The Rio Balsas submarine canyon heads in or near the surf zone of Mexico's Pacific coast, 280 km northwest of Acapulco. One of its tributaries is related directly to the main distributary of a large river of the same name. Seaward, the canyon terminates in the Middle America Trench.

The intricately bifurcated heads of the tributaries were investigated by divers. They are cut into poorly consolidated deltaic sediments. The walls commonly slope more than 30°, and are vertical to overhanging where sedimentary strata are exposed. The tributaries apparently are not controlled tectonically.

Shifts in river discharge cause formation of new tributaries and filling of old ones. The easternmost tributary heads in an embayment formerly associated with the main river discharge. Sediments exposed in three

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overhanging banks, at depths of 7.5, 17, and 32 m, were deposited less than 185 years ago according to C¹⁴ dates. Historic records indicate that the major river discharge shifted away from this tributary about 100 years ago. Thus the canyon apparently was deepened by at least 25 m in 100 years.

It is concluded that canyon cutting can progress at rapid rates, that submarine processes can form tributaries, and that, therefore, a dendritic canyon pattern is not necessarily indicative of subaerial erosion.

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