1. To return, we had to go across a stream, but the water rose very quickly.

2. To return, we had to go across a stream, but the water became high very quickly.

(Are both the same?)

3. To return, we had to go across a stream. Suddenly the water reduced very quickly.
(Can we use 'reduced' as the opposite of 'rose'?)

Yes, both one and two are the same.

Yes, you can used reduced as the opposite of rose.