Consider AB as the breadth of the river. Take a point M at a distance from B. Draw a perpendicular from the point M and name it as N so that it joins the point A as a straight line.Now in △ABO and △NMOWe know that∠OBA=∠OMN=90∘We know that O is the midpoint of the line BMSo we getOB=OMFrom the figure we know that ∠BAO and ∠MON are vertically opposite angles∠BAO=∠MONBy ASA congruence criterion△ABO≅△NMOAB=NM(c.p.c.t)Therefore, MN is the width of the river.