I've often thought about this.

I've wondered; if we built a huge trench from the ocean into the Sahara desert, what would the environmental impact be??

People have been making plans about how to flood parts of the Sahara for about 100 years. Some of the more colorful options involved using buried nuclear warheads as trench-digging tools.

https://en.wikipedia.org/wiki/Qattara_Depression_Project

https://en.wikipedia.org/wiki/Sahara_Sea

The Amazon rainforest would die. [1]

How much of it actually would die is obviously depends on factors like how big the replacement lake is and how much it affects the phosphorus, but I'm constantly amazed by how seemly separate systems are actually intimately interconnected.

[1] http://www.nasa.gov/content/goddard/nasa-satellite-reveals-h...

I can imagine it would quickly be filled in by sand. The Sahara is a slow moving ocean of sand particles.

The sand only moves if it is light. When the weather and climate changes, the make up of the sand changes and so does the way it moves. It is a complicated process and it would definitely have some surprises.

Actually, most of the Sahara is rocky, not sandy.

All the sandworms would die.