It is 2.8 miles ( 4.5 km ) wide at its narrowest ( between Kent County 's Plum Point near Newtown and the Harford County shore near Romney Creek ) and 30 miles ( 50 km ) at its widest ( just south of the mouth of the Potomac River ) .
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 .