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Predicting the occurrence of cold water patches at intermittent and ephemeral tributary confluences with warm rivers
Citation:
Ebersole, Joe, J. Wigington, S. Leibowitz, R. Comeleo, AND J. Van Sickle. Predicting the occurrence of cold water patches at intermittent and ephemeral tributary confluences with warm rivers. Freshwater Science. The Society for Freshwater Science, Springfield, IL, 34(1):111-124, (2015).
Impact/Purpose:
Numerous studies have highlighted the ecological value of intermittent streams during the wet season. Such streams can provide spawning, rearing, or refuge habitats for fish when they contain water and are accessible from downstream waters. However, as we describe, seasonal streams can also have important ecological functions when dry. This can occur when colder sub-surface flow discharges from "dry" tributary channels into warm downstream perennial streams and rivers. In the northeast Oregon streams where we conducted this research, warm water temperatures during the dry summer season are a factor known to limit the survival and distribution of Pacific salmon, trout, and charr, including fish stocks listed under the Endangered Species Act. The presence of cold-water patches in these warm rivers can provide critical thermal refuges for cold-water fishes, sustaining populations that would otherwise be unable to persist in warm rivers through the summer months. Ours is the first study we know of to systematically characterize the occurrence of cold-water patches provided at tributary confluences of "dry" stream channels. Our models successfully predict the occurrence of cold-water patches 75% of the time, based solely upon GIS data. This information could help inform regulatory decision-making under the Clean Water Act, particularly as it relates to non-perennial streams, and will further understanding of the seasonal ecological function of stream networks for endangered fish populations. These findings also may be useful for projections of climate change impacts to fish populations associated with changes in streamflow and water temperature.
Description:
Small, cold tributary streams can provide important thermal refuge habitat for cold-water fishes such as Pacific salmon (Oncorhynchus spp.) residing in warm, downstream receiving waters. We investigated the potential function of small perennial and non-perennial tributary streams, including intermittent and ephemeral channels – some of which were dry – as sources of cold water in northeastern Oregon streams. We used random forests analysis to model cold-water patch occurrence as a function of watershed and climatic characteristics, and tested predictive performance using a two-year dataset of 68 tributary-mainstem confluence zones. Cold-water patches were present in 53% (36 of 68) of the tributary confluences; of these, 14 occurred at tributaries that had no surface water flow at the time of sampling. The likelihood of a tributary contributing a detectable source of cold water to a confluence zone during late July-early August was positively associated with an estimate of the water surplus available in the tributary basin during the end of the preceding wet season. Basin area and the presence of tributary surface flow at the time of sampling were relatively uninformative predictors of cold-water patch presence. Because surface flow was not evident in 39% of the tributaries where cold-water patches were observed, we conclude that the availability of thermal refuges for cold-water fishes like salmon and trout in summer-warm streams is in part dependent upon the continued release of groundwater from tributary basins during mid-summer, even after surface streamflow ceases. These findings highlight a potentially important ecological function of intermittent and ephemeral stream channels as sources of cold subsurface discharge to downstream waters during the dry season.
