Spur dikes installed in series in a river with vegetated floodplain system generate flow separation and recirculation that strongly influence flow and longitudinal transport in rivers. Flow dynamics are affected by riparian submergence during high-flow conditions; however, the combined effects in a river-spurs-floodplain-vegetation on flow patterns and momentum transfer remain insufficiently understood, especially in submerged flow conditions. This study employs large-eddy simulation (LES) to examine flow structure and momentum distribution near spur-dike fields under three configurations: unsubmerged spurs, submerged spurs interacting with the floodplain, and submerged spurs with vegetated floodplain. The unsubmerged case exhibits horizontal separation, forming a large cavity and a sharp interfacial shear layer that promotes flow stagnation. Submergence promotes vertical flow separation, reducing recirculation extent and rotational strength by ~25\%. Floodplain vegetation introduces drag, redirects flow into the cavity, and generates secondary cells that partially restore rotation. Anisotropy analysis confirms that submergence suppresses large-scale coherent vortices and reduces shear-dominated turbulence. Consequently, turbulence shifts toward more isotropic small-scale mixing, with vegetation further reinforcing this behavior. Quadrant analysis evidenced this through sweep–ejection (Q2–Q4) dominance in vertical transfer and Q1–Q3 control in lateral exchange; submergence redistributes events, while vegetation redirects secondary motions toward the main channel. Consequently, exchange coefficient ($K$) increases, enhancing interfacial transport and reducing stagnation. The combined effects of submergence and vegetation promote longitudinal dispersion and interfacial exchange in rivers, but may also elevate local scour risk. Meanwhile, vegetation reduces near-bank turbulence and enhances bank stability. These findings have important implications for spur-dike design and floodplain restoration.