Complex sentence: The stretch from the TSYS building to the Bibb Mill was constructed in the third phase, which was followed by the construction of the fourth phase which extended the RiverWalk from the Bibb Mill to the Lake Oliver Dam.
Split into simpler sentences: The stretch from the TSYS building to the Bibb Mill was constructed in the third phase. The fourth phase extended the RiverWalk from the Bibb Mill to the Lake Oliver Dam.