Well from bulbapedia it says: During certain occasions, the spirit of a deceased young girl will be
standing on the bridge, and will disappear if the player walks towards
her. An old lady in the gate to the east of the bridge will explain
that the young girl used to play around the area with an Abra before
the bridge was built.
Also, in the pokemon black and whte anime, ash and his friends crossed a bridge with the same ghost girl. She used to carry people on her boat accross the river but ever since they built the bridge, she has been alone, only with a gothitelle.