Witnesses spotted the mammals in the channel near the end of the San Gabriel River Trail at about 11 a.m. They said they called California Department of Fish and Wildlife officials after the animals appeared stuck in one spot.
The whales drew a crowd of onlookers as they floated along the surface and splashed and twirled in the low tide. They appeared to be about 100 yards from a nearby bridge.
Shortly before noon, the whales managed to turn around toward open water but remained in the channel.
Officials from the National Oceanic and Atmospheric Administration said they were "en route to investigate the situation." They added that during this time of year whales are known to stay close to the shore when migrating with their calves, keeping them safe from predators.
Experts said the whales appeared to be healthy and were acting normally. There were any number of reasons they made the detour, including feeding or keeping close to shore as they migrated with calves.
By early evening one whale was seen swimming back out to the Pacific and the other was not visible and was believed to have already returned to the ocean.