The River Nile enters Egypt as it flows through Lake Nasser, formed by the building of the Aswan Dam. In its lower reaches, the river is about wide and the alluvial plain about wide. The annual flooding of the Nile no longer occurs and the fertility of the Nile Valley is now maintained by irrigation rather than the deposition of silt. Much of the Nile is bordered by flat land but in some places there are low cliffs. Where the river flows into the Mediterranean, there is an extensive fan-shaped delta area with channels, lakes and salt marshes.