Is an embankment always man-made?

An embankment is man-made, or at least artificial or deliberately created. The em- prefix indicates that the banking is being done.

OED has

em- prefix
1. a. < em- + n., ‘to put (something) into or upon what is denoted by the n.’; also ‘to put what is denoted by the n. into’ (something).

For embankment, OED has

1. The action or process of embanking.
2. A mound, bank, or other structure for confining a river, etc. within fixed limits.
3. A long earthen bank or mound, esp. one raised for the purpose of carrying a road or a railway across a valley.

Note that (2) equates mound and bank with structure, and specifies fixed limits; and (3) specifies raised for a purpose. All of these indicate it's deliberately made.

A natural riverbank is just a riverbank. An embankment is an artificial riverbank.