Find a point A on the other side of the river and a point B on this bank so that AB is perpendicular to the river bank.

Look for a small D and a small O in BD on this river bank.

Then start from D, stay away from the river bank along the direction perpendicular to the river bank, and stop when A and O coincide in your sight. If you mark this as C, then A, O and C are in a straight line.

At this time, it is easy to prove that the right triangle AOB and the right triangle COD are the same.

River width AB=CD, only the width of CD is measured.