The difference in tides on the Pacific and Atlantic entrances of the Panama Canal was once a moot factor in the arguments of the sea level vs. lock level controversy. The lock level advocates pointed out the necessity of a Pacific lock to offset the difference of tide level and resultant current therefrom, i.e., they declared an actual sea level canal was not feasible from the matter of tides alone.
There are now over sixteen years of American tide records available. Accurate sea levels have been determined and many values are now known that had to be approximated in the preconstruction days.
The Panama Canal transits a vessel from the Caribbean Sea to the Pacific Ocean, an oceanic distance of 10,600 miles, in thirty- eight nautical miles. Considering the more usually schoolbook assigned cause of tides alone, the attraction of the sun and moon, it might be expected that the tides at the Canal entrances would vary but slightly. But there are other causes, such as position and shape of land formations, ocean currents, prevailing winds, revolution of the earth (tidal ranges are usually the greater on the west coasts of continents), densities of sea water, and so forth.
Panama Bay is virtually a reentrant angle with a vertex 100 miles from the ocean proper—a very conducive reason for wide tidal ranges. All the year the winds over the Caribbean Sea are from the northeast, being strongest in the dry season from January to May; on the Pacific side the same winds are off shore in the dry season, but are largely from the southeast the rest of the year.
The Caribbean Sea is noted for its small tidal ranges. At Cristobal Harbor there is an average range of but 0.6 feet, with maximum ranges between plus 1.7 and minus 1.06, or 2.76 feet. Furthermore, it has the intermediate form of tides, i.e., part of the lunar month it has the semi-durnal type two highs and two lows and ranging from that to the diurnal type of one high and one low. At Balboa Harbor, which has the semi-diurnal type of tides, the tidal variations are many times the greater; maximum springs range 22.3 feet, between plus 11.2 feet to minus 11.1 feet, and the average range is 12.6 feet. The so-called “low water springs,” used as the reference datum for soundings on hydrographic charts and tide table, is 8.3 feet below mean tide level. Minimum neap tides have about 5 feet ranges. The Coast and Geodetic Survey estimate that for an unobstructed free channel 1,000 feet wide across the Isthmus, there would be a maximum spring tidal current of 3 to 3.4 knots an hour.
One of the odd things about Panama’s tides is that, although geographically the two canal entrances are but forty-two statute miles apart, similar tidal phases of semi-diurnal types on the two sides are usually about three hours apart, i.e., a high or low tide on one side usually is synchronous with a mean tide level on the other. (See Cotidal map and typical Balboa and Colon curves.) Furthermore, the Balboa annual mean sea level is about .68 feet above that of Colon. The mean sea level difference on the two sides, however, varies considerably according to the months. There is but slight difference in February and March, whereas there is nearly one foot difference in October.
One of the peculiar features of Panama Bay hydrography is the probable effect of the meeting near Cape Mala of the southbound California current and the northbound so-called Humboldt current. The higher level of mean tides at Balboa is probably at least a partial effect of this.
The general impression that the surface of all oceans is a uniform level, or more scientifically, an area on a perfect spheroidal surface is, of course, erroneous. There are many reasons why this is not true. Without going into any theory or discussion of these causes, it is significant to note that the Canal Zone is the only place in the world wherein bench marks on mean sea level datums of different oceans may be interconnected with a short line of precise levels, as they have been in the Canal Zone.