Abstract e-Waste recycling using crude techniques releases a complex, yet incompletely characterized mixture of hazardous materials, including flame retardants (FRs), to the environment. Their migration downstream and the associated risks also remain undocumented. We examined 26 FRs (18 brominated (BFRs: 12 polybrominated diphenyl ether (PBDE) congeners, plus 6 alternatives) and 8 organophosphate esters (OPEs)) in surficial sediments of the Lian River. Sampling encompassed the river's origin, through the Guiyu e-waste recycling zone, to its mouth, as well as associated tributaries. OPE exceeded BFR concentrations in most sediments, despite their far greater water solubilities. Among OPEs, tris(1-chloro-2-propyl) phosphate dominated upstream, but shifted to triphenyl phosphate in Guiyu and downstream sediments. For PBDEs, Deca-BDE dominated upstream, but Penta-BDE prevailed in Guiyu and at many downstream sites. Among emerging alternative BFRs, decabromodiphenyl ethane dominated upstream, transitioning to 1,2-bis(2,4,6,-tribromophenoxy)ethane in Guiyu sediments. Penta-BDE (BDE-47 + -99, 668–204,000 ng g−1, ∑PBDEs 2280–287,000 ng g−1), tetrabromobisphenol A (2,720–41,200 ng g−1), 1,2-bis(2,4,6,-tribromophenoxy)ethane (222–9870 ng g−1) and triphenyl phosphate (4260–1,710,000 ng g−1, OPEs 6010–2,120,000 ng g−1) concentrations in Guiyu sediments were among the highest reported in the world to date. The continuing dominance of these e-waste indicative FRs in sediments downstream of Guiyu suggested that FR migration from Guiyu occurred. Hazard quotients >1.0 indicated that the extreme sediment concentrations of individual FRs posed ecological risks in most Guiyu reach and downstream areas. Simultaneous exposure to multiple FRs likely increased risks. However, risks may be mediated if FRs were associated with strong sorbents, e.g. carbon black from burned debris, hydrophobic polymer fragments, or resided as additives within polymer fragments. Graphical abstract Unlabelled Image Highlights • Lian River sediments contained BFR and OPE levels among the highest ever reported. • e-Waste from Guiyu appeared to be the dominant FR source to the Lian River. • While seldom associated with e-waste, Penta-BDE, TBBPA, BTBPE and TPP dominated. • The Guiyu FR profile was mirrored in downstream sediments, suggesting migration. • Some sediment-associated FRs appeared to pose unacceptable ecological risks. [ABSTRACT FROM AUTHOR]