Regulation

Information on lagoon regulation has been received from France for the Mediterranean region.

The reasons for the importance of lagoon regulation in the French Mediterranean region is described on the Rhône delta (EAUX, No 43).

The construction of dikes and channels in the Rhône delta last from the 12th century onwards in order to control flooding, permitting the agricultural use of the fertile delta plain. In the 19th century, the modern delta form was determined using dikes to control two main river channels in the delta - Grand Rhône and Petit Rhône - complemented by a sea-wall dike, restricting marine flooding of the delta. This is the most important wetland area in France and a nature reserve zone was first created in 1927. It is home to the most important colony of pink flamingos in Europe as well as 340 other bird species.

Table B.13 Wetland areas in the Rhône delta.

Modern delta between Grand Rhône/Petit Rhône (wetland area)

Area known as Camargue

170,000 ha

75,000 ha

The special benefits of the Lagoon regulation in the Rhône delta are:

  • Use of fertile agricultural areas (e.g. rice cultivation)

  • Navigation from Mediterranean to Arles and then up the Rhône.

The special effects of the Lagoon regulation in the Rhône delta are:

  • Dikes have reduced freshwater and marine invasions of the delta area, resulting in a progressive desertification effect. Increasing agriculture (especially rice growing) has also contributed to this drying out phenomenon. This has led to the loss of an estimated 40,000 ha of natural wetland areas over the last forty years.

  • Human interventions in the catchment (dams, gravel extraction, erosion reduction in the mountains) have reduced sediment loads to the delta, resulting in coastal retreat and a progressive ‘estuarisation’ of the delta (the channelised Grand Rhône tends to dig deeper into the river bed, so that river influence is greater than marine influence).

  • These interventions have modified the surface morphology of the delta, gradually leading to ‘stable’ delta.

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