The Giver tells Jonas that "memories are forever, and that if he were to fall into the river and drown accidentally, his memories would come back to the people.  Since the people would have been completely insulated from the emotions and feelings associated with the memories, they would be woefully unprepared to deal with them.  The sudden rush of memories would wreak havoc among the people in the community.

The Giver relates that this is what happened when Rosemary, who was to have been the Keeper of Memories before Jonas, was unable to handle the pain of the memories and asked for release.  Rosemary was the Giver's daughter, and her loss was devastating to the old sage.  The Giver was so overwhelmed with his own grief and anger at Rosemary's death that he didn't even try to help the people through the turbulent time when the memories returned to them unchecked.

The Giver reflects that he has already transferred to Jonas many more painful memories than he gave to Rosemary.  If Jonas should die and his memories should return unchecked to the people, the results could be doubly devastating (Chapter18).

What happens to the memories if Jonas falls into the river and drowns?

As Receiver-in-Training, Jonas is tasked with not just containing all the memories of the community but also all of the emotions. When the novel first opens, the reader is unaware that the community is built on only law and order and practicality; as the story progresses and more is revealed, primarily through interactions the Giver has with Jonas, it becomes clear that the community is sheltered from a true and meaningful life experience. They don't hold painful memories (as the Receiver/Giver hold those for them), but they also do not have memories of love. Families are structured based on their compatibility and everyone agrees to cooperate for the betterment of the community. When Jonas realizes how much the community could be, he really struggles with how they choose to interact with each other, devoid of all emotional connections.

The Receiver cannot die before his memories are transferred to the next Receiver-in-Training (which is a highly selective process) because all the memories that the Receiver holds would be released to the community. Not only would they then know of love, but they would also know of hate and sorrow. The community members have no understanding of these feelings, and it would lead to massive confusion as they all tried to simultaneously process these previously unknown emotions. Nevertheless, Jonas continues pondering the idea of somehow releasing the memories to his community because, as he tells Gabriel, "There could be love" (Chapter 16). This idea continues to gain momentum in his thoughts as the novel nears its end.

What happens to the memories if Jonas falls into the river and drowns?

In the unfortunate event that Jonas should fall into the river and drown, the community would be able to access the difficult memories Jonas received from the Giver during his training, and chaos would ensue. The Giver explains to Jonas that the community suffered greatly when the previous Receiver of Memory requested release, which is why Jonas is not allowed to request to be released. According to the Giver, when the previous Receiver of Memory requested release, her memories drifted into the atmosphere, where the community had access to them. The painful, difficult memories caused chaos and panic throughout the community because the citizens could not handle the traumatic memories. In Jonas's community, comfort and safety are paramount, and citizens are not exposed to the difficult memories and experiences that Jonas and the Giver understand. Since being released is simply a euphemism for dying, the community would suffer from Jonas's loose memories the same way they suffered when the previous Receiver of Memory was released.

What happens to the memories if Jonas drowns in The Giver?

Since "memories are forever", those that the Giver had transferred to Jonas would come back to the people if Jonas were to fall into the river and drown.  The results of such an event would be catastrophic because the people, who have been completely insultated from anything out of the ordinary, would be totally unequipped to deal with the memories, especially the unpleasant or traumatic ones, such as pain and war.  A period of chaos would result as it did when Rosemary was released, but it would be much worse in Jonas's hypothetical situation.  Rosemary only had five weeks worth of memories, while Jonas has much more (Chapter 18).