Pond, pool.
A mini lake!
A "bay" is a small protected body of water that is a four-letter word.
pool, lagoon
Loch.
Another word for a small valley is a dell.
Tarn
Noah Brown
One possibility is a tarn.
A Gherkin is another name for a small pickled cucumber.
small
Coronet is another name for a small crown.Tiara is also another word for a small crown.
another word is plantation(depends on what you are looking for)