Initially, humans relied on one of the primary forces of naturegravity-to move water from one place to another. Gravity only works, of course, if the water is moved downhill on a sloping grade. It was soon discovered that the pressure built up by accumulating water behind the water source (e.g., behind a barricade, levy, or dam) moved the water farther. But, when pressure is dissipated by various losses (e.g., friction loss) or when water in low-lying areas must be moved to higher areas, the energy required to move that water must be created. Simply, some type of pump is needed.