Usually we lose them

Because river dolphins (freshwater) are so rare, most of the species are endangered, or in the case of the baiji, the Yangtze river dolphin, extinct.

So it is uniquely gratifying and slightly hopeful that a new river dolphin species has been identified and described in South America.

Brazil river dolphin is first new river species since 1918

The new species lives in the Araguaia River.  Where is that, you ask?

Here;  look southeast of the Amazon.  Weirdly, though this river and the Tocantins, that it officially flows into, are in the Amazon River basin, they aren't tributaries of the Amazon, they have their own outlet to the sea.  Which helps explain why the Araguaia river dolphin could evolutionarily diverge from the Amazon river dolphin.