It's not the return of Humphrey, but like him, two large humpback whales swimming in the Sacramento River near Rio Vista appear to be in trouble.

The mother and calf spent Monday between Rio Vista and a stretch of Cache Slough in the Sacramento-San Joaquin Delta.

As it was growing dark Monday evening, the whales were seen heading upriver near channel marker 49 in the Sacramento Deep Water Ship Channel, approximately 4 to 5 miles north of Rio Vista. No further sightings had been reported to the Coast Guard by 11:45 a.m. Tuesday.

Late in the day Monday, a crew from Alaska Whale Foundation tracking the whales said the mother was entangled in some kind of rope or fishing gear.

"It's some kind of wrap over her head. It looks like a crab pot line," Kathi Koontz, a volunteer research associate, said by cell phone from the foundation's boat Monday evening. "We're trying to monitor her and see how much of the wrap is there and where it's located."

The foundation is based in British Columbia and has an office in Benicia. With help from Solano and Sacramento County sheriff's boats, the group was attempting to coax the whales downstream to deeper water.

The foundation asked the National Marine Fisheries Service on Monday for permission to intervene to help the whales and was waiting for a response.

Koontz said the adult whale is about 45 feet long and her calf, likely born this winter, is about 17 feet long.

The whales were first spotted Sunday. On Monday word spread fast, spawning a sightseeing frenzy as the whales cavorted in Cache Slough, upstream from Rio Vista.

Hundreds of people lined the levees throughout the day Monday on both sides of the slough, near the Ryer Island Ferry crossing, as the whales swam back and forth across the ferry's path as if on parade.

"I never thought we'd be doing this today. This is history in the making," said Carol Paycer of Sacramento, who dashed to the scene Monday morning with her daughter, JaNiece Paycer-Mosher, as soon as they heard the news.

Observers noticed the mother whale's entanglement. In aerial video shot by a KCRA Channel 3 news helicopter, it was barely visible as an immobile object on the whale's right side.

The entanglement wasn't the only mystery surrounding the whales. U.S. Coast Guard officers, based in Rio Vista, say they personally confirmed seeing four whales on Sunday. By Monday, they could confirm only two.

The Coast Guard and sheriff's patrol boats maintained a 100-yard perimeter around the whales to protect them from boat traffic. A 1,000-foot flight ceiling was also in effect to protect the whales from disturbance by aircraft.

The region hasn't seen anything like this since 1985, when "Humphrey" the humpback whale dazzled Rio Vista and the world with a long visit before being helped back to sea. Then he returned to San Francisco Bay five years later, beached himself, and was eventually dragged back into the bay.

John Calambokidis, a whale expert at Cascadia Research in Olympia, Wash., said Humphrey hasn't been seen anywhere in surveys his group has done every year since 1990. He said it is likely that Humphrey is now dead, and probably didn't live long after his stranding due to the stresses involved.

Unlike gray whales, it is rare for humpbacks to enter inland waters such as San Francisco Bay -- let alone 30 miles upriver from the bay.

"To swim upriver is highly uncharacteristic," Calambokidis said. "It's hard to tell if they're stressed. At this point I think the most important thing would be to try to keep them as undisturbed as possible and monitor their movements."

A block party atmosphere prevailed along Cache Slough Monday as people stood up in their pickup beds, or poked up through sunroofs to get a better look at the visiting whale pair.

The two whales, swimming side-by-side, made repeated laps in the slough between the tip of Ryer Island and a point beyond the ferry crossing. The whales would surface for air at regular intervals, then dive, sometimes stirring up clouds of bottom sediments.

A trio of television news helicopters whirled overhead and the riverbanks were lined with people toting cameras and binoculars. Farmworkers getting off work from nearby alfalfa fields stopped to watch from truck beds. A school bus stopped and children leaned out the windows to see.

At around noon Monday, the Solano County Sheriff's Office asked the California Highway Patrol to help with traffic control on River Road, the route leading from Rio Vista to the ferry crossing.

"When do you get to walk out of your front door and then drive up and see a whale?" said Rio Vista resident Mike Neal.

Niko and Lolo Wong drove down from Sacramento when they heard the news. The couple visited Hawaii in January and paid $50 each for a whale-watching tour. This was better, they said.

"We got close, but we definitely got a better view here," said Niko Wong, who also witnessed Humphrey's visit 22 years ago.

The crowd reaction was a mixture of awe at the mysteries of nature and concern for the whales. Many onlookers rooted for the whales to point downstream and swim hard for the ocean.

Humpbacks are an endangered species under U.S. law and are also guarded by the Marine Mammal Protection Act.

Calambokidis said there are about 1,500 humpbacks in the California population. The group has been rebounding at a rate of about 8 to 9 percent annually since commercial whaling ended, he said.

The whales' needs are now in the hands of the National Marine Fisheries Service and the Marine Mammal Center, which works in partnership with the government to help stranded whales.

Aside from the adult's entanglement, heat stress after stranding is the biggest risk the whales face, said Jim Harvey, a marine science professor at Moss Landing Marine Laboratories.

Whales normally cool off by shunting more blood to their fins, flippers and flukes. This lets them pick up coolness from the surrounding water.

But when whales are in warm, shallow water or warm air, that heat exchange function doesn't work, and heat begins damaging the animals' organs.

"I'm concerned with the distance they are from the ocean," said Joe Cordaro, a wildlife biologist with the National Marine Fisheries Service. "If it starts getting more compromised as far as the safety of the animals, their health, that kind of stuff, then we may initiate something."

Human rescuers might help by either making loud, annoying noises behind the whales or playing more appealing whale vocalizations in front of them.

For any whale, Harvey said, "to go that far means they are pretty messed up," and they'll probably keep going until they figure out they've made a mistake, Harvey said.

"They're not really, really bright," he said. "I sort of think of them as the cows of the ocean."

Harvey said he believes, however, that the whales' prospects aren't bleak.

"I think the chances are probably very good, maybe with a little bit of help from humans, that they'll get out of there," he said.

California's humpback population normally winters off the coast of mainland Mexico and Central America. Then starting in May, they spend the rest of the year in the deep ocean well off the California coast.

Their normal diet is krill, a shrimp-like crustacean they eat by filtering through the baleen plates in their mouths. But in recent years, Calambokidis said, humpbacks have switched to eating mostly fish, which has brought them closer to shore than usual.

This may be a result of ocean conditions along the Pacific Coast that have disrupted the krill population, with drastic effects on sea birds and fish.

It's possible, Calambokidis said, that the Rio Vista whale pair followed a school of fish upriver.