The defending champion U.S. won gold in six minutes 10.59 seconds, leading throughout and resisting a late Canadian charge to maintain their stranglehold on the event.
Canada, which had hoped to finally dethrone the U.S. boat, finished in 6:12.06 ahead of the Netherlands in 6:13.12.
The Canadians and the U.S., the defending Olympic and six-time world champions, came into the Games as rivals for gold. The Americans had not been beaten in six years but it was a photo finish in May when the two met at a World Cup in Lucerne, Switzerland, where the U.S. won by just 0.03 seconds.
The Canadian women's eight was shut out of the medals in the last two Olympics. Its last gold was in 1992 in Barcelona.
The Canadian men's eight won silver behind Germany at the Games on Wednesday.
Elsewhere, Rowing Canada's medal hopes took a dent Thursday when Victoria's Lindsay Jennerich and Patricia Obee, world silver medallists last year, failed to advance to the final of the lightweight women's double sculls.
The men's pair of Dave Calder and Scott Frandsen represents Canada's only other medal hope in rowing. They race Friday.
Canadian rowers won a gold, a silver and two bronze in Beijing.
The Canadian women were third after 500 metres but passed the Dutch to take second at the 1,000- and 1,500-metre marks. The Americans powered through the course, showing their class.
The U.S. boat, which consigned Canada to silver at the last two world championships, featured six veterans of the 2008 Olympic champion crew.
The London boat features four Beijing veterans: coxswain Lesley Thompson-Willie and Ashley Brzozowicz of London, Ont., Darcy Marquardt of Richmond, B.C., and Andreanne Morin of Montreal.
London is the 52-year-old Thomson-Willie's seventh Olympics — she was also named to the 1980 team that was part of the Moscow boycott. She won silver with the four in 1984 and gold with the eight in 1992, silver in 1996 and bronze in 2000.
She becomes the first Canadian athlete to win medals in five different Olympic Games.
Janine Hansen of Winnipeg, Rachelle Viinberg of Regina and Krista Guloien of Port Moody, B.C., rowed in the four at Beijing.
It's the first Olympics for Lauren Wilkinson of North Vancouver and Natalie Mastracci of Thorold, Ont.
It was only the second race at these Games for the Canadian women, who won their heat Sunday to advance directly to the final.
The U.S. won the other heat.
The demise of Jennerich and Obee was another example of an Olympic regatta that has seen some established crews struggle to retain their world status.
Only the top three in their semifinal moved on and the Canadians finished fourth. They started well but faded quickly and were fifth after 500, 1,000 and 1,500 metres. They posted the fastest last 500 but it was too little, too late.
China won in 7:10.39, ahead of Denmark and Australia. Canada's time was 7:14.83. Britain, Greece and Germany moved out of the other semifinal.
"A tough one to swallow," said Jennerich, fighting to keep her composure behind wraparound sunglasses as she talked to reporters. "It takes a little while to set in. ... I said to Obee, this might capture it well, it's like 'There's seven minutes of your life that suddenly take you out of a shot at an Olympic medal.'"
Jennerich and Obee had a poor start to the Games, finishing fifth in their six-boat heat. But the duo bounced back to finish second to the U.S. in their repechage Tuesday. The semifinal was another disappointment, however, denying Canada a shot at a medal.
Jennerich, who turned 30 on Monday, shared the boat with both 20-year-old Obee and 37-year-old Tracy Cameron of Shubenacadie, N.S., in the leadup to the Games.
In 2010, Jennerich and Cameron teamed up to win the world championship.
Last year they won in Lucerne, a major regatta ahead of the world championships. But when Cameron was sidelined with a stress fracture of a rib, Jennerich joined forces with Obee, the team spare, to win silver at the worlds.
Healthy again, Cameron won a row-off with Obee to see who would join Jennerich this year. But Jennerich and Cameron struggled when they got back in the boat, finishing eighth in May at a key pre-Olympic regatta in Lucerne.
Cameron, who won the lightweight quadruple sculls at the 2005 world championships and Olympic bronze in 2008 in the pair with Melanie Kok, retired in early June, saying rowing was no longer fun.
Jennerich said the pre-Games preparations were not ideal, "but it's what you make of it."
The circumstances "were handled like champions," she added.
In the women's lightweight class, rowers can weigh no more than 130 pounds and the crew average weight can't exceed 125.
The men's four of Michael Wilkinson of North Vancouver, Anthony Jacob of Victoria, Dean Will of Kelowna, B.C., and Derek O'Farrell of Montreal finished fifth in their semifinal and failed to make the final.
Michael Braithwaite of Duncan, B.C., and Kevin Kowalyk of Winnipeg finished last in a tough double sculls B final that featured 2008 Olympic champion Australia and world bronze medallist France. The Canadians finished the regatta in 12th spot.
NOTES — The men's eight got to eat Big Macs and meet Johnnie Mac after their silver medal performance Wednesday. CTV bought the rowers 140 pounds worth of McDonald's, allowing them to indulge a taste for junk food after months of good nutrition. Then the rowers met former tennis star turned commentator John McEnroe outside the International Broadcast Centre.
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