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Formation of the dark “summer” laminae of the varved sediments from the anaerobic slopes of the central Gulf of California has previously been considered a response to the summer-fall peak in runoff from the major rivers of Sonora. Reconstruction of a 20th century varve chronology for an area off Guaymas, Sonora now provides a well constrained temporal framework for direct comparison of river discharge data to the mass accumulation of material in the dark laminae. A continuous record of dark lamina deposition from 1934 through 1966 shows no indication that construction of dams across the major Sonoran rivers, begun during the 1940s, has had any effect on the accumulation of these laminae. No relationship exists between the mass flux values and the “controlled” discharge of the rivers below the dams. However, a weak relationship was found to exist between mass flux and the “free,” undammed summer-fall river discharge of the Sonora, Yaqui and Mayo Rivers compiled from stations above the dams or for periods prior to dam construction. Based on these results we argue that the transfer of terrigenous material from the mainland occurs principally through eolian processes by removal and transport of desert dust associated with convective summer thunderstorms.

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