11.4 Trends in turbidity in the Slave and Hay Rivers

Last Updated: 
December 17, 2015

This indicator tracks the turbidity of the Slave and Hay Rivers.

Turbidity is often described as the "cloudiness" of water. Turbidity is the scattering effect suspended solids have on light. It is a visual property of water and implies a reduction or lack of clarity resulting from the presence of suspended particles (or solids)1. The higher the concentration of suspended particles in the water, the higher the turbidity of the water.

The Hay River has naturally high turbidity.

The Hay River has naturally high turbidity. Land developments can cumulatively increase turbidity by enhancing sediment run-offs. Photo: GNWT/D. Downing, ENR. 2004 Karst survey.

Particles (solids) can originate from surface runoff, instream bank and bed erosion, algal growth and anthropogenic sources, such as land disturbances, forest harvesting, agriculture and sewage, mining and industrial effluents2. The most frequent cause of turbidity in rivers and other water bodies are an increase in inorganic material as a result of rainfall events or land disturbances such as erosion. There is a seasonal component as the highest rate of influx of particles tend to happen in spring as the snow melts and flows into the rivers.

Higher turbidity can reduce the amount of light penetrating the water, which reduces productivity and the production of dissolved oxygen. Higher turbidity can also increase water temperatures as the suspended particles absorb more heat. Suspended materials can clog fish gills, lower growth rates, affect egg and larval development and habitat. As particles settle, they may blanket the stream bottom and smother fish eggs and benthic macro-invertebrates. These are some of the reasons for measuring turbidity in water samples.

Data, analysis and text for this indicator are taken from studies by Water Resources Division (prior to 2014), Aboriginal Affairs and Northern Development, Government of Canada (and after 2014) Environment and Natural Resources, GNWT. This indicator replaces the archived indicator entitled "11.4 Trends in turbidity and arsenic in the Hay River".

NWT Focus

Turbidity levels will differ greatly among rivers flowing in different ecozones. High turbidity (high sediment loads) is important for the formation and maintenance of deltas where water flows are slow enough to release suspended particles (solids) particles to the river bottom, which form sediment layers3.

Determination of turbidity is a common component of water quality assessments in the NW. It is routinely used as an indicator to assess the condition and productivity of a water body. Although turbidity values vary spatially, increased levels of turbidity can be elevated with human activity. Conversly, turbidity can significantly decline after the construction of a dam.

Water quality parameters are naturally variable with seasons as they change with water flow, ice cover and rainfall. A fairly robust set from past years is needed to assess whether data patterns are indicative of a trend or are within the natural variability inherent in water quality measurements. In the case of turbidity in the Slave and Hay Rivers, the data set spans over 20 years.

Current Status

Slave River saw a range of 16.9-49.3 NTU, as an example of the variation in turbidity levels across the NWT. As a spatial comparison, using a daily average recorded by the ENR's community-based monitoring sonde, turbidity levels in theYellowknife River were between 1.967 and 3.325 NTU and between 10.23-16.89 in the Kakisa River.


Turbidity in NWT Rivers

Figure 1: Illustrating the varying levels of turbidity in the Slave River, Hay River, and Kakisa River (Taiga Plains), and the Yellowknife River (Taiga Shield) for the same dates in 2014. Data from ENR community-based sonde monitoring program.

Current Trend

Turbidity is naturally high in rivers flowing through sediment-rich ecosystems, such as the Taiga Plains and the Taiga Cordillera4. Very slow erosion rates in the Taiga Shield result in very clear rivers and lakes4.

Slave River

Turbidity varies greatly in the Slave River. Mean monthly turbidity concentrations range from low (4 NTU) to high (1.870 NTU) (n=104; 1982-2010) at Fort Smith. At Fitzgerald (AB), levels are between 3-6,400 (n=241; 1972-2010)2,7. Generally at  turbidity levels at Fitzgerald are highest during the open-water season and lower during the winter. At Fort Smith, turbidity levels are highest from May-July and generally decreased from August-April.

