Westminster Abbey: Originally built on Thorney Island (or the "Eyot of Thorns") created by 2 branches of the River Tyburn. The river has since been significantly re-routed
See undercitylondonstories.blogspot.com

Danube regulation in the 19th Century

The original effluence of the River Bievre into the Seine was where the Rue de Bievre is situated today - situated on the south bank at the eastern end of Ile de la Cite. It has since been moved further east. See
See parisisinvisible.blogspot.com

the Western walls of the Kremlin were defined by the River Neglinnaya "The Kremlin was built on a hill west of the Neglinnaya, using the river as a moat" (Wiki) and "At one time the River Neglinnaya, a tributary of the Moskva, flowed along here protecting the Kremlin walls. There were two bridges over the Neglinnaya, a drawbridge by the Pine-Grove Tower and a stone arched bridge linking the Trinity and Kutafia towers".

"Another distinct feature of Guanajuato is the network of underground caverns and built to divert the Guanajuato River underneath the city"