Sea level during the Pleistocene glaciation got to be ~150 m lower than today. You are absolutely right that during times of low sea level, the rivers eroded th

Sea level during the Pleistocene glaciation got to be ~150 m lower than today. You are absolutely right that during times of low sea level, the rivers eroded this part of the shelf just as today. However, these canyons go all the way down the continental slope to ~3000 m below current sea level, with the same basic erosional forms (check out Monterey Canyon in Google Earth, it's gorgeous). These mud flows (called turbidites in the literature) are quite capable of eroding the seafloor when they flow down steep slopes, though at rates well below what rivers do on the surface.