Complex sentence: Starting in the early colonial period, the interconnected lakes of the valley, including Lake Xochimilco were drained and by the 20th century had shrunk down to a system of canals which still connected Xochimilco with the center of Mexico City.
Split into simpler sentences: Starting in the early colonial period, the interconnected lakes of the valley, including Lake Xochimilco, were drained. By the 20th century, the lakes had shrunk to a system of canals that still connected Xochimilco with the center of Mexico City.