By John Douglas

This article is largely adopted from information complied by a USA national river group, American Rivers, and modified to New Zealand conditions, using the Clutha River as the main example.

Information on the awareness on environmental issues of dam engineering mainly comes from a paper titled "Dams and Environment" prepared by the International Commission On Large Dams (ICOLD). ICOLD was founded in 1928, with a total of 81 member countries and now today attach great importance to the environment and social aspects of dams and reservoirs, dam safety that now have as equal importance as the technical, economics and financial feasibility of a dam project.

Environment-conscious planning and construction, requires adequate environment impact assessments and the implementation of mitigating measures. This requires great care to protect the environment from the avoidable harm or interference.

The larger the project, then the greatest the effects it has on the nature and social environment.

Concern for the environment, including both natural conditions and social aspects, must be manifest from the first planning steps, throughout all phases of design and implementation, and during the entire operating life of a project. Special attention should be paid to any affects on biodiversity or the habitat of rare and endangered species

The people directly affected by a project should always be the first to benefit instead of suffering for the benefit of others eg; in 1916, communities in Switzerland are entitled to considerable annual payments and quotas of free energy for granting the rights to hydropower developments on their territory.

Dam promoters must act as mediators and educators with the aim of becoming good neighbors and not intruders.

Today both Contact Energy Ltd and Meridian Energy run power schemes here in the south half of the South Island that have caused considerable damage to the environment which has been allowed in the past in the name of "national interest".

This is now to be tested under the Resource Management Act (1991) as Contact Energy is in the process of renewing its water rights to run the Roxburgh, Clyde and Hawea dams on the Clutha River while Meridian Energy is intending to add more hydro-power schemes on the Waitaki River.

Dam building harnesses rivers for a variety of purposes, including hydropower, irrigation, flood control and water storage.

While dams can benefit society, they also cause considerable harm to rivers. Dams have reduced considerably the material that has been lost out along the coast (long shore drift) eg: Dunedin beaches, raised flood levels as a result of sediment built-up behind dams, degraded river ecosystems, diminished recreational opportunities and degraded fisheries on nearly all our New Zealand rivers. Some of these dams are old, (eg; most of the older Otago irrigation dams), are no longer safe and a few, those built in the early mining period, no longer serve their intended purpose.

To many people, these dams have negative impacts to those on the riverside and riverside communities eg, Alexandra, getting little or no benefits of the dam. Dam removal may be a reasonable approach to restore healthy rivers and riverside communities though most dams could be operated in a better fashion that reduces their current impacts on the river.

