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After part of the Euphrates was dried up due to re-routing its water to the sea , a dam was built so water could not back up from the Tigris and sustain the former marshland .
 Euphrates, Tigris
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
 Euphrates, Tigris