Thinking about Deltas:
The Nile-style "Delta" Delta and the Mississippi-style "Bird's Foot" Delta

by Roger Bourke White Jr., copyright Decemeber 2005

(Note: this is one of my first forays into scientific thinking. I thought this up as a preteen in the late 1950's or early 1960's)

As a youngster, when I played on the beach and built sand castles, I loved to work with running water. One of the things I noticed was the "delta building" of sand being carried into a puddle by a stream of water from a garden hose. I remembered reading in school about deltas and that they came in two varieties: The delta-form, like the Nile delta variety, and the bird's foot-form, like the Mississippi delta variety.

As the water ran into the puddle, it was building a bird's foot delta. It would deposit sand along the edges of the main channel, and the main channel would extend until one of the banks got unstable and broke. The break would then become the main channel, and the extending process would begin again.

Once I had seen and understood that bird's foot dynamic, I tried to make my little river make a Nile-style delta. I swished out all the bird's feet, and let the water flow over a nice flat base. But in spite of the nice flat start, the delta rapidly became a bird's foot delta again. That puzzled me, and I thought about it long after I had finished playing on the beach that day.

Sometime later, I figured out what was missing: For a Nile-style delta to grow, the water must flow out in a big surge -- essentially ignoring all the banks. Then, in between surges, the flow must be so low and sediment free that the delta building effectively ceases. So the flood stage of the Nile River must be large and sudden, while the flood stage of the Mississippi must be slow and gradual.

-- The End --