The ditches of Sung-Shu and Erh-Lung were partially filled.
Long ditches with stone-paved sluices for washing this mineral-bearing material have long been used by the Indians, who also construct stone bars across the beds of the streams to make riffles and hold the deposited grains of gold.
In the ditches and pools common yellow and white water-lilies are seen, as well as water-soldier (Stratiotes aloides), great and lesser reed-mace, sweet flag and bur-reed.
The level at which it is desired to keep the water in these ditches constitutes the unit of water measurement for the polder, and is called the polder's zomer peil (Z.P.) or summer water-level.
The nature of the breeding-place varies greatly according to the species, and while many of the mosquitoes that infest houses will breed even in the smallest accidental accumulation of water such as may have collected in a discarded bottle or tin, the larvae of other species less closely associated with man are found in natural pools or ditches, at the margins of slow-moving streams, in collections of water in hollow trees and bamboo-stumps, or even in the water-receptacles of certain plants.