Blood River (2007)

"The Rio Tinto river in Huelva Province, Spain runs red like blood due to iron oxides from an ancient 5000-year-old mine upstream (that is still mined today). Nearby the anarcho-socialist village of El Membrillos Bajo was razed to the ground by Franco's forces in 1937. All inhabitants who did not escape were murdered. Similar retribution took place across Spain during and after the civil war. (Indeed Nazi leader Heinrich Himmler when visiting Spain in 1940 was shocked at the level of repression by The Nationalists*.)

* newyorker.com/magazine/2005/10/31/the-spanish-prisoner