What natural boundaries helped protect the river valley civilizations?

There were mountains, barriers, and lots of water around to prevent invasion and disease

Mountains, rivers themselves, deserts, etc. A good example is the ancient Indians who were protected by the himalayas

Primary sources are usually  more accurate and are written by someone who experienced it or witnessed it. secondary sources are those who heard of it after the fact that can lose details and or twist the event in a light that benefits them.