When the brothers left Port Elizabeth for Patensie, they spent their first night by a dry river bed where Morris and David, in the story, tried using their Martini Henry rifle for the first time.
For most of the year river beds such as these are dry. The sand is usually very fine and soft making crossing them extremely difficult for wagons. In the summer months it is normally the wet season. This can make traversing these rivers even more difficult, especially if they are inhabited by crocodiles.
Below is a typical scene of the type of bushland the brothers would have had to walk through. I was fortunate enough to come across a pair of White Rhinos when I took this photo.