To Tread

1. (intransitive) To step or walk (on or over something); to trample.
2. (transitive) To step or walk upon.
3. To beat or press with the feet.
4. To go through or accomplish by walking, dancing, etc.
5. To crush under the foot; to trample in contempt or hatred; to subdue.
6. (intransitive) To copulate; said of (especially male) birds.
7. (transitive, of a male bird) To copulate with.

Treaded is not commonly used in the UK and is less common in the US as well. It is apparently used more often in tread water.

Tread is sometimes used as a past and past participle, especially in the US.