After days of water-quality concerns caused by heavy rains last week put the swimming portion of the Olympic triathlons in doubt, the women dove into the Seine River on Wednesday under gray skies that lingered following an early-morning drizzle, followed by the men a few hours later.

The athletes began near the Pont Alexandre III, a bridge that spans the famed Paris waterway. The steady rain tapered off just as the athletes splashed into the water. Some dunked their swim goggles in the Seine before putting them on and heading into the river with the Eiffel Tower in the background.

The decision to go ahead with the swim for the triathlon competitions was a big deal for the city, Olympics organizers and the athletes. Officials undertook an ambitious plan, including 1.4 billion euros ($1.5 billion) in infrastructure improvements, to clean up the long-polluted Seine and have been steadfast in their insistence that the swimming portion of the triathlon and the marathon swimming events next week could safely be held in the river.