Dams and Reservoirs

Dams are constructed across valleys or rivers to store, regulate and divert water for various purposes such as agricultural production, hydropower generation, human and industrial use and flood peak attenuation. Most dams serve multiple purposes. The task of dams to deliver water at the time of need for human use interferes with the normal ecological functions of the river. The construction of dams and reservoirs may lead to adverse impacts on the following aspects of riverine ecosystems:

 

Flow regime:

  • Reduced seasonal variability of flow, i.e. low flows increased and high flow decreased
  • Increased flow fluctuations at hourly and daily timescales
  • Change in frequency and timing of floods

(impacts depend on reservoir capacity and dam design and operation)

Sediment load and channel structure:

  • All sediment but the wash load fraction is trapped in the reservoir
  • Reduced sediment downstream leads to possible accelerated bed degradation and bank erosion immediately downstream of a dam
  • Encroachment by riparian vegetation, decreasing the channelís conveyance capacity
  • Possible coastal erosion

Water quality:

  • Constantly cold water released from deep layers of the reservoir reduces the temperature variability of downstream river water
  • Possible accelerated eutrophication, due to the reservoir incorporating and trapping nutrients
  • Water turbidity is decreased, which can lead to increased primary productivity
  • Reservoir will export plankton downstream, changing availability of food resources

(most impacts on water quality depend on a reservoirís retention time)

Habitat, biodiversity and natural resources:

  • River species largely replaced by lake species in reservoir
  • Native river species reliant on natural flow regime will disappear downstream of the dam
  • Changes in thermal regime affects many species, e.g. invertebrates
  • Short-term flow fluctuations (de-watering) result in stranding of organisms, in case of a hydropower dam
  • Most silt and organic matter is retained in reservoir, instead of fertilizing flood plains
  • Floodplain structure and functioning is changed, as flooding is reduced or eliminated
  • Dams sever the longitudinal connectivity of the river which impedes or hinders the passage of fish and invertebrates along the river course


Possible Mitigation Measures

  • Managed flow releases by reservoir operation, leading to seasonal variability of flow
  • Multiple and/or depth-selective intake structures for maintaining the natural seasonal temperature regime of released flows in reaches below dams, as well as water quality
  • Allowing for fish passage over weirs and dams, in both directions
  • Appropriate sediment bypassing devices
  • Bypassing large woody debris

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