Question

rivers in veld are not navigable give reason

Answer 1

Most of the river is navigable but it cannot be reached from the sea due to the Livingstone Falls, a series of rapids on the river near its mouth.

Answer 2

According to Microsoft Encarta, veld is open grassland:

an area of open grassland which especially lies in the southern region of Africa. Almost all rivers are navigable nowadays.

But it cannot be reached through or by the seas owing to the presence of Livingstone Falls which is a series of rapids on the river located just near its mouth. You can try to navigate and write an exploratory answer.