If you mean the triangular area of land at the mouth of a river:
* Source: The starting point of a river.
* Upland: Elevated land away from a river.
* Plateau: A flat elevated landform.
* Crest: The highest point of a hill or mountain.
If you mean the fourth letter of the Greek alphabet:
* Alpha: The first letter of the Greek alphabet.
* Omega: The last letter of the Greek alphabet.
If you mean a change in a variable:
* Constant: A value that remains unchanged.
* Static: Not changing or moving.
* Equilibrium: A state of balance.
If you mean a specific form of change:
* Regression: Returning to a previous state.
* Decline: A gradual decrease.
To give you the best antonym, please provide the context in which you are using the word "delta."