All aquatic organisms have an optimal range of temperature under which they can successfully maintain populations. Some species may be able to tolerate higher than optimal temperatures and survive, but they will not thrive. For example, brown trout can tolerate temperatures above 68 °F (20 °C) indefinitely, but they would not be successful reproducing.
Maryland classifies its waterbodies into Use Classes. Each Use Class has an associated range of species and species function (e.g., adult brown trout survival, or brown trout reproduction). Acceptable temperature ranges are defined so that these species can survive in their particular Use Class.
Use Class IV waters are waters that brown trout can survive in but not reproduce in, hence Use Class IV waters are meant to designate waters that can support a fishery where the brown trout populations are maintained by periodic stocking. An example of Use Class IV waters are portions of Northwest Branch and Upper Rock Creek. Maryland sets a maximum threshold water temperature of 75 °F (24 °C) for Use Class IV waters.
Class III waters includes waterbodies where brown trout can reproduce. An example of a Class III water in Montgomery County is Paint Branch. The maximum water temperatures does not usually exceed 68 °F (20 °C). Brown trout are able to reproduce here and stocking is usually not necessary to maintain populations with proper regulations in place (e.g., catch and release fishery).
Use Class I waters include all other freshwaters of the state (Use Class 2 is reserved for brackish water) where a maximum temperature of 90 °F (32 °C) should not be exceeded.
In nearly all cases the concern regarding water temperature is when it gets too high or exceeds the maximum value set for a particular use class, not how low water temperature gets. Temperature spikes are often associated with discharges of rainwater runoff from areas with high imperviousness (surfaces where water cannot soak into the ground, such as roads and parking lots). Furthermore, discharges from improperly designed storm water management ponds can cause high temperature spikes. The cumulative effects of these temperature spikes can harm the overall health of aquatic communities.
Learn more about Use Classes in Montgomery County (PDF, 13 pp, 2.5Mb)
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Temperature Monitoring Activities
Given that the concern with temperature monitoring are high temperatures rather than low ones, stream temperatures are monitored from May 31 to September 30, when temperatures are highest. Temperature loggers are placed in specific stream sites during May, and are recovered in October. Loggers are set to record the maximum temperature within a 24 minute period continuously during the time period specified.
DEP monitors temperatures at targeted stream sites where land uses are expected to raise water temperatures (such as in areas that are under construction). Special Protection Areas (SPA) are also subjected to concentrated temperature monitoring. Temperature meters are deployed at specific sites to assess the ability of storm water or sediment and erosion control devices and systems (otherwise known as Best Management Practices or BMPs) to minimize thermal impacts. Typically, a temperature logger is placed in the stream, both upstream and downstream of the discharge channel from the BMP. Generally, the temperature of the stream and the water discharging from the BMP is monitored by a consultant hired by the developer.
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Data and Data Requests
Montgomery County has collected instream temperature data from 1995 to the present. However, the sites selected for temperature monitoring would not be available for every year since 1995. Tables and maps are available showing the locations where temperature was monitored during specific years. Maps can be made to order depending on the request.
Temperature Data Table
||The station field is a nine character code that identifies the station name. The stations are a combination of the two letter code for the watershed+the two letter code for the subwatershed + the single digit stream order code+ the sequential reach number.
||The date and time the water temperature was recorded.
||Water temperature in Celsius.
For proper interpretation, water temperature data should be compared with air temperature and precipitation data from the same time period.
Submitting a Data Request
If you are interested in obtaining data or protocols, please contact DEP at email@example.com. In your request, please provide the following information:
Name, organization (if applicable), phone number, and/or email address
Type of data requested
Time frame requested
Explanation for use of data (helps to personalize the data request)
Preferred method of data retrieval (email, CD by mail, FTP, pick up CD or materials from DEP offices)
Spreadsheet Tools for River Evaluation, Assessment and Monitoring from Ohio Department of Natural Resource Modules
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