The Upper and Lower provinces are a 50-50 split of the total curvilinear habitable surface of the 6 x 9 mile cylindrical interior of the Ark. They were originally divided in this way to serve as a means for convenient geographic referencing of places, akin to how we divide the spherical surface area of planet Earth into a Northern Hemisphere and a Southern Hemisphere.
The Lower Province has vast expanses of natural forestry as well as engineering facilities scattered in the midst of these. Here we find the Farming Complex, the Wildlife Preserve, the cemetery/crematorium, the Educational Complex, the Manufacturing & Robotics Facilities (MRF), the Black Forest and the EVA Bay.
There's also a freshwater feature, named Lake Meghna, which was named after the Meghna river in Bangladesh and designed with a shallow depth profile not dissimilar to Lake Okeechobee in Florida. The tidal forces acting upon its waters are evenly maintained by the uniform rotation of the cylindrical Ark, preventing any over-spillage though floodings do occur seasonally in much the same way as Eridanus itself. One example of such a flood is to be found in Caroline's dream on page 48 of The True Price of Immortality, where the river of Eridanus has been transformed beyond all recognition into a huge sea!
The Flying Fishes of Lake Meghna was a children's TV program aired by Utopia Central Network that became popular with kids attending Redwood Junior School (p. 132 of The True Price of Immortality).