The geography of Southampton is influenced by the sea and rivers. The city lies at the northern tip of the Southampton Water, a deep water estuary, which is a ria formed at the end of the last Ice Age. Here, the rivers Test and Itchen converge. The Test—which has salt marsh that makes it ideal for salmon fishing—runs along the western edge of the city, while the Itchen splits Southampton in two—east and west. The city centre is located between the two rivers.