otherwindow:

Unlike ocean mermaids who have plenty of room to grow their hair, freshwater mermaids prefer shorter styles similar to humans; because of their close proximity to the surface, “freshies” may also be seen wearing hats to block out sunlight and peek out from the water like alligators.

Additionally, due to their small size in even smaller environments like lakes and rivers, freshwater mermaids prefer human speech for communication over the whale-like humming used by their ocean relatives to speak. 

Abyssal mermaid, humming: [Hello cousin…!]
River mermaid, tipping their seashell cowboy hat: HOWDY