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 b 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 reas 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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Although turbidity currents may be the major mechanism for transporting sediments down canyons and seavalleys, the ordinary, almost continuous currents also are important. Commonly, speeds of up to 30 cm/sec exist in these flows which we find alternately moving up and down valley floors. These currents can transport finer sands as well as great quantities of silt. Thus beach sand introduced by longshore currents may be permanently lost to the beaches due to these normal canyon currents. It also is important to know whether waste products such as the heavier sludge from sewer outfalls, which is locally dumped into nearshore canyon heads, will be carried continuously down the canyons into deeper water. Our study of canyon currents provides at least some of the answers to this problem. >

Our current-meter records also provide oceanographers with information which was not previously available. We found that currents flow almost continuously up and down the canyon axes, in cycles that have a length that is related to both depth of water and range of tide. High frequency of reversals-in-flow-direction occurs in most shallow canyon heads, whereas in deep water, reversals-in-flow have a frequency closely related to the semidiurnal tidal cycles. The depth where currents become tidally related depends on the range of tide, being at shallow depth with large tidal ranges and at great depth with a small range.

Internal waves were found to advance along the canyons and other types of valleys, most commonly progressing landward into shallow water, but in some areas progressing into deeper water. The latter usually appear to be related to a source of water moving toward a submarine valley head, as for example where a valley is located off a large river mouth.

Much is still to be learned about these seafloor currents, and some of our information needs more documentation, such as our discovery that at least in one place the currents are essentially unidirectional, moving seaward as a countercurrent under the landward-moving surface currents. We still are puzzled by finding that some records show the fastest currents in a direction normal to the canyon axes. Suggestions that surface winds may play a part in producing such anomalies require more investigation, including the comparison of currents at various heights above the valley floors and their transport potential.

It is surprising to learn how little investigation of bottom currents has been done by geologists and physical oceanographers during the otherwise rapid growth of oceanography in post-World War II years. To petroleum geologists, eager to understand processes of sedimentation, this study would seem to be of paramount importance.

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