If the intention is to develop a tank by holding back a large amount of water, then the dam must be carefully designed (with the assistance of experienced local people or engineers).
The outflow from the dam must be thoughtfully positioned and designed in order to regulate the quantity of flow and the size and shape of the channel. Important considerations include allowing enough water to spill over after a heavy rain so that the dam will hold, yet ensuring that the water spilling over does not damage downstream lands by erosion.
If a dam is built in an area where fish migrate upstream, then the ability of fish to navigate the structure should also be considered.
The following three diagrams show how earthen dams can be constructed using different materials, depending on the nature of the soil and rock near the surface of the land.