The vertical accretion of salt marshes is mainly due to flow reduction and wave damping by vegetation. However, the details of the hydrodynamics are only partially understood, and have been studied mainly in the laboratory. This study presents detailed field investigations of the water flow in a Spartina maritima salt-marsh in the Ria Formosa, a shallow, meso-tidal lagoon in Southern Portugal. Detailed velocity profiles were obtained within and above the 30 cm high canopy using a high-precision velocimeter. Results show that the influence of the bottom becomes negligible a few centimetres above the bed, and that the flow depends on the vegetation density at each level of the canopy. When the canopy is partially emergent or is only slightly submerged, the upward increase of horizontal velocity is roughly linear. A more drastic flow reduction exists when the canopy is well submerged, with a slow, nearly constant velocity in the denser part of the canopy and a faster, logarithmic shaped velocity profile above. This dampening effect of the vegetation is expected to promote sedimentation. However, the short-term sedimentation rate obtained with sediment traps during fair-weather conditions is usually lower in the Spartina marsh than in the surrounding areas. Therefore, the effect of the Spartina canopy for sediment accumulation seems to be more that of erosion protection during storms than of sedimentation enhancement during normal conditions. Using these results, a simple conceptual model is proposed for the sedimentary processes taking place in the intertidal areas of the studied lagoon.