Calculate the water level drop for each 3-day interval.<br><br>
Day 1 to 4, Day 7 to 10, Day 19 to 22, Day 25 to 28.<br><br>
Day 7 to 10 shows the steepest drop on the graph -- the water level falls by the greatest amount in that interval.<br><br>
Final Answer: Day 7 to 10