In this puzzle, the water needs to be lowered seven feet, which requires six columns. Since there are nine places to put the columns, you won't need three of them. Thus there are eighty-four possible combinations of steps.

Here is a picture of all of the possible positions of columns and their heights. The horizontal line segments represent the top of the column, while the number above each segment is how tall that column would be. Remember, to solve the puzzle, you need one column on each level, and the columns should step down, from left to right, one foot at a time.

Notice that the column sections available are in 2-foot and 4-foot sections. This means that an odd height of column (3-foot, 5-foot, or 7-foot) is not possible. Removing any of these positions eliminates all but a few possibilities. We must combine a 2-foot and 4-foot section to create a 6-foot column to be used on the bottom level and on the second level from the top.

Only five 4-foot sections are left. We can combine two of these to form an 8-foot column, and the remaining levels use 4-foot sections.