If you don’t need an introduction to the debate over what the meaning of “head” is in reference to the River Sidon, you can skip down to the bottom of this post.
The Heartland Geographic Model for the Book of Mormon looks something like this:
As you can see, the “head” of the River Sidon is not a “head” at all, but rather a “confluence” of two rivers. At the Mormon Evidence Facebook group, Jake Hilton (who I’m almost certain is the administrator of the group), argues the following:
1828 Noah Webster’s Dictionary: “Head” – Definition #23 = “Conflux.” Look up the definition for “conflux” and you get “A flowing together; a meeting of two or more currents of a fluid.” It also says for you to see “confluence.” If you look up “confluence” in the same dictionary you get “A flowing together; the meeting or junction of two or more streams of water, or other fluid; also, the place of meeting; as the confluence of the Tigris and the Frat, or of the Ohio and Mississippi.” And there you have it. In 1828, as Joseph Smith was translating the record, the conflux of the Ohio and the Mississippi was regarded as a “head” of the river.
So, in order to get from “head” to “a meeting of two or more currents of a fluid” we have to use the 23rd definition, and then follow that definition across the dictionary.
Here is the word “head” in the 1828 dictionary: http://archive.org/stream/americandictiona01websrich#page/878/mode/2up
Here is the word “conflux” in the 1828 dictionary: http://archive.org/stream/americandictiona01websrich#page/446/mode/2up
There have been various arguments against this interpretation, starting with the fact that the river seems to flows from Manti through Zarahemla, and that Manti is south of Zarahemla and the text seems to indicate that Manti is higher in elevation than Zarahemla.
The “Head” of the River in Lehi’s Dream
However there is another bit of evidence in the Book of Mormon that I personally haven’t seen used yet, and which I think ought to be considered. 1 Nephi 8:13-14 says the following:
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.
Here we have Lehi, in a vision, associating the “head” of the river with the place “from whence it came”. This is an obvious blow to the idea that the “head” of a river in the Book of Mormon is anything but its starting point.
I don’t want to overdo it though, because it isn’t the case that this one example of the use of “head” in reference to a river should dictate the way we interpret it in all other cases. The immediate and broader contexts of individual instances should be taken into account. However, we do know for a fact that in at least one case in the Book of Mormon the “head” of a river unambiguously refers to the place “from whence it came”. On the flip side, there are no examples anywhere in the Book of Mormon where the “head” of a river unambiguously refers to a “confluence”.
Lehi's Library 
I have never heard of anyone defining the “head” of a river as a “confluence” and vice versa.
I was taught that the head of a river means where the river starts, or “from whence it came”.
I showed this to a friend who is a hydrologist (not LDS) and has worked with many rivers, and a lot of water above and below the ground, and he could not believe anyone would define a river head the way the Mormon Evidence group did.
I am not trying to be snarky or put anyone down. I wanted an opinion of someone who would know, and if Mormon Evidence could have a valid argument for their explanation. Mormon Evidence has it wrong.
To be fair, Meldrum et al are correct to point out that the meaning of some words has changed since the time of Joseph Smith. We should look at what a word meant in his time, not necessarily or own when studying Book of Mormon vocabulary.
That being said, I obviously think their interpretation is tenuous at best.
Excellent comments, James. It is obvious when you have to jump from the definition of one word to the next (and to the next) in order to make the connection that the connection is tenuous. Even if we ignore that problem, however, it seems to me that more than simply definition of the word, but its usage in context, matters. One could say that “the rivers (plural) come to a head” when referring to a confluence of two rivers. But one would not talk about a confluence of two rivers as “the head of the river (singular).” Different meanings of the same word are not interchangeable and context matters to understand the sense intended. In no case does the Book of Mormon suggest the sense of “head” is “confluence.”
If you look at the 1828 dictionary that is reference above you also find definition #18 for head; the principal source of a stream; as the head of the NIle. At the bottom of the same page in this dictionary is another indention. Head, to originate; to spring; to have irs source, as a river. Therefore to suggest that, “In 1828, as Joseph Smith was translating the record, the conflux of the Ohio and the Mississippi was regarded as a “head” of the river,” is disingenuous and molding things t fit your own assumptions and conclusion. When Lewis and Clark went of find a water passage to the Pacific they found the head of the Missouri not the mouth.