Term for an extremely fast-moving part of a river, caused by a steep descent in the riverbed. The Nubian flow of the Nile had six cataracts, which subdivided the area and formed obstacles for shipping due to the presence of eddies, dangerous blocks of granite and rocky islands. The first cataract at Aswan formed a natural southern border for the Egyptian Empire throughout its history. However, in the Middle Kingdom several Egyptian fortresses were built at the Second Cataract. In the New Kingdom the influence of the Egyptians reached up to the Fourth Cataract. Here, the border station of Napata remained a trade colony for several centuries.