A Landscape Assessment and Associated Dataset of Stream Confluences for the Conterminous U.S. Donald Ebert, James Wickham , Anne Neale, and Megan Mehaffey Research Impact Statement: Stream confluences may be biological hotspots. We present a USA stream con- fluence dataset to stimulate further ecological research. The dataset contains 1,085,629 confluences and 383 attributes. ABSTRACT: Stream confluences are important components of fluvial networks. Hydraulic forces meeting at stream confluences often produce changes in streambed morphology and sediment distribution. These changes often increase habitat heterogeneity relative to upstream and downstream locations, which have led some to identify them as biological hotspots. Despite their potential ecological importance, there are relatively few empirical studies documenting ecological patterns upstream and downstream of confluences. We have produced a publicly available dataset of stream confluences and associated watershed attributes for the conterminous Uni- ted States. The dataset includes 1,085,629 stream confluences and 383 attributes for each confluence organized into 15 dataset tables for both tributary and mainstem upstream catchments and watersheds. Themes in the dataset include hydrology (e.g., stream order), land cover, land cover change, geology (e.g., calcium content of underlying lithosphere), physical condition (e.g., precipitation), measures of ecological integrity, and stressors (e.g., impaired streams). Additionally, we used measures of ecological integrity to assess the condition of the stream confluences. Aside from a generally positive east-to-west gradient in ecological condition, we found that approximately one-third of the confluences had markedly contrasting ecological conditions between mainstem and tributary, catchment and watershed, or both. The dataset should support many, multifaceted studies of stream confluence ecology. (KEYWORDS: EnviroAtlas; headwaters; NLCD; stream networks; StreamCat; watersheds.) INTRODUCTION Stream confluences emerged as important elements of lotic ecosystems as their conceptualization advanced from continua to networks (Rice 1998; Rice et al. 2001; Benda, Andras, et al. 2004; Benda, Poff, et al. 2004). Every stream confluence is conditioned by the hydrau- lic forces of its two or more upstream sources. The hydraulic forces introduced by a tributary can produce changes in the morphology of the streambed (e.g., scours, aggradation) and distribution of sediment at the confluence and further downstream (Rhoads 1987; Best 1988). Common streambed geomorphic changes include the development of avalanche faces at the mouths of the mainstem and tributary, a scour hole, flow separation (water volumes from each confluent do not mix immediately), separation bars (a ridge of Paper No. JAWR-20-0022-P of the Journal of the American Water Resources Association (JAWR). Received February 26, 2020; accepted November 9, 2020. Published 2020. This article is a U.S. Government work and is in the public domain in the USA. Journal of the American Water Resources Association published by Wiley Periodicals LLC on behalf of American Water Resources Association. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any med- ium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made. Discussions are open until six months from issue publication. Office of Research and Development, Center for Public Health and Environmental Assessment, U.S. Environmental Protection Agency, Research Triangle Park, North Carolina, USA (Correspondence to Wickham: wickham.james@epa.gov). Citation: Ebert, D., J. Wickham, A. Neale, and M., Mehaffey. 2020. "A Landscape Assessment and Associated Dataset of Stream Conflu- ences for the Conterminous U.S.." Journal of the American Water Resources Association 1–13. https://doi.org/10.1111/1752-1688.12899. JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION JAWR 1 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION AMERICAN WATER RESOURCES ASSOCIATION