Can Dams Cure or Cause Drought?

The Weather Network reports, “22 of 31 watersheds throughout southern Ontario are reporting some level of drought conditions (July 29). Ten watersheds have advanced to Level 2 (moderate drought), including most watersheds in eastern Ontario, plus the Hamilton and Grand River watersheds. At this level, local residents are asked to voluntarily lower their water usage by 20 per cent. On August 4, three watersheds in southeastern Ontario have posted Level 3 (severe) drought and residents are asked to cut water usage by 50 per cent.


In such times of drought, people often begin to think about dams on our rivers and streams to retain and manage water. But – Dams Have Two Sides.


Canadians have proudly built dams since there were Canadians. Dams and the power they harness are commonly described as “green” and “renewable,” but there are several disadvantages to dams. They are not totally “green” and they can irreversibly change a river. The water may be renewable but these river habitats are not.


This is a story we don’t often hear because dams are very advantageous to one species: the humans.


Dams to Regulate Water Levels


During settlement, dams were the major source of power; they ran sawmills for lumber and grist mills for flour. No longer major power sources, many dams now just regulate water levels for recreational power boaters in our lakes and canals and increase the real estate value of cottages and retirement homes. The role of those dams is no longer vital and it may be beneficial to consider these dams as optional.


Using dams to regulate water levels should be contrasted with the disadvantages. Sound decisions are not possible otherwise.


There is a large base of knowledge that explores the ecological effects of making variable water levels more constant. Most river and lake watersheds on the Canadian Shield have naturally variable water levels. Often variation is extreme, from boulder-rolling spring runoff all the way to just wet rocks in late summer.


Regulatory dams that enforce even slight changes in water level can deliver extreme impacts to the ecological processes in rivers and lakes. Those impacts include:

  • Preventing seasonal oxidation of organic matter exposed to air during low water and allowing organic matter to fill lake beds faster.
  • Decreasing oxygen concentration in the water by heating the water and by reducing the mechanical mixing by whitewater flows.
  • Removal of breeding sites for fast water species and removing the flow that delivers food to species that feed by filtering.
  • Removal of upwelling sites where current flows cause vertical currents to move nutrients up into the sunlight to increase phytoplankton production for the food chain.


These are a few examples of the major ecosystem impacts of forcing water levels to become constant.


Beaver Dams


Beavers were the 0riginal Canadian dam builders, and beaver dams are often cited as having only beneficial effects on ecosystems – that point can be debated, too. Beaver dams do cause an increase in diversity of plant species which causes an increase in bird species diversity. Essentially these effects result from the beavers inserting small lakes into a previously flowing stream. Lake plants and lake birds follow. With time the beaver pond fills with rich organic matter and silt trapped from the stream. A beaver meadow results and has species that differ markedly from the surrounding ecological community. Beaver dams have many of the basic impacts of human dams but the ecosystems are better able to adapt to those impacts because the effects are temporary. As the beaver population changes, so do the effects of beaver dams on ecological processes at the landscape scale.


Like older human dams, beaver dams deteriorate and can be washed out by high water. For both beaver and human dams, poorly planned placement of buildings and bridges allows economic damage. Beaver ponds also trap very large amounts of silty sediment that is nutrient-rich, especially rich in nitrogen. If a backhoe removes all the foundation of a beaver dam right down to bedrock, all the nutrient-rich sediment stored in the bed of the pond can be washed downstream and change the bay of a lake from moderately nutrient-rich to algae-filled overnight.

Dams for Energy


Dams to generate electricity are newer and usually larger but are often built in the same locations as older dams simply because that is where engineers can harness falling water. The electricity that these dams generate is often considered vital but a significant fraction of it is for export – for profit. These dams also are valued as mechanisms for the storage of electricity produced above the ongoing demand. That excess power can be used to pump water back up above the dam to be used to generate more electricity at times of higher demand – usually called pumped storage.


When a dam floods areas of terrestrial or shallow wetland habitats, one of the first important changes is an increase in methylated mercury. The mercury comes from the bedrock. Without methylation, mercury does not enter living systems. Methyl mercury is produced when methane (CH4) is produced by the decay of organic matter without access to oxygen.


High levels of methyl mercury have been found associated with many hydroelectric dams. However, the effects of high levels of methyl mercury on natural systems are not well known, but we have seen the effects of methyl mercury in humans. It affects the neurological system and has been called Minamata disease. First Nations people in the Wabigoon-English watershed of northwestern Ontario have felt the effect of this toxin, although in that case the mercury came from a pulp mill.


The water held back behind a dam is called an impoundment, and this water changes the global water cycle. Water has a very high capacity to store heat, so when the surface area of the water behind a dam is increased, it is heated more by the sun. This causes a significant increase in the evaporation of water vapour and carries more heat into the atmosphere. More water vapour and more heat creates feast or famine rainfall – just what we are seeing. So, rather than being a cure for drought, dams can be part of the cause.


As global warming increases, this effect will increase. The warmed surface water will allow greater algae growth and favour invasion by other algal species, including toxic blue-greens.


If the dam overflow is at the surface, then warm water constantly flows downstream, changing the environment. Coldwater habitats are eliminated. Caddis fly species are among the first to go, followed by Speckled Trout, Atlantic Salmon and (non-native) Brown Trout. Smallmouth Bass take over. Fun to catch but less diversity.


Dams as Barriers


Dams are barriers in streams and rivers. Speckled Trout do not jump over barriers – even low ones. Unlike Pacific Salmon, Atlantic Salmon also are very poor jumpers. Such species are prevented from getting to upstream spawning grounds. Their populations along a stream are broken into smaller subpopulations by barriers. These smaller populations separated by barriers are at higher risk of local extinction due to their small numbers.


Ecologically, the impoundments made by dams are essentially ponds inserted into a flowing stream habitat. This is a fundamental change in the landscape and in the ecological processes there. For human-built dams, this change is imposed over large areas and for long periods of time, and the ecological changes are rarely considered.


Because dams can cause as many problems as they cure, those that are not replacing more damaging ways of generating power should be removed as part of environmental rehabilitation.

Dams as an Option


Dams can change a river significantly. The water used for power applications may be renewable but the impacts on river habitats are not “green”. Dams may affect development of droughts by their effects on evaporation/precipitation rates in the global water cycle. Effects on aquatic plants and animals, including sport fish, are dramatic. The role of many dams is no longer vital to humans and it may be best to consider these dams optional. The balance of importance given to all other species compared to humans needs consideration.



Leave a Reply




You can use these HTML tags

<a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <s> <strike> <strong>