Davey has broken his expedition down into three stages and as of now he should be into Stage 1. He'll begin his adventure by first trekking to the summit of Mount Mismi, a 5597 meter (18,363 ft) peak in the Peruvian Andes. That mountain has been identified as the most distant source of the Amazon River and what starts as a tiny stream there will eventually feed into the largest river on the planet.
After completing his trek, du Plessis will then climb on his bike and begin the second stage of the journey, riding through the Andes as he follows the Apurimac River, which eventually merges with several others before flowing into the Amazon itself. The cycling portion of the expedition will cover approximately 1500km (932 miles) through remote and difficult terrain.
In the third and final stage, Davey will climb into a kayak and begin paddling the river itself. From the seat of his boat he'll cover an additional 5700km (3541 miles) as he follows the massive river through Peru and Brazil to the Atlantic Ocean itself.
As of the last update to his blog, du Plessis was leaving Lima for Mount Mismi and his Twitter feed seems to indicate that he is still en route. The first stage of the journey is underway and this should be an impressive expedition to follow. Be sure to check Davey's website for regular updates, particularly once he gets paddling.
Thanks to Adventure Lisa for the tip on this great story.