Explanation:

An irrigation canal is a man-made channel that is used to transport water to crops and farmland for irrigation purposes. Sometimes, these canals need to pass over natural obstacles such as rivers or valleys. In such cases, a special structure called an aqueduct is constructed.

Aqueduct:

An aqueduct is a structure that is used to carry water over an obstacle such as a river or valley. It is typically made of concrete, stone, or brick and is designed to support the weight of the water as well as the weight of the canal. Aqueducts are commonly used in irrigation systems, but they can also be used to transport water for other purposes such as drinking water or hydroelectric power.

Types of Aqueducts:

There are several types of aqueducts, including:

1. Aqueduct Bridges - These are bridges that are specifically designed to carry water. They are often built with arches or other structural elements that help to support the weight of the water and the canal.

2. Pipe Aqueducts - These are aqueducts that are made of pipes. They are often used to transport water over shorter distances or in areas where it is difficult to build a traditional aqueduct.

3. Tunnels - These are underground aqueducts that are built through mountains or other obstacles. They are often used to transport water over long distances and are typically more expensive to build than other types of aqueducts.

Conclusion:

In conclusion, when an irrigation canal needs to pass over a river, the structure that is constructed is called an aqueduct. Aqueducts are essential components of irrigation systems and are used to transport water over obstacles such as rivers, valleys, and mountains.