Possible contenders:
* Beaver: Beavers are known for their complex family structures and cooperative building projects, making them highly social in a community-oriented sense.
* River Otter: Otters are playful and often seen in groups, especially mothers with their pups. They engage in social interactions like grooming and playing.
* Manatee: Manatees are generally calm and gentle creatures, often congregating in large groups for warmth and protection.
The catch:
* "Most social" is subjective: While beavers have elaborate social systems, otters are more playful and interactive, and manatees form large groups for safety. It's hard to definitively rank them.
* Freshwater vs. marine: Some might argue that dolphins, being highly social marine mammals, are more social than any freshwater mammal. However, the question specifically asks for freshwater.
Conclusion:
Ultimately, there's no definitive answer. All three species – beaver, river otter, and manatee – exhibit high levels of social interaction in their respective ways. It depends on how you define "most social" and which aspect of social behavior you prioritize.