How do you call the lake that was created by flooding a site where sand or gravel was extracted? (And are there different terms for the "sand" lake or "gravel" lake?) Thank you.
If you want to highlight the fact that the lake is not natural but produced through human effort (if that's what "created by flooding a site" means), then it could be called an artificial lake or a reservoir.
From Manitoba's "Artificial Lake Safety Guidelines":
Artificial Lake means an outdoor, artificially constructed basin that
(a) has a sand, clay or mud bottom;
(b) is not lined with concrete, fibreglass, vinyl or similar material;
(c) is intended for swimming, wading, diving or one or more other water recreational activities;
(d) has a water depth greater than 60 cm; and
(e) has a surface area greater than 1,500 m2.
And from Merriam-Webster's definition of reservoir:
1 : a place where something is kept in store: such as
a : an artificial lake
Note that a reservoir need not be an artificial lake that is used for recreational activity. It could be used for the storage of water for other reasons.
Unless you want a technical term for lakes by their origin as mentioned here (Thanks Ronald for the comment), it is possible to form a term for those that are not in the list. In those cases,
any word + lake is possible! Generally, it's understood unless you put a regional word. But if you have it in English, it's well taken!
For instance, you can have a 'crocodile lake' where you find crocs. That said, 'sand lake' may give listener a hint that the lake has something to do with sand. You may later clarify though.
Example? Well, we have Nakki Lake in India. Nakh in a local language is a fingernail. Yes, that lake was dug out from nails! Aww..