I didn't realize until just looking at Google earth that the lake is directly adjacent to Highway 20, and that its essentially right in a neighborhood with really very few trees around. Looking at the location it seems strange that anybody could easily take TWO girls without somebody seeing this, unless they went freely with somebody they know.
This was my thought exactly.
I drive by that lake on Hwy 20 all the time, and it doesn't seem like the type of place 2 kids could drown in easily. there's no current or anything like you get in a river.
Given that it's two of them, and that there is no trace found, I think the most likely scenario is actually a run-away, or abduction by someone they knew, that they went with willingly.