Ahistorical sources can be deemed unreliable if the author appears to unfairly favor a particular side or viewpoint. in this case the source is

s the president shall be commander in chief of the army and navy of the united states, and of the militia of the several states, when called into the actual service of the united states; he may require the opinion, in writing, of the principal officer in each of the executive departments. sooo yeahhh

explanation:

these civilizations were located around fertile rivers, where they could produce an abundance of crops