When I first started reading about this case, I was one who thought suicide. Then when she was found and finding out the location, I changed my mind.
I think when people choose a river, they choose it because of the swift current, they're deeper and they're more difficult to get out of. Ever been in a pool under water for too long? You start to panic and do everything in your power to get to the top for air. If you're in a river, you can't necessarily do that easily. In a drainage ditch that's only a couple feet deep, it would be easy to come up for air when your body and brain are telling you to breathe.
I also don't believe she took pills and then walked all the way there to drown herself. Sadly, I have had the experience of dealing with my child trying to overdose twice. The first was Motrin and the second was a combination of her antidepressant and anxiety med. Both times she started vomiting within a half hour of taking them and that continued for hours anytime she moved, even after receiving fluids through IV. Plus, if she'd taken a prescription drug, her family would be able to tell if a large amount were missing. I suppose there is always the chance she bought some illegally, but I just don't see CD as one who would break the law.