Does anyone know what main body of water feeds the st. petersburg canals in russia?

Answer 1

St. Petersburg was built on swamps and marshes. The systems of canals was built to serve to purposes: 1) drain the swamps and 2) protect the city from spring floods.

Although water in most canals comes from the river Neva, in some cases it comes from smaller rivers or from the swamps. Sometimes the source changes over time (Moyka river and related canals used to be independent from Neva, but now it's connected to Fontanka, which starts from Neva)

Answer 2

It might be River Neva which comes from Lake Ladoga.