This Policy Position Statement reviews the issues concerning the
construction and use of reservoirs. It highlights the main
advantages and disadvantages of reservoirs in both the UK and
global context, as well as emphasising the importance of
comprehensive appraisal and mitigation.
CIWEM's Position on Reservoirs
- CIWEM welcomes the framework for decision-making established by
the World Commission on Dams.
- Reservoirs provide many benefits including water supply,
floodwater control, power generation, irrigation and recreational
- Conversely, there are many disadvantages, such as loss of high
quality agricultural land, displacement of people, changes to
downstream flow patterns and impacts on fish migration and
- Many of the environmental impacts associated with reservoirs
can be mitigated, and such measures should be employed wherever
- All options to meet the required purpose should be considered
in detail before reservoir construction is sanctioned, with full
stakeholder consultation throughout the appraisal and planning
- Due to the long planning, construction and filling timescales
for reservoirs, decisions must be taken in due time.
- Storage of wet season runoff is a sustainable long-term water
resource option. However, proposed schemes must be subject to
full environmental, economic and social impact assessment and all
possible options explored.
The Chartered Institution of Water and Environmental Management
(CIWEM) is the leading professional body for the people who plan,
protect and care for the environment and its resources, providing
educational opportunities, independent information to the public
and advice to government. Members in 96 countries include
scientists, engineers, ecologists and students.
The Institution develops policy through its Technical Panels;
one of these is the Water Resources Panel. Its members are drawn
from all organisations with a concern for water resources,
including the Environment Agency, government, water companies,
consultants and environmental organisations.
Reservoirs have provided water and power for many centuries both
in the UK and throughout the world. The storage capacity they
provide enables water to be captured in greater quantities than
would otherwise be possible, and in times of plenty for use in
times of shortage. Their construction, however, has sometimes
required the relocation of people living in the impounded area, and
some reservoirs have had significant environmental impacts. There
is a lively debate as to whether the social and environmental
impacts outweigh the benefits.
In November 2000, The World Commission on Dams prepared a New
Framework for Decision Making in relation to the choice (or
otherwise) and design of dams and reservoirs. This looked at the
social, environmental and political choices involved. The
Commission established core values of equity, efficiency,
participatory decision making, sustainability, and accountablility
as ones which are to be adopted in the process of selecting - or
rejecting - dam and reservoir options for addressing water
supply-demand, water power, flood control and other problems.
The underpinning principle is recognising entitlements and sharing
benefits. The onus is placed on the State and the developer to
ensure that those affected will enjoy improved livelihoods.
In launching the Report, Nelson Mandela noted:
"The problem is not the dams. It is the hunger. It is the
thirst. It is the darkness of a township. It is townships and rural
huts without running water, lights or sanitation. It is the time
wasted gathering water by hand"
The decision as to whether construction of a reservoir
produces an overall net gain or loss (with all things
considered, over short and long term timescales, and with respect
to the interests of all stakeholders) is a far from straightforward
one to make. This policy statement considers the issues
involved in a global context.
Key Issues - For
Water is a critical resource; without it people cannot survive.
For over 5000 years, reservoirs have provided people with the water
they need to live. Reservoirs have enabled people to collect water
when it was plentiful and save it for dry periods. Reservoirs have
been essential to the establishment of towns. In Victorian times
new towns suffered up to 50,000 deaths a year from water-borne
diseases. The provision of clean water (mostly from upland
reservoirs) supported by better sanitation cured this. Few large
cities today could survive during periods of drought without their
reservoirs. Even where most of their water is obtained by direct
abstraction from a river, reservoirs can still be needed to ensure
that, during drought periods, water is still available without
having to over-abstract from rivers and harm the environment.
The latest climate change scenarios indicate that many parts of
the world will have less dry season runoff and that droughts could
become more extreme. Water demand management such as leakage
reduction, more efficient water-using appliances, metering with
appropriate tariffs, and the reuse of treated effluent can reduce
the need for water.
Today, the most appropriate policy for sustainable water
resources management is the twin-track approach of balancing
economical demand management with water resource development. In
the UK, most sustainable groundwater is already fully committed,
and during droughts there is little water in the rivers that is not
required to maintain a healthy environment. Water transfer between
river basins is expensive, and it can result in the mixing of
different quality waters in varying proportions, which can itself
result in ecological stress and change. It can also lead to
transmission of fish disease and new species to catchments which do
not have them. Thus most future resources will involve the
reallocation of existing resources, the enlargement of existing
reservoirs or the construction of new ones.
