The ancient Romans and Mesopotamians built aqueducts to transport water between a source and areas where it was needed for agriculture or civilization. The Roman system was very extensive, and was constructed throughout its empire. Some portions of these ancient aqueducts are still in use. Today, modern concrete-lined channels transport water hundreds of miles. The most extensive aqueduct systems in the world today are those that bring water to Southern California from the Colorado River in the east and from the Sacramento River in the north.