The Persian Gulf is mostly shallow and has many islands, of which Bahrain is the largest. The gulf is bordered by Oman and the United Arab Emirates to the south, to the west by Qatar and Saudi Arabia, to the north by Kuwait and Iraq, and along the entire east coast by Iran. It was generally thought that the gulf had previously extended farther north and that sediment dropped by the Tigris, Euphrates, Karun, and Karkheh rivers filled the northern part of the gulf to create a great delta. But geologic investigations now indicate that the coastline has not moved and that the marshlands of the delta represent a sinking of the earth's crust as the Arabian land block pushes under Iran. The gulf waters have very slow currents and limited tidal range.