*5.1. Tidal Influence on Coastal Sediment Production*

The most extreme tidal range in the Gulf of California is located in the far north around the delta of the Colorado River, where a macrotidal regime with maximum amplitudes of 12 m plays out over a very low regional slope that results in tidal flats stretching seaward by more than 2 km at low tide [18]. High wave height achieved under wave surge during a major storm that coincides in timing with a high tide can be expected to influence the placement of marine sediments at a higher elevation in any given storm deposit. In the upper gulf region, those sediments are dominated by a peculiar species of marine bivalve (*Mulinia coloradoensis*), the disarticulated shells of which form major intertidal banks called *cheniers* [18]. Tides of this magnitude are unique to the upper part of the gulf. Normally, the daily shift in tidal action during calm weather should have little e ffect on a particular CBB in the central part of the Gulf of California, where the Guaymas Basin is quite deep. In the case of Ensenada Almeja, the daily tidal cycle does influence the sand beach at the south end of the bay. The modern beach and inland dunes that encroach on the fault valley connected to Ensenada San Basilio [9] are dominated by carbonate sand derived from the abundant bivalve mollusks giving the place its name: Clam Bay. Infaunal bivalves that burrow into the bay's sandy bottom are sheltered during low tide but the disarticulated shells of expired bivalves are liable to be uncovered by currents that accompany changes in the tidal cycle. North-facing sandy beaches in other parts of the central gulf region are commonly enriched by carbonate sand resulting from the breakdown of the abundant bivalve mollusks most commonly belonging to the species *Megapitaria squalida* [8,17].
