how blocking of river flow by dams prevent water from mixing and how it leads to unhealthy nutrient levels in water?
When water is stored for some time in large reservoirs like dams, it may become stratified, whereby the warmer waters (that are less dense) float on the cooler (more dense) waters in the deeper parts of the reservoir. The surface waters are further warmed by the sun and, as the density of water decreases with rising temperatures, mixing of the cooler mass of water at the bottom is prevented. As a result, stratification of the water column may occur.
In these circumstances, there is a strong seasonality whereby the water mass at the bottom may become deoxygenated and unable to support life, although the surface waters may be well oxygenated. New reservoirs show an increased tendency to become anoxic in the period immediately after filling when the large quantities of vegetation flooded by the rising water start to decay. The tendency for reservoirs to become deoxygenated is also accelerated in regions where there is a substantial input of nutrients from agricultural fertilizers, animal wastes or human sewage, either directly into the reservoir or into the river feeding it.
The warm upper waters of a reservoir are well lit by the sun's rays and, in contrast to the swiftly flowing river waters, can support abundant phytoplankton that produce oxygen by photosynthesis. In contrast, the deeper waters receive little light, and phytoplankton are scarce. Organic matter carried by the river is deposited on the bottom of the reservoir and provides an energy source for microbes especially bacteria. They consume oxygen during the process of decomposition and, since the water column does not mix, the oxygen in the deeper water is gradually depleted. So most aquatic life becomes confined to the surface waters. This does not happen in normal rivers where continual mixing of surface and bottom waters occurs in even the deepest rivers.
If the dam may hold all water and prevent any flow to escape downstream, then dewatering and perishing of all aquatic life in the drawdown zone occurs. But more commonly, at least a proportion of the natural flow volume is allowed to pass the dam. The condition of this water depends upon whether it has been released from the surface of the reservoir, or if it comes from the reservoir depths. Surface waters are usually clear, warm and oxygen-rich, and contain little detritus but may have much more phytoplankton than would be found in river waters. Water released from the depths of the reservoir, on the other hand, is cool and oxygen poor, and contains little or no phytoplankton. Accordingly, animals downstream of the dam may have to deal with elevated temperatures but enjoy a rich food supply (in the form of phytoplankton), or may have to cope with cool conditions, low oxygen and little food.
Regardless of whether water is released from the surface or depths of the reservoir, the water will contain little suspended sediment. Once the water starts to move downhill in the channel below the dam, it has a tendency to erode and transport material from the river bed or banks creating an erosion zone that often extends for several kilometres below the dam. This means that fine sediments, as well as sand and gravel, may be eroded, and these substrates, which may be important to spawning fishes, are often scarce in river sections downstream of dams. Reservoirs and dams pose a barrier to downstream migration of young fish, as well as to the upstream migration of the adults.