Stream Temperature Assessment

By Russell N. Faux & Bruce McIntosh – Department of Forest Science, Oregon State University and Oregon Department of Fish and Wildlife

Removing or disturbing riparian vegetation and water flow management changes the thermal characteristics of streams — and hence fundamentally alters the habitat for most aquatic species. Measuring temperature variations throughout a watershed provides important information on fish habitat quality, potential refugia, restoration needs, and water quality. Remote sensing using a thermal infrared sensor (also known as forward-looking infrared (FLIR)) is a tool that offers resource managers a means to assess and monitor stream temperatures at multiple scales over a whole stream system.

FLIR is a combination of new and old technologies. It includes a helicopter-mounted infrared sensor along with a video camera and a global positioning system receiver (GPS). FLIR data are collected as a series of images that provide a map of stream temperatures at a single point in time over a broad area. The data obtained by FLIR also can be analyzed at the reach and subreach scales by looking at individual frames. This “snapshot” of thermal conditions complements the point monitors that are now commonly used to measure stream temperature. Point monitors provide data over longer periods of time but are limited to a single location in the stream system. Potential applications of FLIR include the following:

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Calibration and validation of watershed scale temperature models. The Oregon Department of Environmental Quality (ODEQ) has recently begun using FLIR for this purpose. Temperature conditions are modeled and then compared to the FLIR profile for the date and time of the survey.

Modeling and monitoring of different temperature mitigation scenarios for watershed restoration. FLIR establishes a baseline for the temperature profile in the basin. If restoration scenarios are implemented, the stream can be resurveyed with FLIR to monitor restoration outcomes.

Identification of priority areas for refugia and restoration. Cooler reaches can be identified for refugia planning, while warm areas or subdrainages can be targeted for restoration projects.

How Does FLIR Work?
FLIR sensors detect thermal infrared radiation emitted from the water surface and save these measurements as a thermal image. Using a calibrated sensor and knowing how well an object emits thermal infrared energy, the temperature of the object (in this case water) can be determined accurately.

The helicopter, with the FLIR sensor mounted on it, is flown directly over the center of the stream with the sensor aimed straight down. The data, which are fed to a computer on the helicopter, consist of overlapping GPS-tagged FLIR images. The video facilitates the interpretation of the FLIR images and provides a record of the survey. The ground area covered by the FLIR sensor varies, depending on the stream characteristics and flight parameters. The images generally cover a length of stream about 150 meters long and have a pixel resolution of less than 1 meter. Current capabilities allow the coverage of more than 100 kilometers of stream in a single day.

A Typical Analysis
An example image pair (FLIR and video) is shown in Figure 1. The image shows the confluence of the Applegate River and the Little Applegate River in Oregon. The Applegate River flows vertically from the top of the image and the Little Applegate River flows from left to right. Temperatures in the thermal image are color coded on the video screen (shown here in black and white) to emphasize instream temperature differences. The image shows the mixing plume as the Little Applegate River (19.8oC) enters the cooler Applegate River (17.2oC).

Analysis typically begins at a watershed scale and works progressively down to reach and subreaches. A longitudinal temperature profile is developed that provides managers and researchers with a measure of temperature distribution and thermal complexity throughout the watershed. This sampling of individual images also helps identify the location and temperature of surface water inflows and major tributaries.

The analysis enables the user to answer a variety of questions such as the following:

  • Does the stream warm continuously from its headwaters to its mouth, or are there cool reaches in the system?
  • What are the individual and collective contributions of surface inflows to the observed pattern?
  • Are there cool reaches that provide thermal refugia during midsummer temperatures?
  • Are there areas of cooling that are not attributable to surface inflows?
  • Are there warmer areas and sub- drainages that might be targets for restoration?
  • Are there desirable sites for installation of continuous temperature-monitoring stations?

Considerations and Limitations
FLIR data is complementary to traditional instream data loggers that record stream temperatures at set intervals over extended periods of time. Data loggers are very accurate (typically ±0.2∞C) and provide valuable information on daily and seasonal temperature variation at specific points in the stream. However, they cannot provide a picture of temperature changes throughout the watershed. In addition, they often are placed at easily accessible points in the drainage, without specific knowledge of the stream’s spatial temperature patterns. In contrast, FLIR data (accurate to ±0.5∞C ) show how stream temperatures vary spatially within the watershed and identify cool versus warm areas at different scales.

Moreover, FLIR is cost-effective. FLIR imagery currently costs between $125 to $250 per river mile to collect and process. In contrast, instream data loggers typically cost between $100 and $125 each. The ODEQ unofficially estimates the cost of calibrating, deploying, auditing, retrieving, downloading, and organizing FLIR data at 40 person-hours.

One limitation of FLIR is that it only measures temperatures at the water surface. A key property of water is that it is essentially opaque to thermal infrared wavelengths. This is not an issue for well-mixed streams where surface temperatures are representative of the water column. However, on stream systems that have thermally stratified areas, the FLIR data will only represent the surface temperatures. Stratified areas often are identifiable in the FLIR images and can be delineated in the analysis. Other sources, such as field measurements, dam locations, flow, and geomorphology also can identify areas of potential stratification.

Because FLIR data represents temperatures at a single point in time, it cannot be used to assess compliance with water quality standards. For example, the Oregon State water temperature standard is based on a seven-day moving average of daily maximums. However, the FLIR data can provide a measure of the extent of stream that exceeds (or is within) a given temperature criteria at the time of the survey.

The use of FLIR continues to be widely applied to stream temperature monitoring and assessment. In 1999, the ODEQ collected FLIR data on over 1,200 miles of stream in three different watersheds. The data complement current methods and provide a more complete understanding of stream temperature conditions. Current efforts focus on continuing development of data processing techniques and creating useful data products to support resource managers.

This application was developed at Oregon State University (OSU) to assess fish distributions in relation to thermal refugia and reach-scale isotherms. The work has continued at OSU as part of the Environmental Protection Agency’s ongoing Advanced Monitoring Initiative project.

For more information:

Bruce A. McIntosh, Oregon Department of Fish and Wildlife, Corvallis Research Lab, 28655 Highway 34, Corvallis, OR 97333; phone: 541-757-4263

 

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