4. overview of category A interventions
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 leads 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 get 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;
Building of weirs;
Dredging of river channels;
4.1. Damming, Building and Management of Reservoirs
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/allochthonous 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 the former river and, therefore, change the physico-chemical and nutrient balance in the reservoir water and in the downstream river.
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;
Low flow enhancement;
4.1.1. Public Water Supply and Irrigation
In the French Atlantic and Mediterranean region damming for public water supply and irrigation purposes is primarily done because of a lack of freshwater resources (surface water and groundwater) in the receiving areas with growing or huge seasonal population, respectively, and difficult climatic conditions (long drought periods, violent autumn rains, torrential floods). The large reservoirs are used in multi-purpose ways, the smaller reservoirs provide high quality water for public water supply. In the French Mediterranean region this intervention also serves flood protection.
Agriculture is an important part of the regional economy and the irrigation demand is increasing rapidly, because of an increase in irrigated land which is influenced by the Common Agricultural Policy. Additionally the river levels have dropped due to dry weather conditions and the irrigation demand which is growing faster than available supplies. This led to a major reservoir building programme in the Atlantic region to guarantee a stable agricultural economy, security for public water supply and a minimum flow for the rivers, protecting river habitats.
In the northern part of the
Portuguese Mediterranean region public water supply is a major reason for damming because
of a lack of freshwater resources. The main problems can be seen as migration barriers for
migratory fish and impacts on water quality due to eutrophication in the reservoir. In the
southern part reservoirs are mainly used for irrigation purposes. Here the main problems
occur in connection with water quality caused by intensive agriculture and waste
The contributions to this project show that damming for generating hydroelectricity is one of the most significant interventions, especially in the Alpine and Continental region of Austria and France, in the Mediterranean region of Portugal and in Norway.
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, 1992).
In Austria ~70 % of the total electric power is generated by hydropower and the exploitation of the usable hydropower potential amounts 65 %. In France about 13 % of the national electricity production is operated by hydropower and about 75 % of the potential hydroelectricity is exploited. In Norway 100 % of the electricity production is hydropower and about 63 % of the potential is developed, while 20 % is protected against development. In Portugal about 40 % of the total energy production is operated by hydropower.
Especially in the Alpine region there are a large number of hydropower dams, and often are placed in nature protection areas. The reservoirs are characterised as long and relatively deep valley-filling, especially in the French Alps and Pyrénées.
In the Continental region dams for hydropower are multi-functional serving navigational purposes, water supply, flood protection, tourism and stabilising alluvial groundwater levels. The typical river Rhône dam in France affects about 10-20 km of upstream rivers and diverts flow to a short-circuited river.
The major impacts on the natural environment were identified as dams acting as migration barrier and fragmenting aquatic ecosystems. Temporal and spatial discharges of large quantities of water affect long lengths of river reaches and lead to disturbances of the flow regime, the flow velocities, the sediment transport, the physico-chemical properties of the water and of lateral (floodplains), longitudinal (river continuum) and vertical (groundwater) interactions, depending on the reservoir management. Often large volumes of water (about 50 to 90 % in some valleys in Alpine France) were transferred between different catchments, drying out tributaries and downstream river beds. This intervention further changes autochthonous species composition and biodiversity as well as variable habitat structures.
In Norway the natural hydrological regime is characterised by low winter run-off and concentrated melt-floods in spring/early summer, that is the water fluxes are not coincidental with the major fluctuations in the energy consumption. This gives demand for high seasonal regulation and large reservoirs. The impact on runoff distribution is strong, even on river reaches that carry the full volume of water. Rivers with small catchments and a high relief, inter-basin 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. The impacts and influences on the lake and river ecosystems are quite drastic.
In Portugal damming for hydropower
plays a special role north of Tejo river. Approximately 100 % of Portuguese hydropower
will be generated in this area.
4.1.3. Flood Control and Low Flow Enhancement
Damming for flood control and low flow enhancement is a main purpose of the multi-functional reservoirs in the French Atlantic and Continental region beside hydropower and tourism.
The first priority is to defend urban and agricultural areas against floods and the second to guarantee sufficient flow ensuring water supply for public and industrial efforts (especially cooling water for nuclear power plants in the Atlantic region) due to a lack of quality and quantity of freshwater especially in the Parisian region (over 10 million people).
Further, low flow enhancement ensures minimum flows avoiding ecological impacts due to drying out of the river.
In France the major problems are
seen in the artificial regulation of flows and the periodic emptying of large reservoirs
which is required by law every 10 years .