  1. Dams Reduce River Levels. By diverting water for power eg; the proposed Meridian Energy Waitaki canal scheme, dams remove water needed for healthy in-stream ecosystems. Stretches below (irrigation) dams can be often completely de-watered in dry seasons eg; some of the Otagoís irrigation schemes.
  2. Dams Block Rivers. All dams prevent the flow of plants and nutrients, impede the migration of fish, eels and other wildlife, and block recreation use. Fish passage structures can enable a percentage of fish to pass around a dam, but multiply dams along a river make safe travel unlikely. Contact Energy is not making any attempt to put in fish passes
  3. Dams Slow Rivers. Many fish species such as salmon depend on steady flows to flush them downriver early in their life and guide them up upstream years later to spawn. Though there are no stagnant reservoir pools on the Clutha, if there were, disorient migrating fish and significantly increase the duration of their migration.
  4. Dams Alter Water Temperatures. By slowing water flow, most dams increase water temperatures. Other dams decrease temperatures by releasing cooled water from the reservoir bottom. Fish and other species are sensitive to these temperatures irregularities, which often destroy native population.
  5. Dams Alter Timing of Flows. By withholding and then releasing water to generate power for peak demand periods, dams cause downstream stretches to alternative between low or no water and powerful surges that erode soil and vegetation, and flood or can strand wildlife eg; Lake Hawea to Albertown, the Clutha river below Lake Roxburgh.
    These irregular releases destroy natural seasonal flow variations that trigger natural growth and reproduction cycles in many species. Some of the aquatic fauna are now extinct, imperiled or vulnerable. Reduction of the aquatic fauna reduces the feed supply for trout, salmon, eels and other bottom feeders.
  6. Dams Fluctuate Reservoir Levels. Peaking power operations can cause dramatic charges in reservoir water levels, (Lake Hawea in the 1970ís experienced some 20 metres which was forced to introduce controls to limit it no more than a maximum of 10 metres), which degrades the shorelines, create dust storms and disturb fisheries, waterfowl and bottom-dwelling organisms.
  7. Dams Decrease Oxygen Levels in Reservoir Waters. When oxygen-deprived water is released from behind the dam, it kills fish downstream.
  8. Dams Hold Back Silt, Debris and Nutrients. By slowing flows, dams allow silt to collect on river bottoms and bury fish spawning habitat. Silt trapped above dams, accumulates heavy metals and other pollutants. The river bed rises as the silt keeps building-up, increasing the vulnerability to riverside communities eg; Alexandra by some three metres, flooding properties, flood roads eg; Manuherikia Road, flood river walkways, threatened bridges eg; Shaky Bridge, Alexandra Bridge, Clyde Bridge and as well choke / block rivers and creeks of the reservoir tributaries eg; Manuherikia River. The storage capacity of the reservoir reduces as sediment levels build up eg; Lake Roxburgh has now lost more than 40% of its reservoir storage. Gravel, logs and other debris are also trapped behind dams, eliminating their use downstream as food and habitat.
  9. Construction of Levees (floodbanks) Provide a False Sense of Security. Levees cause more damage than good. Riverbeds keep rising due to sediment deposition and during a large flood, inevitably the levees are breached and the flood hazard is then tremendous. In addition the levees prevent the river when in flood, from using its natural floodplain. In some cases, pumps are installed behind flood banks to pump water that is trapped in behind. When the pump(s), fail, the flooding is just as bad as if there is no bank(s).
  10. Flushing is Harmful both at the Reservoir and Downstream. When flushing occurs, it causes dramatic changes in the reservoir water levels which degrades the shorelines, disturb fisheries while that below the dam, the fine silt is highly damaging to pumps eg; Roxburgh orchards, while the silt blankets the river bed, smothering the gravel areas where algae and insects grow, and where fish feed. Further, flushing only works when the silt is fresh and recent and able to be Moved, before it settles and hardens.
  11. Dam Turbines Cut up Fish. Fish following currents downstream, are drawn into and cut up by the power turbines. When fish are trucked or barged around dams, they experience increased stress, disease and decrease their homing instincts.
  12. Dams Increase Predator Risk. Warm, murky reservoirs often favor predators of naturally occurring species. In addition, passage through fish ladders or through turbines that injure or stun fish, eels, making then easy prey for flying predators like gulls, herons and shags.
  13. The Effect of Faulty Design Dam Problems. When dams are not designed to handle extreme flood events; the floodwater is discharged over the dam structure that can be quite destructive to the dam structure and to the riverbanks just below the dam. There is also for the possibility that the dam could collapse, causing a tremendous amount to burst through destroying all in its path.
    Both the Roxburgh and Clyde dams have been designed on underestimating the one in 500 year flood event the Big Flood of 1878. The government engineers at the time, based their dam design calculations on that of the 1878 flood of 3,300 cumsecs, with a 50% safety margin added.
    In the 1990ís, the ORC from a river computer model program, now has the 1878 flood at 4650 cumsecs, which now has both the Roxburgh and Clyde dam only able to safely handle a 1 in 100 year flood. Any water that the dam cannot control, will now discharge over the dam structure itself. Not only is the water destructive to the dam structure itself, but also to the riverbed / riverbanks. Further the powerhouse will be in danger and be forced to shut down. This will mean that the power will be lost to the pumps controlling the water in behind the flood banks at Alexandra.

Dams can benefit the community as long as there is a balance between hydropower productions operating such that there is a minimum impact to ecology of the river and effects to riverside / lakeside communities. This requires all power companies to accept all the responsibilities that come with ownership of a national asset


Honoring the Dam Construction. Often at the dam, at a lake lookout site, can be found a plaque honoring by name the government leader, the engineer, major contracting firm(s), the dams height, its size, commissioning date, volume of water held or diverted, power generation, flood capacity measurements and thatís fine.

But there is never a plaque to what is lost; the names of any species hurt and lost, archaeological sites destroyed, the names of any people displaced, the number of people displaced, communities flooded, the hectares of horticulture and farming land lost, potential loss of new technology eg; vineyards, the cost to the taxpayers, the price of maintenance or decommissioning and why this option was chosen in the first place.

--- From a member of the World Commission on Dams amended to our own local situation.