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All answers having "European river" as clue:

4 Character Answers:

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AARE

ARNO

EDER

EGER

ELBE

ISAR

ISER

ODER

SAAR

URAL

YSER



5 Character Answers:

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LOIRE

MEUSE

RHINE

RHONE



6 Character Answers:

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DANUBE



7 Character Answers:

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CODICIL

OVERDUE