What happens as a river gets older apex

The gradient smooth ens...the waterfalls and the rapids gradually disappears

It has low gradient and low erosive energy

Explanation:

A river can be defined as the natural flow of water course, which is usually a source of water. This water flows towards the another river, ocean or sea.

A older river usually has very low gradient and low erosive energy. They also have flood plains.

Example of older rivers are : Yellow, Nile, lower Ganges, et cetera.

igneous rocks which formed by the crystallization of magma at the surface of the earth called extrusive rocks. they are characterized by fine-grained textures because the rapid cooling at or near the surface did not provide enough time for large crystals to grow.