For anyone who is a Watership Down fanatic (like I am), here are some links to the geography of Watership Down.
The links will place an unmarked arrow at the exact spot, similar in shape to the one at right. Then Google will place a lettered arrow at the nearest place Google thinks is interesting. Ignore the lettered arrow and focus only on the arrow like the one shown to the right.
The “River Test” is about 8 meters wide, not exactly a “river” to us, but a serious river to our heroes.
Another enthusiast has a set of pages with photos here.
The map in the book is included below for reference. NOTE: In this map, North is to the LEFT