Combined Sewer Overflows

Project main page || Data || Relationships || Further resources


Sewage systems are a crucial piece of waste management infrastructure, and when properly treated, the effluent from these sewage systems can have minimal to negligible effects on water quality. However, although treatment facilities are in place and effluents are generally safe in the Ottawa River Watershed, the risk of contamination has not been completely eliminated. During periods of extreme rainfall, the increase in rainwater volume can cause the capacity of combined sewers – sewers that transport both sewage and stormwater – to be exceeded. This results in overflows, introducing raw, untreated sewage into the river. 

Untreated or insufficiently treated sewage can have detrimental effects on aquatic life, and pose health risks to recreational water users. Perhaps the most obvious – at least in terms of public safety – consequence of raw sewage introduced to the river is the risk of exposure to harmful contaminants such as E. coli. However, CSOs also pose other threats to the health of the watershed, such as introducing excess nutrients, and decreasing the amount of free oxygen. The organic material in sewage contains phosphorus, which when present in high enough amounts can have negative consequences such as algal blooms. The additional organic material introduced by CSOs also creates a higher demand for decomposition, a process that requires oxygen, resulting in a decrease in dissolved oxygen available in the water column to organisms such as fish and benthic invertebrates for respiration. The severity and duration of the effects of a CSO event can be influenced by the volume and flow rate of the river or body of water where CSOs occurred, with faster-moving and deeper waters having greater capacities to dilute the contaminants than shallow or slow-moving areas.

The environmental and public health effects of CSOs are also directly and strongly tied to land use, with densely populated and rapidly growing urban centres presenting a larger threat than more natural, undeveloped areas. There are several reasons for this, but some noteworthy causes include urban substrates such as asphalt and concrete that do not absorb rainwater like more natural substrates like soil do. Absorption and infiltration of rainwater in natural substrates increases if this land has plant or forest cover. In developed, urban areas,the constantly expanding area of impermeable land cover means more rainwater ends up flowing into the sewers as runoff, increasing the risk of overflows. In rapidly growing areas, the capacity of sewage systems can be quickly surpassed if they are not updated to keep up with the demand or sewers are not fully separated. With Ottawa-Gatineau as one of the fastest-growing urban centres in Canada, CSOs are of particular interest as an indicator of the health of the Ottawa River Watershed. 

Relationships to other indicators:

Combined Sewer Overflows

Every time there is a Combined Sewer Overflow, sewage, along with stormwater is released into a waterbody introducing pollutants, pathogens and excess organic waste. Untreated or inadequately treated sewage poses health risks for people who use the water for recreational activities, such as swimming, paddling, etc. It also impacts aquatic ecosystems as the presence of organic matter, and its decomposition, decreases the amount of dissolved oxygen available.

Combined Sewer Overflows
Algal Blooms Water Temperature Dissolved Oxygen Chlorophyll-a Total Phosphorus Riparian Connectivity Flow Change in Land Use Benthic Invertebrates Invasive Species Fish Richness Water Mercury Ice On/Off

Algal Blooms

Algae are a group of single-celled and multicellular plant-like organisms. Algal growth contributes and supports the ecosystem in much the same way that plants do, through the production of oxygen and as a potential food source for smaller animals. However, when environmental conditions shift from their normal ranges, this can promote algal blooms, a rapid increase in population size of algae.

Find out more!

Water Temperature

With global temperatures on the rise, water temperature (particularly annual maximum temperatures) will be a crucially important indicator of watershed health moving forward. Water temperature is strongly correlated with a number of other important indicators of ecological status, and can often help to explain, give context to, or even predict changes in habitat quality.

Find out more!

Dissolved Oxygen

Dissolved oxygen (DO) is the concentration of free oxygen in the water column and comes from aquatic plants and algae through photosynthesis, and can also be transferred from the air at the surface of the water. DO is essential for all aquatic life and for the decomposition of organic waste. The amount of available oxygen in the water column is also affected by several abiotic factors including temperature, salinity, pressure and depth, wind, flow, and wave action.

Find out more!


Chlorophyll-a is used in oxygenic photosynthesis and is the predominant form of chlorophyll in green plants and algae. Measuring the presence of chlorophyll-a in an aquatic environment allows for a better understanding of the density and prevalence of algae which can have an impact on the concentration of dissolved oxygen or likeliness of algal blooms.

More information coming soon!

Total Phosphorus

Phosphorus is a fundamental building block of life. It has key structural roles in genetic material (eg. DNA) and cellular membranes, and is an active component of cellular energy production, storage, and transfer, meaning many necessary biochemical processes that all living organisms rely on (such as respiration, and nutrient absorption) can only happen if phosphorus is available.

Find out more!

Riparian Connectivity

Natural, vegetated riparian areas, the unique, transitional space between aquatic and terrestrial ecosystems, provide numerous benefits including nearshore habitat, filtering runoff, moderating local temperatures, and stabilizing shorelines. Measuring riparian connectivity can provide important information on the health of a watershed and a better understanding of the resilience to threats these areas have compared to shorelines that have been developed.

Find out more!


How much water or volume of water present at a given time and how quickly it moves along the length of the river determines flow. Flow can provide context to a number of other indicators and as an important measure to factor when examining water chemistry or understanding impact of pollutants (for example, the same concentration of pollution will impact a river with low flow differently than one with high flow). Comparing flow within the same reach can help identify different trends and, when affected by barriers, can impact aquatic biota.

Find out more!

Change in Land Use

Land use is a classification for all the various ways an area of land be used and includes agriculture, urban, roads, natural and harvested forests, wetlands and protected areas. Changes in land use have implications for both the quantity and quality of water introduced to the river through runoff and can help analyse changes in other indicators in proximity to where land use changes have occurred.

More information coming soon!

Benthic Invertebrates

Benthic invertebrates are the organisms found in soils, rocks and organic materials that make up stream, river, and lake beds. Pollution tolerance varies greatly between different species, so the relative abundances of pollution-sensitive and pollution-tolerant benthic invertebrate species is a good indicator of water quality and ecosystem health.

More information coming soon!

Invasive Species

Invasive species are non-native species that have been introduced to a habitat and are able to outcompete native species. Invasive species disrupt the established ecosystem which can result in a decrease in habitat quality and health.

Find out more!

Fish Richness

Fish richness is a measure of how many different species of fish are present in an ecosystem or habitat. Generally speaking, higher species richness is better, but some confounding factors include habitat type and the presence of invasive species (which can increase richness while threatening ecosystem health).

Find out more!

Water Mercury

Mercury is highly toxic, but naturally occurring concentrations of it are bound by plants and sediment and removed from the water column. However, human activity introduces excess mercury, resulting in free mercury in the water column. Water mercury is therefore a useful indication of human impact on the watershed.

More information coming soon!

Ice On/Off

Ice on is the date when a body of water is fully covered by ice, and ice off is the date when the ice disappears from the water surface. These measurements provide information about several environmental conditions, such as air temperature, water temperature, and flow.

Find out more!

Further resources:


Tackling Combined Sewer Overflows: A Toolkit for Community Action (October 2018)

A detailed toolkit to help communities tackle Combined Sewer Overflows in their local waterbodies, including a step by step roadmap.

Brewery Creek and Beyond: The Problem with Combined Sewer Overflows in Ottawa & Gatineau (October 2017)

The report, “Brewery Creek and Beyond: The Problem with Combined Sewer Overflows in Ottawa and Gatineau,” examines the data collected from five years of studying water quality at Brewery Creek, Gatineau.