Answer: length of water depth =a-2b+ 1.

The first way to solve the problem:

Solution: If the length inserted into the river bottom is b meters, the length exposed to the water surface is b- 1 meter.

Therefore, the length of water depth is: a-b-(b-1) = a-b-b+1= a-2b+1.

Analysis: As shown in the above figure, the length of the bamboo pole inserted into the river bottom is B, because the length of the exposed part is less than that at the river bottom 1 m, then the exposed part is b- 1. The total length of the bamboo pole is a, so subtract the length of the bamboo pole in the river from the total length and then subtract the exposed part.

The second way to solve the problem:

Solution: let the length of bamboo pole in the river be x,

According to the meaning of the question, the length above the water surface is: b- 1 m, then

(b- 1)+x+b=a

b- 1+x+b=a

x+2b- 1=a

x=a-2b+ 1

Summary: In the fourth grade, I began to learn to solve equations with unknowns. I recommend you to use the second method and be familiar with the knowledge of solving equations. Unknown is because of the value to be determined in the equation, and it is also a metaphor for things that are not known yet. In fourth grade mathematics, X and Y are often used as markers, which can effectively help us solve problems in equations. The key point is to get used to understanding topics by drawing, because drawing is more intuitive and can effectively help us set unknowns and equations.