Erosive changes in two rivers and an artificial lake (9)
I believe the answer is:
reservoir
'an artificial lake' is the definition.
(synonyms)
'erosive changes in two rivers' is the wordplay.
'two rivers' can mean two replacements for 'river'.
'changes' is an anagram indicator.
'in' is an insertion indicator.
'river' becomes 'r' (abbreviation for river).
'river' becomes 'r' (abbreviation for river).
'erosive' is an anagram of 'eservoi'.
'r'+'r'='rr'
'eservoi' going into 'rr' is 'RESERVOIR'.
'and' is the link.
(Other definitions for reservoir that I've seen before include "Man-made lake of stored water" , "Lake created to store water" , "Artificial lake" , "Area flooded" , "Man-made lake or tank" .)