The Strid River forms most of the border between Cyrodiil and Valenwood to the south. (map).
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, there are a few Nirnroots, including one on a small island in the middle of the river. There are no other Nirnroots along its entire length, however, and the way the slopes interact with the invisible walls makes it possible to get trapped with no way of getting out without fast-traveling or using the console.
The western tip of the river houses the Ayleid ruin Silorn and the Warrior Doomstone. Fort Istirus and a Reman Rune Stone can be found where the river splits in two. Gweden Farm is located a little northeast of the mouth, and the Jode Stone can be found west of the farm.