In the Gulf of St. Lawrence, short-wavelength, low amplitude magnetic anomalies form dendritic patterns on the shallow (< 100 m depth) water Magdalen plateau. High-resolution seismic data indicate that these magnetic anomalies are associated with 0.6 – 2 km wide incised Quaternary valleys with relatively flat bottoms lying 30 – 70 m below the seafloor. The magnetic signature of the valley is due to high-magnetic susceptibility volcanic detritus derived from the erosion of volcanic rocks that overlie most of the salt bodies in this area. This study shows that magnetic data may be a useful tool for the mapping of surficial sediments. Magnetic data provide the first offshore evidence for the direction of major water discharge. It remains unclear if the valleys are formed by river incision during one or several regional lowstand(s) or from one or several subglacial discharge episode(s) across the Magdalen plateau. The valley evidence should be incorporated into any future paleogeographic reconstruction.