Water is required for food to grow. In many countries this
requires the irrigation of crops. The best growing season is often
the dry season when there is comparatively little water available
for irrigation. For instance storage of flood flows of the Nile in
the Aswan reservoir allows Egypt to grow not just one crop a year
but generally two and sometimes three. It also enabled Egypt to
cope with the droughts of 1973 and 1980. Without the Aswan
reservoir Egypt could only support a much smaller population.
Reservoirs have kept flood waters back so they could be released
slowly to protect people and property in the flood plain
downstream. In China alone probably 200,000 people have drowned in
floods this century. In England, the 1998 Easter floods and
subsequently the 2000 floods resulted in serious damage. Urban
development, with its harder and often impermeable surfaces,
results in more of the rain running off to the river and reaching
it faster. This results in increased flooding downstream. UK
planning policies are increasingly directing development away from
floodplains, and encouraging the use of natural and engineered
washland storarge in rural parts of the catchment.
Reservoirs can also provide water for hydro electric power
generation. This is a renewable and sustainable resource. Its
generation does not result in carbon dioxide emissions and it
contributes little to global warming. Hydro power stations have
long lives, many of those in Scotland having been operating for
about 50 years with much life still left.
Navigation on canals requires water for the operation of locks.
On most canals in the UK, much of this water in dry weather is
provided by reservoirs.
Increasingly, reservoirs are being used for recreation and
conservation and they can enhance the quality of the local
environment and amenity value. This includes fishing, boating,
sailing, bird watching, and walking and cycling around the
perimeter. Several UK reservoirs are now SSSIs, SACs or
internationally recognised Ramsar designated wetland sites. Nearly
a million people a year visit Carsington Reservoir in Derbyshire,
England, for recreation.
Key Issues - Against
There are several disadvantages of reservoirs. The most obvious
is that they require large amounts of land, often much of it in
valleys. This can be good agricultural land. They can also require
the displacement of people and their relocation. Where possible,
sites should be chosen where the minimum number of people are
affected. Any archaeology in the reservoir basin would be lost on
submergence so all sites should be excavated prior to reservoir
Major reservoirs require significant funding, in the order of
billions of pounds. There has to be a high level of certainty that
a reservoir and its water supply benefits are truly needed.
The reservoir dam will change the pattern of flows released
downstream. In Victorian times about a third of the yield of the
reservoirs was provided as a constant compensation flow to power
the water mills downstream. Today releases can be tuned to meet
environmental needs, as has been done at Roadford in Devon, England
to facilitate salmonid runs.
Reservoirs also impact on the surrounding environment.
Microclimatic changes may occur through varying humidity and this
may affect crops and biodiversity downstream of the reservoir.
Dams also inhibit migratory fish movements. Fish passes are now
constructed to facilitate passage, although fish pass design and
research is only in its infancy, and the effectiveness of fish
passes is very variable. Dams can also inhibit navigation but,
where necessary, ship lifts can be constructed. In China these have
been designed to pass ships up to 8,000 tons.
Much can be done to mitigate most of the environmental impacts
associated with reservoir construction, and current practice is to
do that, sometimes adding appreciably to the cost of the scheme.
For instance, landscaping is invariably used to enhance the visual
attractiveness and ecological value of a site. CIWEM emphasises
that mitigation measures should be undertaken wherever
A new reservoir will be a very long term feature in the
landscape, and so extensive considerations must be made prior to
authorising its construction. Demand projections must be carefully
prepared incorporating demand management actions. All other ways of
meeting the resulting demands should be considered. All available
reservoir sites should be investigated, reducing in numbers through
successively more detailed screening. Full social and environmental
studies should be made of the selected site. All the benefits and
disbenefits should be considered when making the decision to
proceed with a new reservoir. All stakeholders should be
consulted throughout the process, to ensure that the selected
scheme has the support and meets the needs of those most affected
However, in the UK, it can take from 15 to 20 years to plan,
carry out environmental assessments, promote, design, construct,
and fill a new reservoir so it is vital that all decisions are
taken in due time.
The need for water for living, for growing food, and for
renewable non-polluting power is paramount for society. Storing wet
season runoff is a long term sustainable method of providing this.
However reservoir schemes need careful environmental and social
impact studies in order to test their viability and all appropriate
measures should be taken to minimise any negative impacts.
Note: CIWEM Policy Position Statements (PPS) represents the
Institution's views on issues at a particular point in time. It is
accepted that situations change as research provides new evidence.
It should be understood, therefore, that CIWEM PPS's are under
constant review, that previously held views may alter and lead to