From the tidal point of view, Torres Strait is probably the most complex area in the world. Its narrow and shallow channels connect two oceans with different mean sea levels caused by the general oceanic circulation patterns. This difference introduces a westward equalising current. In addition, tidal regimes on both sides of the Strait are completely different, with diurnal tides to the west and semi diurnal to the east.
The contrast in regimes is caused by the difference of semi diurnal components of tide at either entrance, the diurnal part being generally uniform in the area. At some phases of the moon it can be high water at one entrance when it is low water at the other. In consequence, marked differences between the levels at the entrances occur resulting in strong tidal streams. While the tides may have a large diurnal component (especially at the western entrance), the tidal streams are predominantly semi diurnal.
Throughout the Prince of Wales Channel and its approaches, from Twin Island in the east to a few miles west of Goods Island, the streams flow at the times predicted for Hammond Rock (see predictions on the following pages). The rates diminish as the channel becomes less restricted and at its western entrance are only about 30 percent of those predicted at Hammond Rock. At Booby Island the rates are comparatively weak and the streams are of different character.
Near Harvey Rock and Saddle Island the streams commence and reach their maximum rates about 30 minutes earlier than at Hammond Rock, but in these more open waters the rates are comparatively weak. In Endeavour Strait the streams commence and reach their maximum rates about 40 minutes later than at Hammond Rock and, except for the more restricted parts of the Strait, their rates do not exceed 30 percent of those at Hammond Rock.
It has to be remembered however, that tidal stream predictions for Hammond Rock do not include any non tidal flows, like the equalising current mentioned above or currents caused by meteorological influences. In addition, the El-Niño Southern Ocean Oscillation can cause a drop of sea level of about 0.5 metres on the eastern side of the Strait in a very short time. The resultant changes to the water levels in the Strait, and to the current and stream direction and rates are impossible to predict.