First, get yourself a lil' pan and fashion a "river" out of Sculpey Clay or something similar. (It cannot be Playdoh because Playdoh will disintegrate.)
Surround your river with sand, which is plentiful in the desert.
Sprinkle grass seed or chive seeds in the black dirt.
Carefully fill your river with water, making sure it spills over a bit and floods the black dirt area. (You know, just like the Nile floods every year.)
In five to seven days, you will look at your fabulous Nile River Diorama, and lo and behold, it has sprouted.
What a great project!
If you'd like to see more pictures of another fabulous blogger (and super awesome friend) doing this, head over to Once Upon a Family. You will see she follows directions much better than me, as I skipped a step that might have made things a little less wet and messy for our Nile River diorama. :)