4.1.4. Fish farming
In the Atlantic and Continental regions of Denmark damming for fish farming has been emphasised as the main intervention in the continuity of Danish rivers, hydrologically as well as ecologically. The total river length suited for fish production and located upstream of dams is estimated to be 2,600 km, about 1,900 km being affected by the dams (Jensen, 1991).
At the typical Danish fish farm,
especially in the (dry) summers, even the total discharge of the river will be abstracted.
Additionally due to the dam changing the function of the upstream river sections into
depositing areas, the transport of organic matter from upstream to downstream areas is
interrupted and hence ecosystem processes are significantly impacted.
4.2. River Channelisation
The contributions show that river
channelisation primarily serves flood control, land drainage and navigation purposes and
is commonly distributed.
4.2.1. Flood Control
River channelisation for flood control occurs in the Alpine and Continental region as a main human intervention in the hydrological cycle changing the autochthonous species composition and biodiversity as well as variable habitat structures of the riverbed, the riparian zone and the flood plains.
4.2.2. Land Drainage
On the Scandinavian peninsula and in Finland, drainage of wetlands in forested areas influence the hydrological regime. The great majority of Denmarks natural rivers have lost their natural physical properties due to channelisation which is among the most important causes of why two thirds of all rivers do not fulfil the politically decided quality objectives.
River channelisation for land drainage secures a high and stable agricultural production even in marginal areas but reduces the physical variability of the rivers, the hydraulic properties and further affects and impoverishes the macroinvertebrate and fish communities and strongly reduces habitat diversity.
In the last decade activities on restoration of rivers and associated riparian areas have taken place re-establishing the natural physical properties of channelled rivers in through re-meandering (and reopening of culverted brooks).
A main reason for river
channelisation in the Atlantic region of France is for navigation.
4.3. Building of Weirs to Improve Fish Habitats
In Norway weirs have been used to
increase the water covered area and to improve fish habitat in rivers with strongly
reduced discharge. Discharge can be strongly reduced because of the operation of
hydropower dams. In salmon rivers there is a long tradition to construct weirs to improve
fishing conditions. In Denmark weirs have been removed for ecological reasons.
4.4. Dredging of River Channels
The primary reasons for river
dredging are for land drainage and mining of river bed gravel which was found in the
Alpine, Atlantic and Continental regions.
4.4.1. Land Drainage
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. It secures a faster drainage of agricultural areas and reduces the risk of floods in the area channelised but may increase the risk of floods in downstream areas. Additionally macroinvertebrates and fish communities are significantly affected and impoverished due to the lowered water level and the less variable physical river environment.
In recent years more environmentally appropriate methods have been used in Denmark as a compromise between agricultural and environmental interests to maintain more habitats for plants, macroinvertebrates and fish.
4.4.2. Mining of River Bed Gravel
The dredging of rivers for gravel is related to wet cuts for mining alluvial gravel in the catchment area and occurs primarily in the Alpine region. It is an important and economically useful source of high quality aggregates but has adverse effects both on river hydraulics, habitat and erosion processes.
In Norway mining of river bed gravel
for private use can be done without licence.
4.5. Lake Regulation
Lake regulation seems to be an
intervention typical of the Alpine region and in Norway.
4.5.1. Lake Shore Modification in Natural Lakes
Most of these lakes play an essential role for tourism and the littoral zones (with reed stands, reclamation areas etc.) of a large number of lakes have been modified by the creation of hotels, second homes and guest-houses with bathing areas, promenades or roads.
These measures affect the biodiversity of these areas in a negative way. The occurrence of many species typical for standing water ecosystems such as 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.
4.5.2. Hydropower Production at Natural Lakes
Especially in Norway natural lakes
are regulated for the generation of hydropower which is the most significant impact on the
hydrological cycle in this area.
4.6. Estuary Regulation for Flood Defence
The contributions showed that
estuaries are primarily regulated for flood defence but also for water storage, land
reclamation, upstream tidal control and sometimes for hydropower. Eliminating the risk of
flooding in inhabited and agricultural areas affects the river by transforming long
reaches from marine-influenced into a freshwater river. This will affect aquatic
biodiversity and change the autochthonous species composition in the estuary. Additionally
the estuary may silt up which will probably requires dredging to clear navigable channels.
4.7. Lagoon Regulation
In the Mediterranean region of France there are very important wetlands where lagoon regulation controls flooding and permits the agricultural use of the fertile delta plain. This regulation and the increasing agriculture lead to a progressive desertification effect due to the reduced freshwater and marine invasions of the delta area which means a loss of natural wetland areas. Human interventions in the catchment may reduce sediment loads to the delta, resulting in coastal retreat and a progressive estuarisation of the delta. The river digs deeper into the river bed and river influence is greater than marine influence.