They chose a pretty spring day in a pretty Virginia town on a sparkling stretch of the elegant old Rappahannock River. They marshaled four boats, a dozen guys with "doctor" in front of their names, a score more of support personnel, a thousand lab bottles and all kinds of technical gizmos.

Then they took a week off and set to work to find out exactly what happens to the massive plug of glop that comes roaring down big rivers every year during spring flood.

It was pure science, Dr. Eugene Cronin agreed happily as the research vessel Aquarius charged up the Rappahannock under bright blue skies.

By that he meant that no one really knew what would come of it. The scientists did know that the question of spring sediment runoff had long intrigued them, and they knew that most of what they found out about it would be new knowledge.

The Chesapeake Research Consortium was on its own. For six years, it has been answering other people's questions and last week it finally got to answer one of its own.

CRC is a mix of scientists from four erudite outfits with a stake in the Chesapeake Bay - The Smithsonian Institution, Virginia Institute of Marine Science, University of Maryland's Chesapeake Biological Laboratory and Johns Hopkins University.

Its mission since 1972 has been to study the Bay and its tributaries, but until last week the consortium was a slave to its contractors. CRC took stream samples for the State of Maryland, measured sediment for the Army Corps of Engineers, checked water quality for private companies and generally did things at other folks' bidding.

Operation Hiflow 78 marked "the first time the consortium has generated its own idea and its own support," said CRC director Cronin.

CRC scentists picked the wide waters of the lower Rappahannock because it's a critical area, Cronin said as the RV Aquarius nosed past the old city of Tappahannock. This is where river meets sea, and under the glassy calm is a busy world of erosion, siltation and saltwater intrusion.

What happens in these waters affects the success of spawning fish, the life cycle of shellfish, the transportation and disposition of pesticides, bacteria, heavy metals and plain old American dirt.

Since most sediment that washes down rivers is carried during flood, Cronin and his colleagues wanted to follow spring high water and find out where things ended up.

They took a guess timing and hit it square on the head. "We put in for those weekend storms," Cronin said.

Monday, they ordered up clear skies and gentle northwest winds. The research vessels went to work Tuesday with scientists on each boat measuring salinity, temperature and sedimentation in samples of river water at five-mile intervals from the mouth up to mile 95.

They found the big glob of sediment from the spring runoff at miles 65 that day, and through the week they watched it move downstream and calculated the variations as sediment dropped out along the way.

They saved samples to take back to the labs for analysis of weight and character of sediment.

That meant that every five miles all hands were on deck manning machinery and taking readings, and the rest of the trip the scientists could lean back and soak in the sights and sounds of spring on the river.

They watched as great flocks of black cormorants gathered at fishermen's stake nets and Canada geese rafted in flocks at bends in the stream, hawks circled lazily and ducks scattered as the boats drew near.

Oyster tonger's sleek workboats bobed in the gentle swell and somewhere off a bluff an eagle was seen on the prowl.

Nice dirty, but what is the practical value of all this?

Cronin isn't sure. He is sure that science will end up with a better understanding of what makes a river work. "And if you understand the system," he said, "your efforts to pretect and manage the resource are bound to be better, quicker and cheaper."