City of three rivers - Dreiflüssestadt Passau

In hydrological point of view the German section of the Danube offers many interesting places. First of all: where does the Danube come from? There are two branches, the Breg and Brigach, and a wonderful well at Donaueschingen. Then there is the “Donauversickerung” phenomenon. This is the place where the Danube simply decides, that this time it flows right into the North Sea, rather than the Black Sea. And at Passau, the city of three rivers we find the estuary of the Inn river. You have to know, Inn has a discharge 7% more than the Danube. Moreover at first sight Inn is wider and somehow its color is different. Despite all, the Danube can keep its name and the Inn dissolves in the smaller river.

 

Three rivers and three colors of Passau. In the south, the almost white (bit greenish) Inn, in the middle, the brownish, flooding Danube, and in the left corner, the dark-colored small Ilz. What makes these rivers so colorful? All three colors are results of the different amounts of suspended material and sediment type, besides the current water level also contributes to this phenomenon as well. The river Ilz comes from the swampy countryside, its water is full of organic sediments, and momentary has a stagnant water level. The flooding Danube has a brownish color as a result of the high amount of organic materials and sediments from the banks. But why is the Inn so bright? Also from the floating sediments, taken from the Alps, which contain only a little organic material. While the Danube flows though a hilly landscape, built up by mainly limestone, the Inn collects its water from glaciers. The melting glaciers contain so much glacier rock sediment that is no wonder why it received the name, glacier-milk. This floating material cannot consolidate, even there are 24 (!) hydro electrical power plants on the river.

The Danube River basin is twice as large as the Inn catchment area at Passau, but due to the “Donauversickerung-leakage” on permeable limestone its discharge is somewhat smaller than the Inn. Therefore it is also possible, that the 62,5 mile (100 km) shorter inn delivers more water into the Danube.

 

The Danube, Inn and Ilz at Passau

If we have to decide whether a river can keep its name or not by its discharge, we would be in a quite difficult situation at Passau. In the roman-age, when the river name Danubius became commonly used all over the empire there was no way to measure rivers discharge or calculate catchment areas in the Alps. It is also problematic to make conclusions of the width of a river, because a wide stream can also be shallow as a narrow river can be very deep. In this situation, the Danube bed is three times deeper than the Inn, therefore the broad, white glacier milk of the Inn can cover the narrower, brownish “Blue” Danube easily. 

Presumably it was not the geographical factor, which decided who wins the name-debate between the Danube and Inn, but rather political rationality. The Danube was separating the Barbarians and Romans for nearly four hundred years, along the 1762 mile frontier (not a limes, but the ripa). Long enough to forget the greek synonym “Istros” used for the lower section.