Between 1900 and 1911, Philadelphia built a system of five sand filtration plants on high ground along the Delaware and Schuylkill rivers, near the city's upstream boundaries (to access the cleanest water within city limits). High ground was preferred because gravity flow was more cost-effective than pumping, but even so, to get water to the highest parts of the city, a new reservoir in Oak Lane and five high-service pumping stations also had to be constructed. A new pumping station was also constructed at Lardner's Point (at that time the largest such pumping station in the world) to take water from the new Torresdale filters (then the largest single filter plant in the world ) and distribute it to reservoirs throughout the city.