FIELD, B.C. -- Parks Canada says it has scaled back the recovery operation for an 11-year-old Calgary boy who fell into the Yoho River in southeastern B.C.

Two boys ended up in the river near the Takkakaw Falls on Friday evening while visiting Yoho National Park with their family.

One of them was quickly rescued, but the other was swept downstream, Parks Canada spokeswoman Tania Peters said.

Dwight Bordin, also of Parks Canada, said helicopters were continuing to monitor the area on Monday after an extensive search on the weekend.

He said a 20-kilometre stretch of river includes some challenging terrain.

"The water volumes are high and the water is milky. There's about 4.5 kilometres of canyon with rock shelves that are very narrow, and downstream there's lots of debris, boulders and log jams."

A Parks Canada dog handler was among 35 people from various agencies involved in the aerial and ground search on the weekend.

Everyone's thoughts are with the missing boy's family, Bordin said.

The incident is now being handled by the Golden-Field RCMP as a missing persons case.