The Brena River forms a large part of the border between Cyrodiil and Hammerfell to the northwest.
There are very few landmarks or anything else of note along the length of the river, since most of it lies beyond the invisible wall, preventing you from exploring its shores. Near its mouth, however, lies a shipwreck, with a nirnroot nearby. Also, if you explore a little to the left of the wreck, you will find a couple of broken chests and litter, as well as one lootable chest (minor loot). There are no other nirnroots along its entire length, however, nor much of anything else, and the way the slopes interact with the invisible walls means that an unsuspecting player could find themselves trapped with no way of getting out without fast-traveling or using the console.
The Lord Doomstone lies south of the river, about where the river splits. Near the stone, the Ayleid ruin Niryastare lies between two bandit camps. Malacath's Shrine and Fort Sutch can be found southeast of the river's mouth.