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 Impoundments and their effects

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Chris Daphne

Male Posts : 141
Join date : 2010-01-28
Location : Highlands of Scotland

PostSubject: Impoundments and their effects   Fri Mar 26, 2010 1:08 am

Impoundments can cause a lot of problems similar to those described in abstraction. Impoundments are usually built for HYDROELECTRIC, drinking water supply, provision of irrigation, flood control, fisheries and usually a combination. They fall into two main types that differ in the downstream flow conditions.
In Hydro electric schemes Water is discharged to generate power, the discharge is usually faster than than the existing natural flow or in finely controlled schemes used to respond to the daily ups and downs in demand, downstream flows may be erratic. Impoundments designed to hold up water tend to have steadier flows downstream with even seasonal changes levelled by a compensation flow. Many impoundments are linked to inter river transfer schemes so the problems of water quality and biosecurity are big issues.

Obstruction to fish migrations in both directions. Changes to the rivers natural regime may prevent fish re entering the system, natural cues to move like surges are obscured and chemical cues may not be detected. Flows which encourage fish to move upstream may be reduced. Severe water quality changes may be enough to act as a pollutant and deter fish. Dams provide a physical barrier, also increased concentrations of gases - dissolved nitrogen below outfalls can cause gas bubble disease ( Like bends in humans)

Changes in flow may mean velocities may be too high or too low for the species to tolerate. Increased flows wash out animals and plants, destroy spawning areas, juvenile refuges and change habitats along the channel. Low flows may prevent seasonal inundation of riparian breeding habitat resulting in sedimentation, destroying coarse substratum and allow other species to invade and compete with natural stock.

Thermal regime changes downstream of impoundments especially if cool hyperlimnion water is discharged. Not only is the size of the change important but also the rate in which it changes. Plants and animals may adapt to a gradual change but where a rapid change occurs this may not be tolerated. Nutrient levels also change, some impoundments may act as nutrient sinks as the stillwater allows some settling and algae also use up the supply. Enriched waters may be released. Oxygen levels also may alter, discharge water may be oxygenated by the turbulence when released but the hyperlimnion water may be poor in oxygen.

Downstream effects can be seen at species level. Work on European Mayflies ( Ephemeroptera) has shown that regulation creating low flows encourages species associated with slow or static water and reduces those that are found in fast systems. Adaptable species thrive almost anywhere, these changes probably reflect basic tolerance levels of the individual species to different flows.
Many effects are very complex. Invertebrate communities downstream of an impoundment in the US have been monitored. No single factor, natural or man made dominated the patterns but the impoundment caused effects that had a knock on effect through the ecosystem. Temperature changes excluded certain grazers allowing algal biomass to increase, this physically altered the habitat and basic food webs.
Another example was in Norway where the submerged rush Juncas bulbosa increased to nuisance levels below a hydroelectric installation while in nearby unregulated rivers numbers did not change.

Effects on reservoirs

After the initial impoundment there is usually an increase in the biological productivity with a risk of severe eutrophication due to leaching of the nutrients from the flooded ground and decaying vegetation. The production of planktonic crustaceans increases as a result. The forage area of fish increases and the population density is lowered resulting in higher growth rates for the first few seasons.
Fluctuating water levels have an impact on on the littoral plants and animals, fringing vegetation will be lost so animal species associated with it will also be lost or reduced. Other species may increase. The overall result is a decline in food for fish. On swedish hydroelectric schemes fluctuating water levels caused a 75% reduction in invertebrate numbers in the zone directly exposed to water level changes, 25% in the remaining area. The maximum number of invertebrates occurred just below the lowest level to which water was drawn down.
The breeding success of fish such as Pike, Perch and Roach will be impaired by these fluctuations as they require submerged vegetation to spawn whereas Charr need open gravel and may benefit.
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James Page

Posts : 2153
Join date : 2010-01-21

PostSubject: Re: Impoundments and their effects   Fri Mar 26, 2010 7:07 am

m8, thats superb, I think we can all understand some of the problems, the big plus is we have examples of how this hydropower stuff destroys/ruins, or in plain language mucks up our rivers
nice one fella
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