No. There are actually several rivers called the Avon. (It's the Welsh word for river.)
In all there are 50 tributaries of the Thames, 10 Backwaters and 9 linked Waterways
there over 20 tributaires
Over 20 tributaries. One of the major tributaries is the Chinwell river
Historians are unsure of its original name but the Romans called it Tamesis - and that was nearly 2000 years ago.
It is named Thames after its Roman name - Tamesis.