Confluences are locations of complex hydrodynamic conditions within river systems. The effects on hydrodynamics and mixing of temperature‐induced density differences between incoming flows are investigated at a small‐size, concordant bed confluence. To evaluate density effects, results of eddy‐resolving simulations for a densimetric Froude number
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Stream confluences are ubiquitous interfaces in freshwater networks and serve as junctions of previously independent landscapes. However, few studies have investigated how confluences influence the transport, mixing, and fate of organic matter (OM) and inorganic nutrients at the scale of river networks. To understand how network biogeochemical fluxes may be altered by confluences, we conducted two sampling campaigns at five confluences in summer and fall 2021 spanning the extent of a mixed land use stream network. We sampled the confluence mainstem and tributary reaches as well as throughout the mixing zone downstream. We predicted that biologically reactive solutes would mix non‐conservatively downstream of confluences and that alterations to downstream biogeochemistry would be driven by differences in chemistry and size of the tributary and upstream reaches. In our study, confluences were geomorphically distinct (e.g., wider, deeper, unique erosional, and depositional features) downstream compared to reaches upstream of the confluence. Dissolved OM and nutrients mixed non‐conservatively downstream of the five confluences. Biogeochemical patterns downstream of confluences were only partially explained by contributing reach chemistry and drainage area. We found that the relationship between geomorphic variability, water residence time, and microbial respiration differed between reaches upstream and downstream of confluences. The lack of explanatory power from network‐scale drivers suggests that non‐conservative mixing downstream of confluences may be driven by biogeochemical processes within the confluence mixing zone. The unique geomorphology, non‐conservative biogeochemistry, and ubiquity of confluences highlights a need to account for the distinct functional role of confluences in water resource management in freshwater networks.
more » « less- NSF-PAR ID:
- 10465635
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Water Resources Research
- Volume:
- 59
- Issue:
- 9
- ISSN:
- 0043-1397
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Confluences are a classic feature in riverine networks with important ecological and morphological functions. A method to characterize the hydraulic complexity of a river based on velocity gradients was applied, for high and low flow conditions, to the Negro and Solimões Rivers confluence in the Amazon basin. The applied metrics
M 1 andM 2approximate the drag forces imposed on aquatic organisms moving between 2 locations and may identify potential habitat zones and edges. MetricM 2 corresponded best with the hydraulic and morphological patterns in the confluence hydrodynamic zone, with the largestM 2 values in the entrance of the confluence, centered at the mixing interface, andM 2 values generally decaying laterally toward the banks and longitudinally with downstream distance. Seasonal decreases in discharge magnitude in the Amazon, and decreases in discharge between other river basins analyzed in this study, led to increases in hydraulic complexity metricM 2 . The hydraulic complexity metrics can characterize some aspects of habitat heterogeneity and contribute to an explanation for observations of increased species richness at Amazon basin confluences and the larger ecological patterns of diversity increasing at nodes in riverine networks.
