Why capitals were built near rivers and marshland

Why were important capitals such as Washington and Berlin established on swampy or marshy ground?

Their rivers made communication and transport easier in the era before modern roads and railways.

Capitals such as Washington and Berlin were built on difficult, marshy ground partly because river access made them easier to reach. Before railways and modern roads, navigable water was often the most effective transport network available. A river could connect an otherwise inconvenient site to the territory it governed. Difficult ground could therefore accompany a decisive advantage in movement and administration.