If the Great Pyramid was a water pump, it had to pump water from somewhere. Where?

As has been known for centuries (literally), the ancient Egyptians used an extensive system of shallow lagoons and ponds for capturing the annual flood of the Nile. This water was then distributed via a system of canals to fields, and canal building continued until today.

Farmers also used swapes, or shadufs (the spelling may be wrong and probably is), to move water from the Nile or canals to their fields, and do the same today. The swape is a long pole mounted on another pole or between two poles so that it moves up and down and in a circle. The horizontal pole is mounted off-center, and has a bucket on one end and a weight (often a ball of mud) on the other. Water is scooped up in the bucket, and the arm rotated to the field, where the water is dumped.

Egyptian engineers also built dams. Perhaps as far back as Khufu's time they dammed the Wadi Garawi, southeast of Cairo. This dam was about 33 feet high, between 200 and 370 feet long, and between 150 and 270 feet thick. Egyptian engineers also dammed the Orontes River in Syria, creating the Lake of Homs; the dam is still in use and is about a mile and a quarter long.

Given their mastery of irrigation canals and dam making, why would they go through the trouble of moving 2.5 million blocks of stone to build a pumphouse?