The Great Basin's internal drainage results from blockage of water movement by high fault-created mountains and by lack of sufficient water flow to merge with larger drainages outside of the Great Basin. This internally-drained area occupies approximately , including most of Nevada, a large part of Utah, and portions of Idaho, California, and Oregon. Much of the present-day Great Basin would drain to the sea, just as it did in the recent Ice Ages, if there were more rain and snowfall.