Celebrating its 25th anniversary, the American River Parkway Foundation’s “Down River Day” will bring hundreds of rafters together for a fun-filled ride down the American River on Saturday. The event will serve as a fundraiser for the foundation’s stewardship programs.   The festivities start at River Bend Park in Rancho Cordova (formerly Goethe Park) Saturday morning, where participants will be picked up by bus and taken down Sunrise Boulevard where American River Raft Rentals will have boats on shore and ready to board. Rafts will depart around noon and return to River Bend close to 2:30 p.m.   “We supply them with drinks, water guns and anything they could possibly need,” said Chris