The Nile is a permanent river, and people lived in its valley no matter how dry the adjacent desert. The behavior of the Nile is influenced primarily by the climate in the area of its headwaters in the highlands of East Africa, where, during cold glacial maxima, there was reduced vegetation cover, more frost action and less rainfall. Thus, there was less water in the Nile and the water carried a heavy sediment load, which was deposited on the floodplain until the valley became choked with silt. This process occurred at least three times during the Middle and Late Pleistocene, with intervening episodes of downcutting. In Upper Egypt and Nubia, remnants of these accumulations stand 20-30m above the modem floodplain and include many Paleolithic sites. The earliest alluvial episode is associated with rare Lower Paleolithic artifacts, the second is late Middle Paleolithic, and the third is Late Paleolithic. Other Paleolithic sites occur near rock outcrops along the margins of the Valley, and there are a few sites in wadi gravels below, between and sometimes within the silt remnants.