The Brena River serves as the border between Cyrodiil and Hammerfell. We can see it north of the independent Gold Coast. Unlike the Brena River in TES IV, ESO is not continuous like the Strid River: there is a waterfall at the northern edge of the map.




A piece of land questions however. Being on the other side of the Brena river, it seems obvious that it is Hammerfell. On the game map, this piece of land is much further away subject to change when a local Hammerfell region becomes available (example with Vvardenfell and the Telvanni Peninsula which were redesigned when the Morrowind chapter was released).
On the composite map, we see that this piece of land does belong to Hammerfell and that the river is Bréna.





Yet the "new" map of Tamriel presented during the Reveal presents us with this piece of land as belonging to Cyrodiil. The Ayleid Ruin of Varlaisvea is now located here.



Despite this map, we can see that the terrain continues without the slightest river mouth towards the north (seen from Varlaisvea) and that it could well be in Hammerfell.