If the Atlantic and Pacific Oceans are at sea level, why are locks needed in the Panama Canal?

Because it has to overcome a height difference. The ground is higher than sea level at the middle of the canal. So instead of digging the canal to an impossible depth the locks are used to lower and raise the ships instead.



http://wiki.answers.com/Q/Why_does_the_panama_canal_have_locks

https://en.wikipedia.org/wiki/Panama_Canal_Locks

Sea Level is about eight inches higher at the Pacific end than the Atlantic end. Tidal variation is greater on the Pacific side as well, so it would be a good idea to have locks even if the canal was level. In addition it's a bad idea to have marine wildlife from the two oceans mix.

http://www.straightdope.com/columns/read/1091/if-the-locks-on-the-panama-canal-were-blown-would-one-ocean-pour-into-the-other

There is a difference in level at each end, simply because of the tides. High tide occurs at different times in different places. Also, there is a difference in tidal range - low to high tide is about 1ft in the Atlantic but 20ft in the Pacific.

In 1883 it was realised there was a tidal range of 20 feet at the Pacific, whereas, the Atlantic range was only about 1 foot. It was concluded that this difference in levels would be a danger to navigation. It was proposed that a tidal lock should be constructed at Panama to preserve the level from there to Colon. This plan would save about 10 million cubic metres of excavation.

eclipse.co.uk/~sl5763/panama.htm#The Locks webpage no longer exists
http://www.yourpanama.com/panama-canal.html

The site tides4fishing.com/pa/canal-panama gives tide details for Cristobal and Balboa, at either end of the Panama Canal. You can see the differences in time and height. Today for example:

In the high tide and low tide chart, we can see that the high tide was at 2:20 am and the low tide was at 10:46 am. The tidal coefficient today is 71, a high value and therefore the range of tides and currents will also be high. The tide heights today are 0.3 m and -0.1 m. We can compare these levels with the maximum high tide recorded in the tide tables for Cristobal which is of 0.5 m and a minimum height of -0.2 m.

In the high tide and low tide chart, we can see that the first high tide was at 6:06 am and the next high tide at 6:16 pm. The only low tide of the day was at 12:13 pm. The tidal coefficient today is 71, a high value and therefore the range of tides and currents will also be high. The tide heights today are 4.7 m, 0.5 m and 4.7 m. We can compare these levels with the maximum high tide recorded in the tide tables for Balboa which is of 5.7 m and a minimum height of -0.9 m.

http://www.tides4fishing.com/pa/canal-panama

In brief:
Cristobal: high tide 02:20, 0.3m
Balboa: high tide 06:06, 4.7m

Also, the 0.5m low tide at Balboa is higher than the 0.3m high tide at Cristobal!

This web page at the BBC has a map of Panama and the canal, and a cross section to the same scale, showing the height differential of the land; referred to in the first post by AdamM7

http://news.bbc.co.uk/2/hi/americas/4941126.stm