In Puget Sound the tidal heights from lowest low to highest high vary approximately 16 feet maximum in Seattle, 22 feet at Olympia. The farther south into Puget Sound the greater the tidal range due to the restricted water area of South Sound. This is a moderate exchange compared worldwide. The greatest exchance takes place in Canada's long Bay of Fundy (52 ft.), while some in Alaska are over 30 ft. as are the tides at the northern end of the Sea of Cortez in Baja, Mexico. Hawaii sees only tidal exchanges of a few feet. (for readers outside the U.S.A. please excuse our inability to switch to the metric system) :).
Animals, seaweeds and sea grasses are affected by the constant rise and lowering of the tide. While some intertidal organisms are adapted to withstand wide ranges in salinity, heat, light, and drying others are less adapted. Some species a restricted to tidal areas where predators or competitors are not too great a threat.
In short, two factors determine which organisms live where in the intertidal: Biological and Physical.
Animals well protected from drying, UV radiation, cold, and excessive heat can thrive in the highest intertidal levels...being able to take long naps doesn't hurt either. Highest living barnacles may be able to feed only a few hours per month, napping the remaining time away...the ultimate couch potatoes. Most intertidal sea stars have heavy complete hard skeletons to protect from excessive water loss during low tide exposure.
Intertidal crabs are almost exclusively flat in shape to allow easy access to damp spaces under rocks where they spend the low tide protected from drying and predators above....you know, the gulls.
So if you should happen to turn over small rocks, make sure you turn them gently back to their original position to protect the critters living under and attached to the bottom of the rock and living on the top of the rock....better yet turn over few or no rocks.
For additional information a slide show presentation can be accessed under ZONATION in the BUZZ LINKS section.