The agents were performing boat operations Tuesday when they spotted a toddler and a baby in a carrier alone along the riverbank.
U.S. Customs and Border Protection shared a photo of the children where they were found.
Under the baby's carrier, agents found a note that said the 2-year-old girl and 3-month-old boy were siblings from Honduras.
Agents conducted a search of the area and found no additional individuals.
Amazingly, the children were not in need of any medical attention. They were taken to Uvalde for processing.
It was unclear how long the siblings were alone before they were discovered.