The River Sidon is key to locating the southern border of the Land of Zarahemla. The River Sidon ran through, not along the Land of Zarahemla:
15 And it came to pass that the Amlicites came upon the hill Amnihu which was east of the river Sidon which ran by the land of Zarahemla and there they began to make war with the Nephites. (Alma 2)
Due to the uninspired punctuation modelers misread the above verse. After removing the punctuation it is clear that the Hill Amnihu is a hill range that ran along Zarahemla.
There were banks which armies could fight upon:
34 And thus he cleared the ground or rather the bank which was on the west of the river Sidon throwing the bodies of the Lamanites who had been slain into the waters of Sidon that thereby his people might have room to cross and contend with the Lamanites and the Amlicites on the west side of the river Sidon. (Alma 2)
The River Sidon was not far from the City of Zarahemla and is where the people were baptized. No other river is mentioned.
4 And they began to establish the church more fully yea and many were baptized in the waters of Sidon and were joined to the church of God yea they were baptized by the hand of Alma who had been consecrated the high priest over the people of the church by the hand of his father Alma. (Alma 4:4)
The City of Zarahemla was west of the River Sidon, and was surrounded by hills and valleys.
7 And now it came to pass that when Alma had made these regulations he departed from them yea from the church which was in the city of Zarahemla and went over upon the east of the river Sidon into the valley of Gideon there having been a city built which was called the city of Gideon which was in the valley that was called Gideon being called after the man who was slain by the hand of Nehor with the sword. (Alma 6:7)
This river was not so deep that you could not cross it, and did not have cliffs that you could not walk upon or fight:
40 And they were pursued by Lehi and his men; and they were driven by Lehi into the waters of Sidon, and they crossed the waters of Sidon. And Lehi retained his armies upon the bank of the river Sidon that they should not cross.
41 And it came to pass that Moroni and his army met the Lamanites in the valley, on the other side of the river Sidon, and began to fall upon them and to slay them. (Alma 43:40-41)
The River Sidon started in the southern wilderness and flowed north into a sea. How do we know that? Because the head of the river is located near Manti:
27 …through the borders of Manti by the head of the river Sidon (Alma 22:27)
22 …took their journey round about in the wilderness away by the head of the river Sidon that they might come into the land of Manti and take possession of the land; (Alma 43:22)
And Manti was south of Gideon:
1 And now it came to pass that as Alma was journeying from the land of Gideon southward away to the land of Manti, (Alma 17:1)
And Gideon was east of Zarahemla:
7 …he departed from them yea from the church which was in the city of Zarahemla and went over upon the east of the river Sidon into the valley of Gideon there having been a city built which was called the city of Gideon which was in the valley that was called Gideon (Alma 6:7)
What does “head” of the river mean? We take our context from Lehi’s dream, which meant source:
13 And as I cast my eyes round about that perhaps I might discover my family also I beheld a river of water and it ran along and it was near the tree of which I was partaking the fruit
14 And I looked to behold from whence it came and I saw the head thereof a little way off and at the head thereof I beheld your mother Sariah and Sam and Nephi and they stood as if they knew not whither they should go. (1 Nephi 8:13-14)
The “head” of a river in the Book of Mormon meant its source, not its mouth. That is why the armies would travel to the head so they could cross on dry land.
HEBREW – RO’SH
In the Bible, “head” means the same thing:
And a river went out of Eden to water the garden; and from thence it was parted, and became into four heads… name of the first is Pison…name of the second river is Gihon…name of the third river is Hiddekel…And the fourth river is Euphrates. (Genesis 2:10-14)
The Hebrew is ro’sh also means “top” as in “tops of the mountain.” (Genesis 8:5) There is no mistaking its meaning.