I have used the translation "Sungai ini sumber makananmu.", and it was rejected. I have made a report to be added.

Translations suggested by Duolingo:

Sungai ini adalah sumber makanan kalian.

Sungai ini sumber makanan kamu.

The translation "Sungai ini sumber makanan kalian." is already accepted.

I have not tried other variations, with "Anda" for "your", "ialah", "merupakan" for "is".