Sadly I think it is highly likely she has had a tragic accident and fallen in the water.
But just humouring the idea that isn’t the case, what else could have happened?
She could have left the area by herself. If it were unplanned (eg she had spontaneous amnesia or a mental episode) - how did she leave the area without being seen and then not been seen since. Why did the dog stay and not follow her. Completely implausible.
She could have staged her own disappearance. There’s seems no evidence at all she would want to disappear. And if you were going to stage your own disappearance, why would you choose to do it like that? How would she get her dog to stay while she walked off? Why do it in an area where you’re highly likely to bump into someone you know who will question why you’ve just left your dog and phone by the river? Why wasn’t she seen by anyone leaving the area? This is also completely and utterly implausible. It’s just not even thinkable she has left on her own.
So the only conceivable alternative to her having fallen in the river is that she left the area because someone else took her away against her will. Which means someone would have needed to enter the area without being seen by anyone, despite there being numerous dog walkers in the area. Either incapacitated or gained control of her without any loud noise or sign of struggle. And somehow got her away from the area without any witnesses, within a few minutes, in broad daylight at a busy time of day. It’s incredibly unlikely.
Whereas people do sadly drown by accident after walking near rivers fairly frequently. And it is common for bodies not to be found for a long time afterwards. Drowning can be quick and silent, particularly in cold water.
It’s incredibly tragic and my heart breaks for her poor family but the chances of it being anything else other than accidental drowning are tiny.