when dutch farmers wanted to create more usable land, they would build a huge dam or dyke in the shallow water. the new land that was once under water is called polders. because the polders are usually below sea level, the dutch have had to build a system of canals and ditches to carry the water.the banks of the canals are built high so that water will flow to the sea,and ditches are dug through the low fields , or polders ,to collect the water. then the water is pumped from the ditches into the canals.