Children fiction village is evacuated prior to flooding for reservoir

From memory it's the story of two children who return to either resolve some problem, or solve a mystery.

More details needed, like when did you read the book?
Cover, any vague memory?
Hardcover or paperback?
Was it new at the time?
To eliminate/check:
Flood, dam/reservoir -- children's/young adult fiction
https://www.librarything.com/list/9410/all/Flood-dam%25252Freservoir-childrens%2...

Suggestion from the list.
Berlie Doherty: Deep secret.

Hi bookel, thanks for your reply.

I read the book in the very late '70s very early '80s. It was a paperback book that wasn't new. I found it in my brother's bookcase, so I'm not sure how long he'd had it. But I don't think it was very recent to when I read it.

I've looked at the link you posted, and I don't think it's Deep Secret by Berlie Doherty. I seem to remember the children being friends - a boy and girl (but could be wrong). From the list They Drowned a Valley by Margaret Paice seems to sort of ring some bells, but there's not much description of the story line for me to be sure.

I believe Margaret Paice is an Australian author. Paice: They drowned a valley was published 1969.
Summary from NLA: "Slowly the upcountry valley of the Upper Wambidgee is dying as the dam for the new reservoir nears completion and the flooding of the valley's farm-land draws nearer. Many families have already moved out and within the general sense of dissolution each member of the community reacts differently to the crisis which the new reservoir represents for them."

Cover https://caerwenbooks.com/products/33434
Can also see it on abebooks.com

Looks like Loeff wrote one too.
https://badgerbooks.com.au/book/40516-theyre-drowning-our-village
Loeff: They're drowning our village