I find it hard to believe that the girls were taken against their will from the spot where the bikes were found. It seems too risky a spot to plan an abduction - parking on the highway would be noticed immediately, the trail itself is apparently used quite a bit - maybe you can drag two girls off without being seen but why risk it when there are undoubtedly other spots where you are less likely to be seen?
The same holds true of ditching the bikes there after abducting the girls elsewhere.
The most likely scenario that I can envision is that they drowned. They rode their bikes out to the lake, it was hot, and they wanted to cool off. Perhaps they chose the secluded spot because they knew there was no swimming in the lake, and they didn't want to be seen? They laid the bikes down or perhaps leaned them against the fence and walked around the end of the fence to the water. The purse is set where it (and the phone) won't get wet. Maybe they initially only planned to splash some water on themselves or wade a little.
They go in a little deeper than they intended - maybe one gets the idea of walking out to the rocks. If the lake bottom is muddy flip-flops or even shoes could easily get stuck in the mud. I'm under the impression that this area is where the culverts (?) are that connect to the river? If so, that could create currents - they might not be strong now, but they could be during seasonal melt-off or storms - that could cause the lake bed to drop off abruptly and unexpectedly. One step the water is knee-deep, the next over a child's head.
Reportedly both girls could swim, but were they strong swimmers? Swimming in a controlled area while wearing bathing suits is different than suddenly ending up over your head while wearing clothes and shoes - and maybe getting a mouth full of water. One girl tries to help the other, and.....
If the lake isn't used for swimming, there's probably a reason for it.