Very good question that deserves an explanation. We know that the mouths of tidal estuaries, river mouths and beaches change shape both horizontally and in profile seasonally, and they can silt-in over time. Drake was on our coast in the Summer of 1579. In 1595, sixteen years after Drake, the Portuguese explorer Sebastian Rodriquez Cermeño led a Spanish expedition to explore and map the California coast. Records from the Cermeño expedition indicate they found three Spanish fathoms (sixteen-and-a-half English feet) of water over the estero bar at high tide. So, there’s evidence that in 1579 it’s likely there was enough depth for the Golden Hind to enter the estero.