Arrowheads have edible tubers that were an important food for Indians. I wade around in the muck just for fun. For the Indians around Watkins Lake, it must have been a way of life. Another of yesterday's finds was wild rice, which also grows in shallow water. The presence of plentiful food sources made this a very active area for Indians. Many artifacts, including fire circles, have been found.
Sagittaria is a nearly worldwide genus. It's likely a primitive food almost everywhere.