Barren mud and salt flats comprise a low-lying coastal plain at the north-western end on the Gulf of California, between the mouth of the Colorado River and the town of San Felipe, Baja California. The region is arid, and characterized by a maximum spring tide range of 8 to 10 m. Three morphologic or environmental units are distinguished in the coastal mud flats: (1) the high flats, approximating the level of extreme spring tides; (2) the intertidal flats, dipping seaward at gradients of 0.1–0.2 degree from spring higher-high to spring lower-low tide level; and (3) the subtidal mud flats, extending 11–12 m below mean sea level. Deposits of the coastal mud flats cover an area of 2000 km2, and are about 16 m thick. Examination of surface sediments in terms of color, minor structures, texture, and composition of the coarse fraction reveals a zonation of sediment types across the mud flats related to variable exposure to subaerial drying and evaporite crystallization, wave action, tidal currents, and the activity of burrowing organisms. In a seaward direction, i.e., with decreasing elevation, the sequence of sediment types includes: (1) chaotic muds and evaporites; (2) moderate brown, well-laminated clayey silts; (3) brown to gray, mottled silty clays; and (4) gray, poorly laminated silty clays and clayey silts. Mineralogy, grain size, and the areal distribution indicate that the silts and clays are derived from suspended load of the Colorado River, and are carried to the site of deposition by Gulf tidal currents. Borings encounter a similar sequence of muds beneath the high flats, indicating development of the mud flats through depositional regression. Seaward growth was initiated during the final stages of late Wisconsin sea-level rise when accretion of tidally supplied muds exceeded the ability of small Gulf waves to rework and disperse. Subsequent growth resulted in onlap of the tidal mud flats across the Pleistocene piedmont plain to the west, and progradation over sandy tidal current ridges of the deeper Gulf to the east. Mud supply diminishes toward the south, and wave effects are accentuated accordingly. Waves truncated the Pleistocene piedmont plain causing rejuvenation and entrenchment of the piedmont washes. Coarse sand supplied thereby is carried northward to form prominent longshore spits which finger out into the intertidal muds. Restriction to tidal flooding imposed by the spits initiated erosion of prominent tidal channels which dissect the southern mud flats. Depositional regression in the southern area has been limited in extent, and has occurred by strand plain development in response to mudflat encroachment from the north. Sediment supply has been much reduced for the past 50–60 years due to diversion of the Colorado River into the Salton Sea and the subsequent construction of Hoover Dam. Consequently, waves have winnowed the poorly segregated mud-flat deposits, piled coarse mollusk remains into beach ridges fringing the northern high flats, and developed a fine sand and shell veneer over the intertidal zone. Older beach ridges, now largely encased by intertidal muds, record an earlier period of low mud supply and reworking which was probably initiated 1000 to 1500 years B.P. by diversion of the Colorado River into the Salton Basin to the north.