Why could the Zambezi expedition not use the Zambezi River as a route for ships?

The Zambezi expedition led by David Livingstone found it impossible to use the Zambezi River as a reliable route for ships due to the river's numerous natural physical barriers.12

Specifically, the river is obstructed by several features that prevent consistent navigation:
* Sandbars: The mouth of the river is characterized by a wide, marshy delta that is heavily obstructed by sandbars.23
* Shallowness: Large sections of the river are too shallow for standard vessels.2
* Rapids and Cataracts: The river’s course is frequently interrupted by dangerous rapids and waterfalls, such as the Victoria Falls, which create impassable conditions for ships.23

While the river contains stretches that can be navigated by shallow-draft steamers, these segments are separated by these natural obstacles, rendering the river ineffective as a continuous trade route for the purposes of the expedition.2