A park is ... than a forest. (small)
smaller
The Pacific Ocean is ... in the world. (deep)
the deepest
A city is ... than a town. (big)
bigger
A mountain is ... than a hill. (high)
higher
Your example: An ocean / a river
...
Rio de la Plata is ... river in the world. (wide)
the widest
This river is ... now than it was in the past. (dirty)
dirtier
This village is ... of all. (small)
the smallest
Your example: A lake / of all
...
... waterfall on Earth is in Venesuella. (big)
The biggest
Our city was ... one hundred years ago. (clean)
cleaner
Mount Everest is ... on our planet. (high)
the highest