Citation:

Mejia, F., C. Torgersen, E. Berntsen, J. Maroney, J. Connor, A. Fullerton, Joe Ebersole, AND M. Lorang. Longitudinal, lateral, vertical and temporal thermal heterogeneity in a large impounded river: implications for cold-water refuges. Remote Sensing. MDPI, Basel, Switzerland, 12(9):1386, (2020). https://doi.org/10.3390/rs12091386

Impact/Purpose:

Many rivers and streams in the Pacific Northwest are currently listed as impaired under the Clean Water Act as a result of high summer water temperatures, and these impairments can be further complicated by river regulation by dams. Much effort is being expended to improve conditions for fish, and methods are needed to project anticipated benefits of water temperature improvements, or costs of impairments. This paper describes how thermal imagery and channel surveys can be used to describe and characterize cold water refuges that may be important to sensitive fish populations.

Description:

Dam operations can affect mixing of the water column thereby influencing thermal heterogeneity spatially and temporally. This occurs by restricting or eliminating connectivity in longitudinal, lateral, vertical and temporal dimensions. We examined thermal heterogeneity across space and time and identified potential cold-water refuges for salmonids in a large impounded river in inland northwestern USA. To describe these patterns, we used thermal infrared (TIR) imagery, in situ thermographs, and high-resolution 3-D hydraulic mapping. We explained the median water temperature and probability of occurrence of cool-water areas using generalized additive models (GAMs) at reach and sub-catchment scales, and we evaluated potential cold-water refuge occurrence in relation to these patterns. We demonstrated that (1) lateral contributions from tributaries dominated thermal heterogeneity; (2) thermal variability at confluences was approximately an order of magnitude greater than of the main stem; (3) potential cold-water refuges were mostly found at confluences; and (4) the probability of occurrence of cool areas and median water temperature were associated with channel geomorphology and distance from dam. These findings highlight the importance of using multiple approaches to describe thermal heterogeneity in large impounded rivers and the need to incorporate these types of rivers in the understanding of thermal riverscapes because of their limited representation in the literature.

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