Düsseldorf is named after the Düssel river, which used to be a mighty torrent flowing into the Rhein. Somewhat improbably, there is an English-language Wikipedia entry for it:
The Düssel is a small right tributary of the River Rhine in North Rhine Westphalia, Germany. Its source is between Wülfrath and Velbert. It flows westward through the Neander Valley where the fossils of the first Neanderthal man were found in 1856. At Düsseldorf it forms ariver delta by splitting into four streams (Nördliche Düssel, Südliche Düssel, Kittelbach, Brückerbach), which all join the Rhine after a few kilometers.
Düsseldorf takes its name from the Düssel: Düsseldorf means "the village of Düssel". The name Düssel itself probably dates back to the Germanic thusila and means "roar" (Old High Germandoson).
Nowadays the Düssel is much-reduced, and is routed underground in many places. Nevertheless, it's allowed to surface pretty often, and when it does, the city planners have done the most with it, using it to create ponds, lakes, mirror pools, and babbling brooks. Here's a GoPro timelapse of the southern tributary which runs through my neighborhood, yesterday, at sunset: