Riverside is either a district of the city or a subdistrict of South Quarter in The City, both make up the South Quarter/Riverside region. It is the location of the Docks. It is the home of the Eelbiters gang. Erin's Hideout is located here.
This may be located near Wayside.
The quest objective says Erin's hideout is in South Quarter, but Garrett mentions it is in Riverside. Further adding to the confusion between the unified region.
When leaving sub areas in the Riverside region (such The Siren's Rest), the game says you are leaving to go to South Quarter, thus also suggesting Riverside is technically a major sub-district of South Quarter.