A reach up the river? (7)
I believe the answer is:
stretch
This might be a double definition.
'a reach' is the first definition.
(I know that reach can be written as stretch)
'up the river?' is the second definition.
'stretch' can be an answer for 'up' (thesaurus). I'm not sure about the 'the river?' bit.
Can you help me to learn more?
(Other definitions for stretch that I've seen before include "extended" , "Expand" , "how much time one's been given" , "Strain" , "Make (something) longer" .)