Randy L. Comeleo

Relationships between watershed stressors and sediment contamination in Chesapeake Bay estuaries

Three methods for assessing the relationships between estuarine sediment contaminant levels and watershed Stressors for 25 Chesapeake Bay sub-estuaries were compared. A geographic information system (GIS) was used to delineate watersheds for each sub-estuary and analyze land use pattern (area and location of developed, herbaceous and forested land) and point source pollution (annual outflow and contaminant loading) using three landscape analysis methods: (1) a watershed approach using the watershed of the estuary containing the sampling station. (2) a ‘partial watershed’ approach using the area of the watershed within a 10 km radius of the sampling station and (3) a ‘weighted partial watershed’ approach where Stressors within the partial watershed were weighted by the inverse of their linear distance from the sampling station. Nine sediment metals, 16 sediment organics and seven metals loading variables were each reduced to one principal component for statistical analyses. Relationships between the first principal components for sediment metals and organics concentrations and watershed stressor variables were analyzed using rank correlation and stepwise multiple regression techniques. For both metals and organics, the watershed method yieldedR2 values considerably lower than the partial and weighted partial watershed analysis methods. Regression models using Stressor data generated by the weighted partial watershed landscape analysis method explained 76% and 47% of the variation in the first principal component for sediment metals and organics concentrations, respectively. Results suggest that the area of developed land located in the watershed within 10 km of the sediment sampling station is a major contributing factor in the sediment concentrations of both metals and organics.

Willamette River Basin surface water isoscape (δ18O and δ2H): temporal changes of source water within the river

Determining how water sources for rivers vary over time can greatly enhance our understanding and management of land use and climate change impacts on rivers. Because the stable isotope composition of precipitation can vary geographically, variation in the stable isotope composition of river water may be able to identify source water dynamics. We monitored the stable isotope values (δ18O and δ2H) of river and stream water within the southern Willamette River Basin in western Oregon over two years. Within this basin, eighty-four percent of the isotopic variation in small tributary streams was explained by the mean elevation of the catchments, whereas seasonal variation was minimal. However, water within the Willamette River had distinct isotopic seasonal patterns that likely occurred because of changes in the mean elevation of source water for the river. River isotopic values were lowest during summer low flow and highest during February/March when snow accumulated in the mountains. We estimated that the m...

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.