At its narrowest point between Kent County 's Plum Point ( near Newtown ) and the Harford County shore near Romney Creek , the bay isabout 2.8 miles ( 4.5 km ) wide . At the widest location , it is 30 miles ( 50 km ) wide . At its narrowest point between Kent County 's Plum Point ( near Newtown ) and the Harford County shore near Romney Creek , the Bay is 2.8 miles ( 4.5 km ) wide ; at its widest point , just south of the mouth of the Potomac River , it is 30 miles ( 50 km ) wide .