Dozens of biologists and environmental activists have rescued two freshwater dolphins after they became trapped in a river with low water levels.

The two were among nine dolphins that have been stranded in the river for more than a month. The two dolphins were caught in nets and brought to shore on Sunday.

They were put in padded, water-filled aluminium containers and driven about two miles downstream, where they were put in a pen in deeper water.

Mariana Escobar, a biologist at the Noel Kempff Centre, said the dolphins had not been hurt and were breathing normally. The team kept the dolphins' skin damp by wrapping them in wet towels.

The two rescued dolphins will be kept in the pen until they could be reunited with the other seven, according to a member of the rescue team. The operation manager estimated it will take two or three more days to round up all of the dolphins.

The nine dolphins were trapped in early July in a tributary of the Rio Grande, north of the city of Santa Cruz, when drought caused water levels to drop.