Nonsense and obscurity about river amphibian (8)

I believe the answer is:

bullfrog

I'm a little stuck... Click here to teach me more about this clue!

'river amphibian' is the definition.
'bullfrog' can be an answer for 'amphibian' (bullfrog is a kind of amphibian). I am unsure of the 'river' bit.

'nonsense and obscurity' is the wordplay.
'nonsense' becomes 'bull' (I've seen this before).
'and' means one lot of letters go next to another.
'obscurity' becomes 'frog' (I can't justify this - if you can you should give a lot more credence to this answer).
'bull'+'frog'='BULLFROG'

'about' is the link.

Can you help me to learn more?

(Other definitions for bullfrog that I've seen before include "Large leaping amphibian" , "Creature with a deep croak" , "pond life" , "Deep-booming croaker" , "Jumper" .)

I've seen this clue in The Independent.

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