Canada stormed to gold in the final of the men's eights, pushing Great Britain into second place.
The North American crew, coached by Briton Mike Spracklen, were a length ahead at half-way and withstood late pushes from GB and the US eight.
The US took gold in the women's eights in imperious fashion to end Romania's run of three Olympic wins.
Great Britain came fifth ahead of Australia, after Natasha Howard and Alison Knowles pulled out with illness.
Romania's women were forced to accept bronze after the Netherlands came through in the last few strokes to take silver by 0.03 seconds in a photo finish.
GB women's eight crew member Beth Rodford said of the illness and substitutions: "It hasn't been the easiest of weeks
"I think we've dealt well with what has been thrown at us and made the most of the situation."
"I'm disappointed - I thought we had the Canadians, but it turns out that we didn't have enough left in the tank"
In the men's race, GB's Alex Partridge won his first Olympic medal, four years after a punctured lung forced him out of the four, which went on to clinch Matthew Pinsent's fourth gold.
But he and crew-mates Tom Stallard, Tom Lucy, Richard Egington, Josh West, Alastair Heathcote, Matt Langridge, Colin Smith and cox Acer Nethercott were clearly dissatisfied as they accepted their silvers.
"There was a stage where we started losing it," said Nethercott. "But you need momentum and Canada had it."
Heathcote said: "It came down to great force and a gutsy performance. Unfortunately it wasn't quite there to beat them.
"I'm disappointed. I thought we had the Canadians, but it turns out that we didn't have enough left in the tank."
However West, who was part of the men's eight that finished ninth in Athens, added: "We were out there racing for the gold, but silver is still amazing.
"It's a very different experience being out there at the top rather than at the bottom."
Canada finished fifth in 2004 despite entering as double world champions and favourites, and many of the crew kept on until to Beijing to make up for that result.
Australia, who had six-times Olympian James Tomkins on board, finished outside the medals.
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