Here the Book of Mormon puts a requirement on the river Sidon. If bones are dumped into the river and are deposited "in the depths of the sea" (Alma 3:3), then obviously the river Sidon must flow into a "sea." The word "sea" seems to imply something more than a lake, and because the river Sidon flowed by the local land of Zarahemla, and because the local land of Zarahemla is later described as being "in the center of the land" (Helaman 1:27), then the river Sidon must have been rather long and large. [Alan C. Miner, Personal Notes] [See the commentary on the river Sidon in Alma 2:15]
“Their Bones Are in the Depths of the Sea”
In Alma 3:3 we find the following:
And now as many of the Lamanites and the Amlicites who had been slain upon the bank of the river Sidon were cast into the water of Sidon; and behold their bones are in the depths of the sea, and they are many.
According to Brant Gardner, there is a contrast between the Nephite treatment of their own dead and their treatment of the dead of their enemies. In Alma 3:1 we read that the Nephites buried their own dead. But here in verse 3 we are told that the dead of the Lamanites and Amlicites are thrown into the river. This distinction in the treatment of the corpses tells us some important information about the Nephites. First the apparent "proper" way to honor the dead was burial. Thus, the throwing of bodies in the river to be carried downstream is not accidental but a conscious denial of respect to Lamanites and Amlicites.
In societies where modes of burial are attached to beliefs about the afterlife of the spirit, the denial of the proper treatment of the dead affects not only the corpse, but the soul of the departed--perhaps even a denial of afterlife. In this light, the actions of the Nephites is very natural and an echo of an ancient people and ancient practices. [Brant Gardner, "Book of Mormon Commentary," [http://www.highfiber.com/] ~nahualli/LDStopics/Alma/ Alma3.htm, pp. 3-4]