First, we learn that King Mosiah led his people from the land of Nephi to the land of Zarahemla to avoid the approaching Lamanites. In Omni 1:12, Mosiah was "warned of the Lord that he should flee out of the land of Nephi." In verse 13, "they were led by the power of his arm through the wilderness until they came down into the land which is called the land of Zarahemla."
By now, I hope everyone can envision this as leaving the Chattanooga area for Illinois, like this map shows:
|Mosiah (blue arrows) leaving Nephi for Zarahemla to avoid Lamanites (black arrows)|
Although the text doesn't specify that they went to the city of Zarahemla, we assume they eventually did. Reciting from memory because his people had no written language, Zarahemla related the history of his people, going back to the time of Zedekiah, king of Judah. They (we call them the Mulekites) crossed "the great waters into the land where Mosiah discovered them and they had dwelt there from that time forth."
This fits nicely with D&C 125 because an ocean-going vessel can travel all the way up the Mississippi until it reaches the Des Moines rapids, just south of Nauvoo. The Mulekites would have had to stop there, and it's such a favorable location, they would have no reason to migrate elsewhere (although they expanded beyond the city of Zarahemla throughout what was known as the "land of Zarahemla").
|Mulekites cross Atlantic, sail up Mississippi, settle in Zarahemla across from Nauvoo|
|Corps of Engineers graphic of Mississippi River bed,|
showing the Des Moines rapids as the first impassable part heading north
The next cool thing is Coriantumr's encounter with the people of Zarahemla. To understand this, we need to go to Ether a little.
The next section is taken from Moroni's America: