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How does sewage affect river ecosystems?

Stormwater Outfall in BanffWe all know that flooding is a needed event for healthy aquatic ecosystems. But flooding can also lead to negative consequences for rivers — due to human development. In Canmore, Alberta, a combination of events nearly flooded Canmore streets with sewage and required an emergency discharge of raw but diluted sewage into the Bow River for 19 hours from June 8 to 9. While it appears the discharge of sewage to the Bow River was short-lived, there was still a need to warn local citizens of the potential health threat. But we also wanted to know what kind of impact sewage has on river and lake ecosystems and wanted to share this information with you.

A State of Emergency in Canmore

Heavy rain, rising river and groundwater levels, and a system incapable of keeping up with the river and groundwater levels led to sewage backups in south Canmore residents' homes and the possibility of sewage running down Canmore streets. EPCOR attempted to resolve the problem with a second semi-complete lift station and other lift stations, with trucks pumping to reduce pressure on the first lift station, and with trucks moving sewage from one catchment to another — to little effect. One power blip and sewage would be in every south Canmore basement and running down streets. Under a declared state of emergency, the Town, through a temporarily constructed pipeline, began pumping sewage (5%) diluted with the high amount of groundwater (95%) into the Bow River on Friday June 8 at 10 pm. This pumping of diluted sewage into the Bow River stopped Saturday at 5 pm and the state of emergency ended Monday June 11 as the river levels stopped rising and groundwater levels dropped (McRoberts 2007).

While these efforts certainly avoided a disaster for Canmore residents, dumping sewage into a river still has negative impacts. First, sewage is a health concern. If certain bacteria are present, it can create human health issues — illness and death in the short term, or long-term effects on reproduction and other bodily processes. In the case of Canmore, the sewage was highly diluted with groundwater. But, in general, how does untreated sewage disposal affect river and lake ecosystems?

Effects of sewage in freshwater ecosystems

Dumping sewage into water bodies, such as rivers or lakes, creates a human health hazard but can also negatively disrupt the river and lake ecosystems. The sewage contaminates the water, spreads disease, and leads to environmental degradation (WQM 2004). Here is a list of effects of untreated sewage disposal into freshwater ecosystems:

  1. Increased organic matter (from the sewage) breaking down in the river reduces the amount of dissolved oxygen in the water body as the decomposition process uses up the available dissolved oxygen. Fish and other aquatic life need that dissolved oxygen in the water to live.
  2. Sewage heightens the levels of nutrients, increasing the bioavailability of nutrients, which can increase productivity of plankton near the sewage outfall (Munawar et al 1993) and increase the chance of algal blooms.
  3. Contaminants present in the sewage might be toxic for some already existing phytoplankton (Munawar et al 1993).
  4. Sewage can increase the turbidity and amount of suspended sediments. This effect reduces light available for plant growth, can smother in-stream habitats, and damage fish gills and respiratory structures of other species (WQM 2004).
  5. Sewage (and stormwater runoff) can introduce pesticides, other chemicals, and heavy metals into the water column. It may also introduce fine sediments, which have the potential to (bio)accumulate within animal tissues and have long-term toxic effects. Sewage and run off may increase acidity, such as from acid sulphate soils which kill fish and crustaceans, cause fish red-spot disease, damage or cause death of oysters, and interact with sediments to release heavy metals (WQM 2004).
  6. Industrial effluents (often a complex mixture of chemicals) can negatively affect fish by impairing growth and reproduction and by reducing immune competence. These effects have the potential to impact fish populations (Environment Canada).
  7. Microbial pathogens introduced by sewage into surface or groundwater can threaten public health, as well as affect ecosystem health and function (Environment Canada).
  8. Sewage can release water that is either warmer or cooler than the receiving water body. Because aquatic life has optimal temperature ranges within which it lives, too warm or too cool water temperatures can harm the aquatic life. For example, cold waters reduce ecosystem productivity, eliminate temperature-sensitive biota, and decrease survival of eggs, larvae of fish and aquatic insects (WQM 2004).
  9. Release of sewage can degrade vegetation and soil by depositing harmful chemicals in bottom sediment, for example.
  10. With sewage comes water that has some degree of chlorine or similar agent. The chlorine or other disinfectant react with organic matter (such as what's in sewage) to create different end-products, such as chloroform or haloketons, which can be harmful in either the short or long term. These reactions happen faster in warmer water (LennTech 2007).

Release of untreated sewage into freshwater bodies is sometimes necessary. Yet, it not only creates a human health risk but damages the health of the receiving water bodies in over short or long time periods. Our responsibility to ecosystems means we should have the capacity to deal with our own waste rather than expecting the rest of the ecosystem to do it for us.


Bow River Basin Council (BRBC). 2002. Urban Stormwater Bow Basin.

Environment Canada. 2004. Water pollution.

LennTech. 2007. Disinfection byproducts.

McRoberts, Dan. June 14, 2007. Diluted sewage pumped into Bow. Rocky Mountain Outlook, Vol. 7, Issue 24

Munawar, M., I. F. Munawar, L. McCarthy, W. Page, and G. Gilron. 1993. Assessing the impact of sewage effluent on the ecosystem health of the Toronto Waterfront (Ashbridges Bay), Lake Ontario. Journal of Aquatic Ecosystem Stress and Recovery (Formerly Journal of Aquatic Ecosystem Health). Springer Netherlands. Volume 2, Number 4 / December, 1993

Canada Centre for Inland Waters, Fisheries & Oceans Canada, 867 Lakeshore Road, L7R 4A6 Burlington, Ontario, Canada

Water Quality Monitoring (WQM). 2004. A Toolkit for Water Quality Monitoring for Local Government. Two pages: Onsite Sewage Monitoring, Aquatic Ecosystem Health

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