Chapter 13 – Water Temperature

Release Date: March 2013

Aquatic organisms such as fish and benthic macroinvertebrates are adapted to a specific range of temperatures and changes in water temperature can affect the composition and function of aquatic ecosystems. Just as air temperature helps control where plants and animals live on land, water temperature has a major influence on aquatic organisms. For example, warmer water is less oxygenated than colder water and some fish can tolerate these conditions (e.g. Smallmouth bass; Micropterus dolomieu) whereas others cannot (e.g. Brook trout; Salvelinus fontinalis) (Figure 1).

Figure 1. A stream in an urbanizing section of the Credit River Watershed. In streams such as this, with little riparian cover and potential for warm stormwater runoff, water temperature can increase potentially altering freshwater ecosystems.

Water temperature is largely controlled by air temperature; however, a number of other factors have an important influence on water temperature (Table 1). With climate change water temperatures are predicted to get warmer, potentially changing the types and amounts of plants and animals that live in the Credit River Watershed.

Based on fish communities many of the streams in the Upper and Middle Credit River Watershed are managed as cold or mixed water rivers according to the Credit River Fisheries Management Plan (MNR and CVC 2002). Management decisions based on water temperature monitoring data will help preserve these important coldwater habitats in the face of ongoing climate change, aiding in the protection of a valuable natural resource.

Table 1. Important factors influencing water temperature in the Credit River Watershed.


Potential impact on water temperature


Warmer air temperatures can increase water temperatures


Groundwater input can reduce summer water temperature

Riparian cover

Trees provide shade to streams and reduce maximum water temperature

Stream morphology

Deep and narrow streams are generally cooler than shallow and wide streams

Land use

Impervious cover can increase the temperature of surface runoff to streams


Water Temperature Monitoring

In 2004, Credit Valley Conservation (CVC) developed the Credit River Water Temperature network. This monitoring network consists of 23 stations (Figure 2): nine located in the Upper Watershed, eight in the Middle Watershed and six in the Lower Watershed. Water temperature at these stations is measured automatically every 30 minutes from June 1st to September 30th, producing over 5,800 temperature measurements per station, per year. To evaluate water temperature status and trends two parameters are calculated: absolute maximum summer water temperature and average daily maximum summer water temperature. Absolute maximum summer water temperature is the greatest water temperature recorded from June to September and provides a measure of short-term temperature stress experienced by aquatic organisms. Average daily maximum summer water temperature is the average of daily maximum temperature recorded for each day of the June-September monitoring period and provides a measure of long-term temperature stress experienced by aquatic organisms.

Figure 2. Location and status (2011) of water temperature monitoring stations in the Credit River Watershed. Status is shown as colder (blue) or warmer (red) than CVC’s maximum summer temperature target.

Derived from known temperature preferences of fish communities, CVC has developed water temperature targets for both absolute maximum and average daily maximum summer water temperature for the Credit River Watershed (MNR and CVC 2002; Table 2). Based on these targets, all monitored streams in the Upper Watershed are managed as coldwater systems. Coldwater systems contain fish which are impacted (e.g. thermal stress) when water temperatures are above 20°C. Monitored streams in the Middle Watershed are managed as either cold or mixed water based on their fish communities. Mixed water communities have some fish that can tolerate temperatures of up to 23°C for brief periods of time. Monitored streams in the Lower Watershed are classified as either mixed or warmwater systems. Warmwater streams have some fish that are tolerant of temperatures up to 26°C for brief periods of time.

Table 2. Water temperature targets for Credit River Watershed

Water temperature

Coldwater (°C)

Mixed water (°C)

Warmwater (°C)

Absolute Maximum Summer Water Temperature




Daily Maximum Summer Average Water Temperature




Water Temperature Status (2011)

Overall, water temperatures in 2011 were above CVC’s water temperature targets with nearly 70% (16 of 23) of monitoring stations exceeding the absolute maximum summer water temperature target (Figure 2 and 3). Nearly half of the monitoring stations (11 of 23) also exceeded the average daily maximum water temperature target (Figure 4). There was also a clear division between water temperature stations on the Credit River versus its tributaries. Whereas 15 of the 17 (88%) temperature monitoring stations on the Credit River exceeded their maximum summer water temperature target, only 1 of the 6 (17%) monitoring stations on tributaries exceeded their maximum summer water temperature target (Figure 2). These temperature monitoring results indicate that water temperatures in 2011 were largely warmer than those preferred by fish communities in the Credit River Watershed and suggests that fish with a preference for coldwater habitat (e.g. Brook trout; Salvelinus fontinalis) were likely stressed for periods of 2011, particularly in the Credit River.

Figure 3. Range of water temperature values for the 23 monitoring station from June to September 2011. Red stations are above whereas blue stations are below their respective absolute maximum summer temperature target. Stations are arranged from north to south within the watershed. Boxplots show the minimum (lower whisker), lower quartile, median, upper quartile and maximum (upper whisker) of the range of water temperatures from June to September 2011.

Figure 4. Temperature difference between the average daily maximum temperature from June to September 2011 and their temperature target for the 23 monitoring stations.

Water Temperature Trends

Similar to climate, robust statistical analysis of time series trends in water temperature requires long-term monitoring (typically more than 20 years). To date, the water temperature monitoring record is too brief for statistical trend analysis. There are still patterns however, that are evident in the water temperature monitoring record. Although the water temperature of an individual station is influenced by a number of factors (Table 2), fluctuations in water temperature through time closely follows air temperature (Figure 5A). Given that some of the warmest air temperatures of the last 40 years have occurred during the water temperature monitoring period (2004-2012) it is therefore not surprising that many of the water temperature stations have regularly exceeded their temperature targets for both absolute maximum (Figure 5B) and average daily maximum (Figure 5C) summer water temperature. In fact, it was only in the relatively cool summers of 2008 and 2009 that over half the monitoring stations were below their temperature targets for both absolute maximum (Figure 5B) and average daily maximum (Figure 5C) summer water temperature. These results indicate that since 2004 water temperatures have regularly exceeded the preference of coldwater fish in the Credit River Watershed for a good portion of the monitoring record.

Figure 5. A) Average daily June to September maximum air (red) and water (blue) temperature. B) Maximum June to September air temperature (red) and the percent of the monitoring stations warmer than their absolute maximum temperature target. C) Average daily maximum June to September air temperature (red) and the percent of the monitoring stations warmer than their average daily maximum temperature target.



Water temperatures in 2011 were generally warmer than the absolute maximum temperature target. This is similar to what was observed over the entire monitoring period (2004-2012) where at least one temperature target was regularly exceeded by over half the monitoring stations. As climate change continues to warm the Credit River Watershed it is anticipated water temperatures will become warmer, further reducing the availability of coldwater habitat for aquatic organisms (e.g. Brook trout). This predicted warming of the Credit River and its tributaries is troubling and we may face the potential loss of brook trout and other coldwater fish species in future. It is therefore important to properly manage the Credit River Watershed to reduce the cumulative effect of climate change with other stressors on the Credit River ecosystem. Ensuring adequate riparian cover and groundwater levels and minimizing artificial pooling and warm stormwater runoff will help to protect important coldwater habitat against future stressors like climate and land use change.

In the next chapter we will dive below the water and explore the chemistry of the stream sediment. Nutrients and contaminants can build up in sediment and therefore the quality of the sediment can strongly influence the aquatic ecosystem.

Did you know?

Animals can accumulate contaminants in their body when feeding on contaminated organic matter in stream sediment?



Credit River Fisheries Management Plan:

Real-time water quality information:


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