Predicting the occurrence of cold-water patches at intermittent and ephemeral tributary confluences with warm rivers

Cold water provided by small tributary streams can contribute to thermal heterogeneity in downstream receiving waters, thereby providing important thermal refuge habitat for cold-water aquatic taxa residing in warmer mainstem rivers. We investigated the potential function of small perennial and nonperennial tributary streams, including intermittent and ephemeral channels (some of which were dry) as sources of cold water to warmer receiving rivers. We used random forest analysis to model occurrence of cold-water patches at tributary confluences as a function of watershed and climatic characteristics and tested predictive performance with a 2-y data set of 68 tributary-mainstem confluence zones in northeastern Oregon, USA. 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 to early August was positively associated with an estimate of the water surplus available in the tributary basin at 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 refugia for cold-water taxa, such as Pacific Salmon (Oncorhynchus spp.), in summer-warm streams is, in part, dependent upon the continued release of ground water from tributary basins during mid-summer, even after surface stream flow 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.