Turbidity (NTU)

Figure 2. Mean monthly turbidity levels (NTU) from Fitzgerald (averaged over 1972-2010). No statistically significant annual trends were found.

Mean monthly turbidity levels (NTU) from Fort Smith (averaged for 1982-2010).

Figure 3. Mean monthly turbidity levels (NTU) from Fort Smith (averaged for 1982-2010). No statistically significant annual trends were found.

Trend analysis for non-flow-adjusted turbidity concentrations in the Slave River at Fitzgerald (1972-2010) was conducted and no statistically significant trends were found. However, because the flow regime has changed in the Slave River since the Bennett Dam was built (1968-1971) (see indicator 11.2), the total amount of sediments carried today by the Slave River has tripled during winter and reduced by almost half during the open-water season3.

Hay River

Trend analyses for non-flow-adjusted turbidity concentrations in the Hay River (1988-2010) show no trends. There are no dams on the Hay River and this level of turbidity can be considered natural5.

Hay River near AB/NWT Border - Turbidity (1988-2010)

Figure 4. Non-flow-adjusted turbidity concentrations (NTU) from Hay River (1988-2010). No statistically significant annual trend was found.

Looking Forward

No predicted temporal trends were significant during the evaluation for turbidity. Turbidity does have a significant relationship with flow and total suspended sediment (TSS). Since turbidity is a useful measure of stream water quality and can be used to monitor change in the aquatic environment, it is recommended these parameters continue to be monitored.

There are literally dozens of water quality parameters that can be measured as part of a water monitoring program and many may be suitable as indicators. It is important to note that as long as monitoring programs continue and water samples are collected and analyzed, other -- or additional - parameters may be selected as indicators in the future.

It may be useful to measure turbidity as an indicator of climate change6. In the NWT, there is already evidence of change in ice-free dates for rivers and lakes, warming permafrost and increasing average annual temperatures (see Focal Point 1 Big Picture). As climate modeling predicts river basins will become warmer in winter and as permafrost melts, an increase in turbidity would be anticipated.

Find Out More

Slave River near Fitzgerald surface water quality data.

Technical Notes

Sonde measurements are taken using a YSI Sonde 6600 and captures data every two hours. The sonde measures: temperature, conductivity, pH, oxidation/reduction potential (ORP), dissolved oxygen, turbidity, and chlorophyll. Information on the unit can be found here.


Found an error or have a question? Contact the team at NWTSOER@gov.nt.ca.


Ref 1 - CCME 2007.  Canadian Environmental Quality Guidelines.  Canadian Council of Ministers of the Environment, Environment Canada. Ottawa, ON.                

Ref 2 - AANDC. 2012. Water and suspended sediment quality of the transboundary reach of the Slave River, Northwest Territories. 317pp.

Ref. 3 - Mackenzie River Bason Board. 2003. Mackenzie River Bason: state of the aquatic ecosystem report 2003. 213pp.

Ref. 4 - ENR. 2007-2013. NWT Ecosystem Classification reports. GNWT.

Ref. 5 - Environ. 2012. Status and trends of hydrology, water quality and suspended sediment quality of the Hay River. Prepared for Aboriginal Affairs and Northern Development. Prepared by Environ EC (Canada), Inc. 

Ref. 6 - SENES Consultants Limited. 2011. NWT environmental audit and status of the environment report. Produced for Aboriginal Affairs and Northern Development Canada, Cumulative Impact Monitoring Program, Northwest Territories.

Ref. 7 - Glozier, N., D.B. Donald, R.W. Crosley, and D. Halliwell. 2009. Wood Buffalo National Park water quality: status and trends from 1989-2006 in three major rivers: Athabasca, Peace and Slave. Prairie and Northern Office, Water Quality Monitoring and Surveillance Division, Water Science and Technology Directorate, Environment Canada.