Dams are structures built across a river for the storage of water and to provide the regulated reliable source of water during the demand and water scarcity. Mainly dams are built for providing services such as drinking water supply, Irrigation, Hydropower generation and also to provide flood control. According to International Commission on Large Dams (ICOLD) the dams are classified based on the height of the impoundments into large dams and small dams. The dams which has the height of impoundment more than 15 m are classified as large dams and less than 15 m are classified as small dams. According to World Commission on Dams (WCD) there are about 45000 large dams in the world.
The water spread area behind the dam is generally termed as reservoir. Reservoirs supply a major source to irrigation, the seepage of water from the irrigation canals recharges the ground water. Few other uses of reservoir storage water are for culture of reservoir-based fishes and recreation. These are some of the beneficial uses of dam and reservoir but its beneficial uses are outweighed by the impacts such as the negative effects on the upstream and downstream ecosystem, modification of riverine ecosystem, sedimentation of reservoir, eutrophication of reservoir, changes in the inflow and outflow water quality and hydrological effects. The environmental impacts of dam will be more when compared to the beneficial uses of dam and reservoirs.
Effects of dam on Upstream and Downstream Ecosystem
Damming of river disturbs the natural biogeochemical cycles and majorly it affects the downstream ecosystems, upstream ecosystems and also it modifies the aquatic ecosystem components. In recently built dams, the reservoirs are young and anoxic conditions usually occurs in the upstream of the reservoirs. The vegetations submerged under the water in young reservoirs, decompose and they are the important source for carbon dioxide (CO2) and methane (CH4) emissions. Emissions of carbon dioxide (CO2) and methane (CH4) will be more during the initial period of flooding. In upstream major effects can be witnessed on the land which gets flooded as reservoir. Sedimentation of reservoir, soil erosion in and around catchment areas and aquatic weeds are some of the disruptions in the ecosystem in the upstream.
In downstream, the outflow from the reservoir will have less dissolved oxygen concentration is this due to the deep outlets provided in the dam. A sufficient quantity of dissolved oxygen concentration is required for the downstream ecosystem and also for self-purification process of rivers. In downstream changes occurs in the morphology of the river due to the dam built in it and these in turn cause reduction of bed load sediment and the clear water in the downstream can coarsen the streambed due to lack of supply of bedload sediment. The coarsening of the stream bed makes it unsuitable ecological habitat for native and alien species. Bank erosion rate increases and make it prone to channel degradation and that reduces the navigational depth of the river, there by affecting the recreational use of the river or channel at the downstream.
Effects of Dam and Reservoir on Water Quality
The water stored in the reservoir for a longtime or even for a short duration can induce physical, chemical and biological changes in the water and that in turn affects the overall quality of water. The chemical composition of stored water in a reservoir will be different to that of the flows received to the reservoir. The stored water in the reservoir is influenced by several factors that can affects the quality of water. The stored water quality depends on the size of the dam, its location in the river system, its geographical location, the detention time of the water and the source of the water.
Recent researches have witnessed that eutrophication is one of the main water quality problems in reservoirs that are caused due to dissolved nutrients in the stored water. Excess nutrients in the stored water may lead to increased primary production and can cause eutrophication and also other environmental problems connected with it. Eutrophication is caused by the major dissolved nutrients such as phosphate (P) and nitrogen (N) which are dissolved in the water. The main sources of P and N are from leaching of soil during high flows within the catchment and these suspended stream sediments and total dissolved solids transported by the rivers which enter the reservoir. Additionally, nutrients can enter through other sources such as resuspension of sediments with in the reservoir due to bioturbation and also due to atmospheric deposition.
Modification in Inflow and out flow by Dam and reservoir
The quality and chemical composition of water entering the reservoir will be different from the water released or stored in the reservoir. The water entering the reservoir will be highly turbid and it generally contains suspended and dissolved organic and inorganic sediments and nutrients.
Generally, changes occur in the inflow and out flow water quality such as pH and salinity as well as in the concentration of nutrients, and dissolved oxygen. The outflow structure and its elevation determine the concentration of dissolved oxygen and quality of water that is released from a reservoir. The water released from the reservoir at the top surface outlets will be well-oxygenated, warm and nutrient depleted water whereas the water released from the bottom outlets of the reservoir will be cold, oxygen depleted and nutrient rich water. Therefore, the hydraulic design of the dam and its structure has also influence in the water quality of the reservoir.
Reservoir and its contribution to sedimentation
Rivers during the natural flow transports heavy, fine sediments and silt in suspension and when it reaches the reservoir, the velocity and turbulence of flow is reduced and suspended material gets deposited. The finer particles may take long time to settle and usually it moves throughout the reservoir in suspension. Mostly large fraction of organic matter and nutrients are delivered to reservoirs by the inflows in suspended, particulate or dissolved form which contributes to the sedimentation processes. The suspended organic particulate material settles by physical process and sedimentation occurs while the excess dissolved nutrients in the water will pave the way for algal growth. This algal growth will in turn contribute to organic material when they die and sink. Thus, both external input and changes within the reservoir can contribute to the total organic sediment load.
Reservoir and its contribution to sediment load
The sediment load entering the reservoir inflow water vary in quantity and quality depending upon the nature of catchment. The suspended load and dissolved load are together considered as sediment load. The suspended load is the major part and it is an important seasonal contributor, it depends on bed slope of the inflow river and also the velocity of flow entering the reservoir. The suspended sediments contain nitrogen, phosphorus and silica along with the sediments. The nitrogen and phosphorus are the important nutrients which are required for the growth of algae and other aquatic microorganisms.
Hydrological Effects of Dam
Hydrological effects mainly related to the residence time of the water. The hydrological effects of a dam become less significant with distance downstream as the length increases. The increase in residence time affects salt concentrations due to evaporation, changes in temperature and oxygen concentration. Both, increased residence time and production of biomass will alter redox conditions and subsequently effect nutrient cycling and promote potential hazardous release from the sediments. The actual sedimentation in a reservoir may depend on these factors and or interaction between them.
The dam has a major impact on fish populations, especially on fish migrations. Few species of fishes migrate for reproduction or migrates to prevent them from predators. The fish movements from upstream to downstream and vice versa are stopped or delayed when dams are built across the river. Fish can suffer during their transit through hydraulic turbines or over spillways. Changes in outflow and water quality has also indirect effects on fish species and its population.
Building and operating large dams may be viewed as beneficial. Environmental impacts associated with dams and reservoirs influence the design and success of engineering projects. The consequences of environmental impacts of dam may not be immediate and the severity may be recognised after a long period of time.