Most of the shops in my local area have a message board, either on-line or a bulletin board on the shop wall where you can post a message.
Show up at your favorite run and offer to drive shuttle or pick up the folks hoping to thumb a ride back to the put-in. Offering a cold drink is a nice way to start a conversation with strangers.
Meet fellow boaters by taking a local class in something river related. Pool rolling sessions or a Z-drag and self rescue primer.
Volunteer for a river cleanup, check local clubs