Stream Temperature Monitoring Project

Water temperature is very important to aquatic organisms as it can drive chemical reactions and physiological activity. In addition, water temperature is an important factor in determining which species of aquatic organisms, such as fish, are able to live in a particular section of river or stream.

The Water Monitoring Group routinely monitors water temperature at wadeable river and stream locations across Connecticut.  Data are collected to complement routine and probabilistic ambient biological monitoring data, to identify high quality watersheds as part of the DEEP Healthy Waters Initiative, and to support evaluation of temporal trends, quantification of natural variability, and testing of hypotheses and predictive models related to climate change and water resource management.

Stream Temperature Overview 

River and stream temperatures naturally vary daily, weekly, and seasonally, in large part in response to daily and seasonal changes in air temperatures. Water temperature can also vary longitudinally from headwaters (cold) to mouth (warm) of a river system. Environmental variables such as elevation, canopy cover, gradient, and quantity of groundwater input all play a role in determining natural water temperature at a given location.  In addition, human activities like damming and permitted thermal discharges may influence water temperature regime by both increasing (surface releases and groundwater withdrawal) and decreasing (hypolimnetic withdrawal) water temperatures. To determine the magnitude of any human influence on the water quality of a waterbody it is necessary to determine how far from natural has the stream temperature changed. Unfortunately, this is a very difficult question to answer as data for pre-disturbance are often not available.

Program Development

To better characterize water temperature conditions in Connecticut rivers and streams, the DEEP Water Monitoring Group partnered with the Fisheries Division to deploy continuous temperature data loggers at more than 300 locations between 2006 and 2010. At each location hourly data was collected allowing for analysis of daily temperature changes as well as longer-term changes. 

Multiple temperature metrics including maximum daily temperature, duration and frequency of maximum temperatures, maximum low temperatures (the highest temperature that the stream does not drop below), and exceedance of critical thresholds for various aquatic organisms were calculated for each site. Based upon this information, a method to classify stream segments as cold, transitional (‘cool’) or warm water habitat location was developed for Connecticut. 

All three water temperature categories naturally occur within Connecticut streams. DEEP is particularly concerned with identifying and protecting cold water stream segments, however, as increasing air temperatures associated with climate change threaten to reduce the extent of this important habitat type along with the aquatic organisms that rely upon it (e.g., wild Brook Trout populations and sensitive macroinvertebrate taxa.)

Current Monitoring Efforts

The Water Monitoring Group continues to conduct ongoing stream temperature monitoring.  A long-term network has been established at which ongoing, year-round hourly stream temperature monitoring will continue indefinitely.  The data generated by this network will allow for long-term trend analysis, including evaluation of the impacts of climate change on freshwater aquatic habitat in Connecticut.

Water temperature data loggers are secured into PVC tubes and placed into a section of stream predicted to have adequate flow to keep the probe submerged. Probes are secured by a heavy iron weight and then covered with large rocks to shade from sunlight and to conceal from passersby. Loggers are ‘swapped’ out each spring and fall and the data are uploaded to a regional stream temperature database.

The Water Monitoring Group’s temperature monitoring effort is supplemented by volunteer monitoring efforts each year. The Volunteer Stream Temperature Monitoring (VSTeM) Network trains individuals to utilize similar protocols to conduct targeted monitoring at streams in their town or local watershed.  Emphasis is placed on guiding volunteer efforts towards monitoring of smaller, headwater streams, with priority given to segments that have not yet been monitored for temperature.

Effort is made to collect paired fish community and benthic macroinvertebrate community data from the temperature monitoring network at regular intervals.  Whenever possible these data are also collected from volunteer monitoring temperature locations.  Combined, these data sets will inform the aquatic life assessment process as well as provide support for potential future refinement of the water quality standards.

Related Documents

Related Pages

For More Information Contact:

Ansel Aarrestad
Stream Temperature Monitoring Project Lead
(860) 424-3349

Content last revised April 8, 2021