If both sides are at sea level, why do canals need locks?

Senior Moments

Canals are man made and it is sometimes more economical to construct locks that allow for shipping to follow the contours of the land than to excavate millions of tons of rock and earth. http://www.czbrats.com/CuPA/bfall/ProfilePC.htm gives a section along the length that will help to explain my reply. Dec 03 03, 2:05 AM

Gnomon

The Suez canal has no locks, even though the two seas at each end (the Red Sea and the Mediterranean Sea) are not at the same level. This causes a constant current in one direction. If the two ends were at exactly the same level, there would be no current and the water would become stagnant and would go bad. To keep the canal clean, it is a good idea to feed the canal from a source of fresh water and to make it flow to the sea in both directions from this point. Locks are needed for this. They're also needed because the furthest point on the canal from the sea might be many hundred feet about sea level. To run the canal at sea level all the way would involve it being in a trench with cliffs hundreds of feet high on either side. This has been done at the Corinth Canal in Greece. Dec 03 03, 3:12 AM