(a) The boundaries of the district are coextensive with the boundaries of the city as those boundaries existed on January 1, 1973, and as the district territory may have been modified under:
(1) this subchapter or its predecessor statute, Section 7, Chapter 619, Acts of the 63rd Legislature, Regular Session, 1973;
(2) Subchapter J, Chapter 49, Water Code; or
(3) other law.
(b) An invalidity in the fixing of the boundaries of the city as they existed on January 1, 1973, does not affect the boundaries of the district.