This is a dangerous assumption. If the dog is well-trained then it may do as it is told with regard to the water or it may not have known the owner had fallen in. It could easily be as simple as the dog sees a squirrel and chases off behind a tree, the woman (distracted by her team meeting) suddenly realises the dog isn't around and quickly checks over the river bank to ensure the dog hasn't gone over the edge, loses her footing and by the time the dog returns it has no idea where she is. Or it could be 100s of different scenarios.