The most obvious and dominant geographic feature of ancient Egyptian civilization (as well as today in Egypt) to me would be the Nile River. It's the world's longest river, stretching 2700 miles from headwaters in Sudan to the delta in Egypt on the Mediterranean Sea.
In ancient Egypt, it was mythical and spiritual, gave them life in the form of irrigation for crops and drinking water, and references to it in hieroglyphics number in the hundreds. Animals were drawn to it as a water source, which made for plentiful and easy hunting on its banks. Egyptian cities grew up along the river's banks and allowed its population to grow rapidly, making it the dominant civilization of its time.
There was even an Egyptian god, Hapy, that was dedicated to the Nile and the annual flooding that brought them fertile soil.