That spot where he claims he saw it is completely built up. The one little patch of woods on the river is the patch where bigfoot was taking a bath. All around are housing developments. So obviously that explains how he remains undetected.
My theory: on the nearby highway, twenty-five years ago, was a popular little burger joint called "Mighty Joe's." Joe, the guy who owned the place, got caught banging a waitress and his wife Joline set fire to the restaurant in anger. When the fire was out all that was left was the plastic statue of Mighty Joe that Joe had paid $250 for in 1970, standing out at the corner of the parking lot. When the crews came to clear the debris they tossed the statue into the woods out back. Over time the area was built up and more water channeled into the river, and the statue was eventually covered to the shoulders with water. It is there to this day, gently bobbing back and forth in the current.