Question

Why were river systems important in supporting early civilizations and in helping them flourish?

Answer 1

Rivers supply water to drink and irrigate crops, they are also used for washing, fishing and sewerage. In addition, rivers form obstacles to movement making fords, bridges and other crossing points important social focus points causing townships to grow up. Rivers also form a transport system, connecting towns and eventually to the sea and hence to other countries.
Finally, rivers form a natural part of a defence system, preventing or obstrubcting attack from that direction.

Answer 2

That was the main thing they needed for farming and cultivation. Also it helped grow their population by keeping a steady source.