It's a really strange case, in that there doesn't seem to be many things that could have happen, but all of them seem very odd. It's most likely that she fell in the river and drowned sadly. It seems odd that she did, but these things do happen unfortunately.
There was a potential route out of the area, unseen by CCTV but in broad daylight you'd be surprised she just left her phone and dog and went with someone under duress. She could have gone with someone she knew, but again, you'd imagine she'd take her phone and dog.
Maybe she was having some mental health problems and wanted to escape, so left her phone and dog and disappeared of her own accord. I find that very strange though. Wouldn't you leave your dog at home in that case? Wouldn't you take the kids to school and then just go? You wouldn't be thinking logically but I doubt you'd join a work call and then decide to do it.