A previously unreported process was observed at the head of a tributary to the Rio Balsas submarine canyon system in Mexico. During a period of large surf, river discharge deflected a pulsating longshore current [peaking at over 7 km/hr (2 m/sec)] seaward over the tributary heading in the surf zone. This pulsating flow occasionally entered the river mouth, causing rhythmic fluctuations with amplitudes of at least 30 cm and a period of about 3 minutes within the mouth, as recorded by a partially filtered tide gage. In diving to the bottom of the tributary at a depth of 18 m, we encountered current pulses with estimated velocities of 4 km/hr (more than 1 m/sec) transporting large amounts of suspended sand down an axial slope of 26°. This bottom flow was at least 3 m thick, and was characterized by pulses separated by quiet periods in phase with the surface rip current. The upper 20 cm of the canyon fill during this time consisted of sand smoothly laminated parallel to the bottom, indicating net deposition on the steeply sloping floor. Estimated water budget suggested that the entire water column below the rip current at 18 m was not flowing seaward during the pulses. The bottom flow landward of the dive site probably separated from the surface flow and was propagated downslope as a turbidity current. Its magnitude was sufficient to erode the canyon walls. These observations substantiate that rip currents play a role in the formation of some submarine canyons. Surf-beat induced rhythmic flushing of the river mouth, however, did not cause density currents in the main canyon head. The need for future canyon studies under extreme conditions is pointed out.