ARTICLE Longest sediment flows yet measured show how major rivers connect efficiently to deep sea Peter J. Talling 1 ✉ , Megan L. Baker 2 , Ed L. Pope 2 , Sean C. Ruffell 3 , Ricardo Silva Jacinto 4 , Maarten S. Heijnen 5,6 , Sophie Hage 7,8 , Stephen M. Simmons 9 , Martin Hasenhündl 10 , Catharina J. Heerema 3 , Claire McGhee 11 , Ronan Apprioual 4 , Anthony Ferrant 4 , Matthieu J. B. Cartigny 2 , Daniel R. Parsons 9 , Michael A. Clare 5 , Raphael M. Tshimanga 12 , Mark A. Trigg 13 , Costa A. Cula 14 , Rui Faria 14 , Arnaud Gaillot 4 , Gode Bola 12 , Dec Wallance 15 , Allan Griffiths 16 , Robert Nunny 17 , Morelia Urlaub 18 , Christine Peirce 3 , Richard Burnett 19 , Jeffrey Neasham 19 & Robert J. Hilton 20 Here we show how major rivers can efficiently connect to the deep-sea, by analysing the longest runout sediment flows (of any type) yet measured in action on Earth. These seafloor turbidity currents originated from the Congo River-mouth, with one flow travelling >1,130 km whilst accelerating from 5.2 to 8.0 m/s. In one year, these turbidity currents eroded 1,338- 2,675 [>535-1,070] Mt of sediment from one submarine canyon, equivalent to 19–37 [>7–15] % of annual suspended sediment flux from present-day rivers. It was known earthquakes trigger canyon-flushing flows. We show river-floods also generate canyon-flushing flows, primed by rapid sediment-accumulation at the river-mouth, and sometimes triggered by spring tides weeks to months post-flood. It is demonstrated that strongly erosional turbidity currents self-accelerate, thereby travelling much further, validating a long-proposed theory. These observations explain highly-efficient organic carbon transfer, and have important implications for hazards to seabed cables, or deep-sea impacts of terrestrial climate change. https://doi.org/10.1038/s41467-022-31689-3 OPEN 1 Departments of Geography and Earth Science, Durham University, South Road, Durham DH1 3LE, UK. 2 Department of Geography, Durham University, South Road, Durham DH1 3LE, UK. 3 Department of Earth Sciences, Durham University, South Road, Durham DH1 3LE, UK. 4 Marine Geosciences Unit, IFREMER Centre de Brest, Plouzané, France. 5 National Oceanography Centre Southampton, SO14 3ZH Southampton, UK. 6 School of Ocean and Earth Sciences, University of Southampton, Southampton SO14 3ZH, UK. 7 University of Brest, CNRS, IFREMER, Geo-Ocean, 29280 Plouzané, France. 8 Department of Geosciences, University of Calgary, Calgary, AB T2N 1N4, Canada. 9 Energy and Environment Institute, University of Hull, Hull HU6 7RX, UK. 10 Institute of Hydraulic Engineering and Water Resources Management, TU Wien, 1040 Vienna, Austria. 11 School of Civil Engineering and Geosciences, Newcastle University, Newcastle upon Tyne, UK. 12 Congo Basin Water Resources Research Center (CRREBaC) and Department of Natural Resources Management, University of Kinshasa (UNIKIN), Kinshasa, Democratic Republic of the Congo. 13 School of Civil Engineering, University of Leeds, Leeds LS3 9JT, UK. 14 Angola Cables SA, Cellwave Building 2nd Floor Via AL5, Zona XR6B, Talatona-Luanda, Angola. 15 Subsea Centre of Excellence Technology, BT, London, UK. 16 O&M Submarine Engineering, Vodaphone Group, Leeds, UK. 17 Ambios, 1 Hexton Road, Glastonbury, Somerset BA6 8HL, UK. 18 GEOMAR Helmholtz Centre for Ocean Research, Wischhofstraße 1-3, 24148 Kiel, Germany. 19 School of Engineering, Newcastle University, Newcastle upon Tyne NE1 7RU, UK. 20 Department of Earth Sciences, South Parks Road, Oxford OX1 3AN, UK. ✉ email: Peter.J.Talling@durham.ac.uk NATURE COMMUNICATIONS | (2022)13:4193 | https://doi.org/10.1038/s41467-022-31689-3 | www.nature.com/naturecommunications 1 1234567890():,;