Adequate data collection and an environmental assessment process that identifies potential water quality problems prior to dam design are essential to problem avoidance and mitigation.  Where warranted, there are a number of design and operational systems that can be employed to minimise negative water quality impacts within the storage and downstream: 

• Selective or multi-level offtakes in deep reservoirs can limit the amount of water drawn into the power station from cold, anoxic depths.  Seasonal management of lake levels can also be utilised to ensure offtake of oxygenated water at seasonally appropriate temperature.
  
  
• Downstream gas supersaturation can be addressed through the use of stilling basins, spillway design, or through structures that favour degassing.
  
  
• Air injection facilities and aerating turbines can be considered to avoid de-oxygenated water being delivered to the downstream river system.
  
  
• Reservoir clearing prior to inundation can limit the amount of organic decomposition in the reservoir, and thus the consumption of oxygen.
  
  
• Reducing the residence time in reservoirs limits and ensuring flow-through can improve water quality conditions.
  
  
• In shallow lakes, baffles can direct circulation and ensure adequate water flow-through and mixing, and can also to inhibit wind-induced resuspension.  Planting of appropriately selected microphyte communities (aquatic vegetation) can control turbidity due to sediment resuspension and erosion.  Raising minimum lake levels to minimise wind-induced resuspension.
  
  
• Catchment management to improve water quality of run-off, and collection/treatment of pollutant-laden inflows can have significant water quality benefits for hydro reservoirs.  Water pollution control measures such as sewage treatment plants or control of industrial emissions may be required in particular cases.