SOLVED. A murder mystery in London with an old river underground maybe the River Fleet [s]

[Kate]

I read this about 10 years ago. It has a male detective, maybe Scotland Yard, investigating a muder. The solution involves an underground waterway

[SamSpayedPI]

Midnight Riot (AKA Rivers of London) perhaps? It's paranormal, though.

[Cheryl A]

The Water Room by Christopher Fowler meets a few of your criteria - underground waterway, male detective(s), more than 10 years old.

[Sue]

"Thrones, Dominations"? It's a Peter Wimsey book, written by Jill Paton Walsh. "After some plot twists, a second murder and a scene involving the hidden rivers and Victorian sewers that run under London"

[SamSpayedPI]

Thrones, Dominations for Sue's link.

[Kate]

Thank you, everyone. Thrones, Domination is it!