The Nuclear Research Submarine NR-1 was used to study morphology, sediment, and sediment—water interactions off the northwest coast of Puerto Rico, New detailed bathymetry from the surface-support ship, USS “Portland”, shows several submarine canyons in the area, some of them unreported previously. The north coast canyons, Arecibo, Tiberones and Quebradillas, are primarily erosional features although no recent turbidity-current evidence is seen. The canyons are presently filling with river-transported sediments. High-resolution seismic reflection profiles show the tectonic control of Mona Canyon. Fault zones can be outlined. The effects of sediment input, turbidity currents and bottom currents act to smooth and shape the canyon topography. River alluvium and biogenic debris are draped around the canyon and on the narrow benches between fault scarps on the north wall. Turbidity currents have carried large amounts of sediment from shallow water to the lower reaches of the canyon where the axis is being filled and leveled. Current measurements from NR-1 show oscillating flow with tidal frequency in the head of Mona Canyon.