The Nile flows north from the mountains of Kenya to the Mediterranean, where it forms a fan-shaped estuary much like that near New Orleans on the Mississippi River. It is broken periodically in its flow by cataracts (rapids and waterfalls) that prevent easy passage to its source. Thus travel routes, guiding merchants carrying frankincense from Arabia and other exotic products, followed the Nile. However, due to a series of cataracts along its southern stretch, travelers seldom sailed its full length.