Substantial changes in mainland river discharge to the Gulf of California in historic time present a useful constraint in establishing major sources of fine-grained terrigenous sediment in the Gulf. A sharp decrease in water discharge of the Colorado River over the last 60 years, caused by extensive irrigation and damming, did not have a strong effect on the accumulation rate of terrigenous silt and clay in the Delfin Basin of the northern Gulf. Resuspension of fine-grained sediments from the broad, shallow shelf of the northern Gulf and/or eolian-transported silt and clay components from the mainland might be responsible for the constant supply and accumulation of sediments in the Delfin Basin.

In the Guaymas Basin of the central Gulf, no consistent changes in the accumulation rate of terrigenous sediments are observed. The mineralogy of the sediments at the core site generally reflects mainland drainage sources which, like the Colorado River, were dammed and diverted for irrigation during the middle 1900s. Because the central Gulf lacks broad shelf reservoirs of sediment, the apparent constant supply of sediment to the Guaymas Basin implies eolian transport of sediment from mainland provenances to the east, as suggested by Baumgartner et al. (1991).

Finally, cores from the southeastern slopes of the Gulf do show a significant decrease in the mass accumulation rate of terrigenous sediment since the 1930s. There are no indications of long-term

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climate (rainfall) changes in the southeast Gulf area during this period. We conclude that damming and diversion of the mainland rivers entering the southern Gulf are the major causes of the historic decrease in sediment mass accumulation rate in the eastern slopes of the southern Gulf.