Lakes and tanks in Bangalore refer to reservoirs of varying sizes constructed over a number of centuries for rainwater harvesting. The three main gentle natural valley systems have allowed for the creation of interconnected lakes and wetlands where water flows downstream through a series of channels or drains. This entire network of waterbodies in the modern administrative regions of Greater Bangalore, and the two districts of Bangalore Urban and Rural (formerly the unified Bangalore district), have seen accelerated change caused by urbanisation in the past four decades.