[eplanit]

They seem too well-educated to not be aware of toxic algae (very common throughout northern CA), and would know to not drink the water -- and certainly not give it to the child. Dogs do often tragically die every summer from drinking from streams, though. Similarly, it's odd for a responsible couple with a child and dog to not know to bring water, etc.

Heat or alage aren't impossible, but it sure does seem odd.