The 'rivers' are not clued separately but can be identified in the overall clue.
For example-
12a So free not to vote for police department.
P AIRD D....MP's are paired so don't have to vote. AIRE (river) in PD (police department)
16a Xerophyte is king...TAMARISK ( a xerophyte)
TAMAR (river) IS K(ing)
Good luck