Let the two endpoints (original positions) of the reed be AB, and the lower end B be D after blowing.

Then set the point where AB intersects the water surface as C.

Of course AB is perpendicular to the horizontal plane.

Now let BC be X, that is, the water depth, and the length of BD be 1+x(AB=BD).

Because BC is perpendicular to CD, according to Pythagorean theorem BC 2+CD 2 = BD 2.

And CD = 5bc = xbd =1+x.

That is, 25+x 2 = 1+2x+x 2 can get x= 12, so the water depth is 12 and the reed length is 13.