Constance Villiers Stuart says it best.
Humayun's garden, apparently level, in reality slopes to the south, where the slight difference in the ground has been cunningly made use of to introduce tiny carved chutes down which the water ripples. At one or two places in the side walls there are longer water-chutes, where the water, which has been lifted up from great wells outside, rushes foaming down the carved stones into the garden. These marble or stone chutes were carved in various patterns, cut ingeniously at an angle so that the water running over them was thrown up and broken into ripples and splashes. Shell and wave designs were the favourites, and their name was as prettily fashioned as their carving they were called chadars, meaning white "shawls" of water. These water-chutes are a very characteristic feature of the Mughal gardens, and were used with much effect where the ground allowed of the garden being laid out in a series of high terraces. But in small gardens, or in the plains, even the slightest slope was made use of; only a foot or two of difference sufficed to create one of these charming little waterfalls, whose inspiration was directly drawn from memories of the dancing spray and white foam of mountain rivulets in the builder's northern home.1
1. C.M. Villiers Stuart, Gardens Of The Great Mughals (London: Adam and Charles Black, 1913), 99.
Villiers Stuart, C.M. Gardens of the Great Mughals. London: Adam and Charles Black, 1913.
Wescoat, J.L. Jr. "Waterworks and Landscape Design in the Mahtab Bagh." In The Moonlight Garden: New Discoveries at the Taj Mahal, edited by Elizabeth B. Moynihan, 59-78. Washington, D.C.: Arthur M. Sackler Gallery; Seattle: University of Washington Press, 2000.