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Abstract

It is a widely held opinion that submarine canyons were cut during the glacial stages of low sea level and now are essentially dormant features, disturbed only on rare occasions by high-speed turbidity currents of torrential proportions. Our research contradicts this concept of the canyons, citing nine years of measurements of currents in the axes of canyons and in other types of seavalleys. Approximately 200 records from continuously operating current meters (during periods of several days to as much as a month) have shown that currents rarely cease flowing alternately up and down the floor of the valleys and frequently attain speeds sufficient to transport sand-sized sediments along the valley axes. Furthermore, turbidity flows with speeds rarely exceeding 3 km/hr are by no means uncommon events and in fact may occur every few days in localities where rivers introduce large quantities of sediment to the sea near the heads of submarine valleys.

Our early studies of currents in canyons offshore southern California may have failed to reveal the importance of these low-velocity turbidity currents, partly because here beaches are the chief source of sediment and there are no large (and few small) rivers entering near the canyon heads. Furthermore, the occurrence of turbidity currents would not ordinarily be recorded in many southern California canyons because great masses of kelp and sea grasses are carried downcanyon by the currents, entangling the instruments, stopping their operation, and frequently causing current meters to be swept away. However, in canyons and valley heads off large rivers (or off small rivers during flood conditions) with no kelp debris on the bottom, we found a very different story. In three out of four areas of this sort even our brief periods of current measurement yielded examples of slow turbidity currents, and in the fourth area, off the Fraser delta, the failure to record a turbidity current may have been because of the almost continuous dredging at the river mouth which removes great quantities of available sediment.

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