Every fall on the third Sunday in October, they flock to Cambridge, oar in hand, awaiting their turn upon the Charles. Then early on that Sunday morning thousands will line the river banks to watch crew's most majestic autumn Regatta.

For almost six and one-half hours, the shells will leave the starting line every ten seconds. At various points throughout the three-mile course, the continuous wave of boats will battle the arches and winding turns of the Charles.

The Head of the Charles will once again bring all the beauty and power of crew to Cambridge this Sunday, as the Regatta enters its 13th year. And as the history of the event grows, so does the traffic.

Last year, more than 2800 entrants rowed in 670 shells, but this year's Head will boast an even larger field: over 3000 racers will man 720 boats in the 18-event program.

Because of the large number of entries for this year's Head, some women's boats failed to make the field. There are 620 women slated to compete in this year's Regatta, the largest number of women ever entered, but Yale's and Princeton's women's teams did not make the field.

In an effort to include the two Ivy teams in the rowing weekend, Radcliffe coach Carrie Graves organized a new race for the "Big Three" women's teams, which has been dubbed "Ahead of the Charles."

The Elis and Tigers will race with Radcliffe Saturday morning at 11 a.m. in the pre-Sunday get-together.

But the weekend's main event, under the auspices of the Cambridge Boat Club, will occupy the river from 10 a.m. to almost 5 p.m. And to think that all this started as just a small, local regatta--it's almost unbelievable.

Back in December 1964, three members of the Cambridge Boat Club, Howard McIntyre, D'arcy McMahon and John Vincent, got together with British rowing great Ernest Arlett to talk about organizing a new regatta. The idea was to integrate collegiate rowing with club rowing, and the result was the Head of the Charles.

In October 1965, the Cambridge Boat Club, with funding from The Boston Globe, sponsored the first Head, which attracted about 100 boats. That turnout encouraged sponsors, and insured the development of what is today one of the world's largest single-day regattas.

The race is patterned after the English "Head of the River" events and is designed to test both the steering of the coxswain and the endurance of the oarsmen. Three miles of rowing along the winding Charles River accomplish that goal.

With staggered starts ten seconds apart, each boat will race against the clock. But the many bridges and turns of the Charles provide boat-against-boat excitement as the shells compete for position through the arches and on the turns.

For the spectator, the Weeks, Anderson and Eliot Bridges provide the best vantage points to view the action. The boats will race upstream on the Cambridge side, so ideal positions will be close to the Cambridge shore-line.

Each race will be filled to capacity, with 40 boats entered in every event. One of the day's best contests should come in the women's singles, where Liz Hills and Lisa Hanson, the women that teamed together to win a bronze medal in the doubles of the 1977 World Games, will oppose each other. Coach Graves will also row in that event.

Harvard crew coach Greg Stone, who represented the U.S. in the Olympics as a singles sculler, will row in the men's singles event.

Harvard-Radcliffe's only winning boat in the `76 Head, the victors of the women's four with coxswain race (White Stag Trophy) will return as the top-seeded boat in this year's four event. Becky Goff, Ruth Colker, Cynthia Strong, Karen Oberhauser and cox Diana Shaw will defend the title in this year's boat. Colker is the only returner from last year's winning boat.

After a full day of rowing, the Charles will slowly clear, and not until next October will it again enjoy such attention. But the one autumn day will remain as a glorious celebration of a grand, old sport. Event  Estimated Starting Time 1. Veteran's Singles (Mowatt Trophy)  10 a.m. 2. Women's Fours with Coxswain (White Stag Trophy)  10:25 a.m. 3. Double Sculls (Cromwell Trophy)  10:50 a.m. 4. Lightweight Eights (Boston Herald American Trophy)  11:10 a.m. 5. Elite Fours with Coxswain (Schaefer Trophy)  11:30 a.m. 6. Novice Singles  11:45 a.m. 7. Intermediate Fours with Coxswain (M.D.C. Trophy)  12:10 a.m. 8. Women's Singles  12:25 p.m. 9. Youth Eights  12:55 p.m. 10. Intermediate Singles (Louis B. Hawes Trophy)  1:10 p.m. 11. Pairs Without Coxswain  1:35 p.m. 12. Lightweight Singles  1:55 p.m. 13. Intermediate Eights (Boston Mayor's Trophy)  2:20 p.m. 14. Master's Singles  2:35 p.m. 15. Lightweight Fours with Coxswain (Coleman Cup)  3 p.m. 16. Women's Eights  3:20 p.m. 17. Elite Singles (Lawrence Curtis Trophy)  3:35 p.m. 18. Elite Eights (Boston Globe Challenge Trophy)  4 p.m.