To call this a river would be a bloomer (6)
I believe the answer is:
flower
This might be a double definition.
'to call this a river' is the first definition.
'flower' can be an answer for 'a river' (a river flows). I am not sure about the remainder of the definition.
'a bloomer' is the second definition.
(I know that bloomer can be written as flower)
'would be' is the link.
Can you help me to learn more?
(Other definitions for flower that I've seen before include "River" , "the best one can pick" , "Blossom" , "Garden plant" , "Bloom" .)