Spanish speakers on horseback named a great many California creeks for what they saw in them: Arroyo de los Sauces for willows, Arroyo Seco for nothing at all. Ours they called Los Berros, the watercresses, and it is one of the more precise names on the map.
Watercress does not grow just anywhere. It wants water that is cold, clean and moving, the kind that comes out of springs and keeps going through a dry August when the rest of the country has gone to dust. A creek named for watercress is a creek that runs all year and runs clear. That was worth knowing if you were moving cattle across a rancho the size of Nipomo, and it was worth putting in the name so the next rider would know it too.
The plant is still here. Look for it in spring in the shaded, gravelly reaches, in dense green rafts with small white flowers. Leave it where it is: the creek is on private land, and watercress from open water is not safe to eat raw anyway. But it is a fine thing to find a place still living up to its name after a century and a half.