The African lake was super-saturated with carbon dioxide from volcanic gases in its lower depths. Nearly 700 feet deep.
But a shallow flowing river wouldn’t super-saturate.
Maybe murder-suicide, we will all go to sleep in this beautiful place? A natural poisoning would not likely be peaceful or quick; they likely would have used a phone to call for help.
Many areas in and around the Sierras don’t have cell access. Too remote. Not saying this is the case, only it’s possible they had no access.
Close to the mark concerning drinking water, probably from a canteen.
One or the other adult had some sick reason to take out the family.
This is where I would put my money. Murder/suicide with some poison. Or perhaps Covid-19.