Given the task definition, example input & output, solve the new input case.
Given a disfluent sentence, modify the sentence to it to its equivalent fluent form, preserving the meaning of the sentence.
Example: Who did the Han Chinese want to help the Khitan no I mean the Mongols fight?
Output: Who did the Han Chinese want to help the Mongols fight?
The input initially means Khitan and then changes to Mongols, hence this is correct.

New input case for you: Why does or where does the cold water mix with Lake Constance?
Output:
Where does the cold water mix with Lake Constance?