Geography:
"The Nephite army departs "out" of the valley headed full tilt downhill to a precise point on the river upstream from the city, obviously a ford (probably the ford across that stretch, for they knew just where to head.) If they left the valley in the morning (v. 23, "morrow"), headed on the shortest route to the river, they had time to fight at the crossing and then pursue the enemy to the wilderness of Hermounts all in the one day. So from the valley to the river would be a distance perhaps a bit less than .. 20 miles." (Sorenson, 1990, p. 246).
Military: The nature of the engagement at the river for would appear to be a rather limited one. The Nephite army must necessarily be forced into a more narrow presentation rather than a wide one due to the nature of fords (they had to pass where the river was passable, not at deeper locations). This creates a very concentrated military front with the reinforcements coming up narrowly from the rear.
Militarily, the Nephites were in a terrible position to meet the full Lamanite army. Note that at the time of the engagement, the majority of the Nephites had not yet emerged from the river (see verses 33 and 34).