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This article is part of the June 2007 issue of the Sound Waves newsletter. 

A decades-old U.S. Geological Survey (USGS) current drifter was recently found by a 5-year-old boy on the shore of San Pablo Bay, California, triggering a flurry of e-mails and memories for members of the USGS Western Coastal and Marine Geology Team (now, the Pacific Coastal and Marine Science Center) and the USGS Water Resources Discipline (now, the California Water Science Center). The drifter—a convex yellow plastic disk attached to a faded red plastic stem with a brass weight near its end—was one of thousands of "near bottom drifters" released in 1970-71 to track currents that carry suspended sediment into and out of San Francisco Bay. This particular drifter was found by the boy on March 11, 2007, at McNears Beach in San Rafael.

USGS hydrologists and geologists undertook the drifter study as part of a larger effort to understand natural processes in the San Francisco Bay system (of which San Pablo Bay is a part), so that scientists could better predict the likely impacts of human activities. A report of the study, "Drift of Surface and Near-Bottom Waters of the San Francisco Bay System, California: March 1970 through April 1971" (USGS Miscellaneous Field Map 333), contains maps showing where the drifters were released and where some of them were found after washing ashore. The authors described the drifter as "a saucer-like plastic disc…that has a serial number, return address, and a statement of reward [50 cents] to be paid with the return of the serial number, and the date and place at which the drifter was found. A brass weight attached to each drifter carries it to the bottom where it is moved with the near-bottom water. Without the weight, the drifter floats and indicates the movement of the surface water."

More than 1,000 near-bottom drifters were released on each of six release dates about two months apart from March 1970 through April 1971. Additionally, approximately 1,000 surface drifters (without the brass weight) were set adrift during the study's last two releases. Some of the drifters were released inside San Francisco Bay and some in the adjacent Pacific Ocean. Within 60 days of their release, about 10 to 21 percent of the near-bottom drifters had been recovered and reported, and about 27 to 33 percent of the surface drifters.

The near-bottom drifters released in the Pacific Ocean showed a pronounced eastward drift into San Francisco Bay, which persisted throughout the year of the study. Near-bottom drifters released within the San Francisco Bay system followed one of three dominant flow patterns:

1. a year-round drift westward from Rio Vista to eastern San Pablo Bay;
2. a year-round drift eastward from the Golden Gate, with virtually all drifters turning northward into San Pablo Bay; and
3. a seasonally reversing drift in the south bay, dominantly northward during summer and southward during winter.

The surface drifters followed somewhat more complex flow patterns. Excluding several drifters released right at the Golden Gate, no near-bottom drifter released within the bay system was recovered on the ocean beaches, and no surface drifter released seaward of the Golden Gate was recovered within the bay system.

Although the results of the study were published more than 35 years ago, every few years another of the old USGS drifters is recovered and reported. The most recent discovery started a chain of e-mail messages that began with an inquiry from the discoverer's mother and eventually made its way from USGS Science Information and Library Services in Rolla, Missouri, to the USGS office in Menlo Park, California, where many scientists remember the drifter study. Although the data are no longer being compiled, finders still receive the promised reward: USGS scientists recently mailed a box containing USGS pencils, postcards, and publications (and 50 cents) to the boy who found the drifter last March.

Today's drifters are more sophisticated than they were some 30 years ago. For one thing, many electronic components are much smaller, making it fairly easy to outfit drifters with global-positioning-system (GPS) units that allow scientists to track the drifters throughout their journeys. But for the simple old USGS drifters that turn up occasionally on San Francisco Bay area beaches, the exact paths they have followed over the past 3 decades remain a mystery.