Assessing stream temperature variation and its relationship with urbanization in the Pacific Northwest

Jing Tan, Purdue University

Abstract

In the Pacific Northwest, water quality degradation due to elevated stream temperature and the threat it imposes on cold-water fish species, especially the economically important salmon fisheries, has been of great concern during recent years. Stream temperature, like other water quality parameters, is believed to be largely affected by a number of factors, but land use change, especially urbanization is of particular concern. The accurate measurement of stream temperatures is important for both identifying environmentally detrimental temperatures as well as being able to monitor compliance with water quality standards relating to water temperature. Two different measurement systems for stream temperature, thermal infrared (TIR) imagery and in-stream temperature loggers, are used to relate the spatial and temporal variability in water temperatures to environmental conditions. This research first utilizes both five-meter and fifteen-meter airborne TIR imagery collected for the Green-Duwamish River basin in Washington State, USA to assess how the spatial variability of stream temperature is affected by land use within the watershed. As part of this analysis, we also explore how stream temperature changes over different time intervals from minutes to hours to days, so that we can better interpret changes in observed temperature that were collected over a span of several hours. In the second part of this research, daily in-stream temperature data for the mainstem Green-Duwamish River basin and a tributary, Soos Creek, were analyzed to identify relationships between urban development and stream temperature. Analysis of overlapping TIR images indicated that stream temperature change was negligible on the order of a few minutes between neighboring images, but that temperature increased on the order of 2-3 °C between the first and last image of the five-meter dataset. Despite the increase, mean reach segment stream temperature was lower in areas with less urbanization and thus more riparian vegetation, while it increased with the urban fraction within each watershed subsection. Spatial variability was found to decrease with urbanization, and increase with distance from the reservoir. This suggests that increased riparian zone shading and additional in-stream materials such a rocks and woody-debris that are more prevalent in less urbanized areas contribute to an increase in stream temperature variability and the likely presence of cold water refugia which are very important to the sustained health of the aquatic ecosystem. In-stream temperature observations in the Soos Creek watershed identified a statistically significant increase in cool season water temperatures and decrease in variability with increased urban land use within the riparian buffer zone upstream of the sampling sites. This is directly related to an increase in cool season minimum water temperatures, which is indicative of increased overland flow due to impervious surfaces. Both observation methods found an increase in average water temperature and a decrease in spatial and temporal variability with an increase in urban area. Each measurement system provided insight into the thermal regime of the Green-Duwamish River basin but together they provided more insight into the temporal and spatial variation of the system than either could alone.

Degree

M.S.

Advisors

Cherkauer, Purdue University.

Subject Area

Environmental Health|Agricultural engineering

COinS