Editing List of rivers of Europe (section)

==Caveats==
The measurements shown are drawn from sources deemed most reliable, but still are often uncertain, especially when other sources disagree wildly. For example, the [[Siret (river)|Siret]] in Romania and Ukraine is 726&nbsp;km long with a basin of 44,000&nbsp;km<sup>2</sup> according to the [http://bse.sci-lib.com/article102578.html Great Soviet Encyclopedia], 647&nbsp;km (44,811&nbsp;km<sup>2</sup>) according to a Romanian management plan for the Siret basin <ref>[http://www.rowater.ro/dasiret/planul%20de%20management%20al%20bazinului%20hidrografic%20sire/planul%20de%20management%20al%20spatiului%20hidrografic%20siret/plan%20management%20sh%20siret%20-%20vol.%20i.pdf Planul de management al spațiului hidrografic Siret], Administrația Națională Apele Române</ref> and 559&nbsp;km (47,610&nbsp;km<sup>2</sup>) according to the extensive transboundary rivers study by the [[Economic Commission for Europe]].<ref name=ece>United Nations. Economic Commission for Europe, [https://books.google.com/books?id=L73CJCH-ZuIC Our Waters: Joining Hands Across Borders : First Assessment of Transboundary Rivers, Lakes and Groundwaters] {{dead link|date=September 2023 |bot=InternetArchiveBot |fix-attempted=yes}}, United Nations Publications, 2007] and {{cite web|url=https://www.unece.org/fileadmin/DAM/env/water/publications/assessment/English/ECE_Second_Assessment_En.pdf|title=ECE Second Assessment|publisher=Economic Commission for Europe}} (note: large pdf file)</ref>

Length estimates for rivers depend on a number of variables:
*River Source - The choice of the source obviously has an impact. Here we attempt to list the most distant source. When that involves a tributary nearer the mouth of the river, the length of the nominal river is listed as well, if it meets the above criteria.
*River End - Rivers flowing into estuaries have an arbitrary lower end. The channel of such a river through an estuary is usually included in the length when it is exposed at low tide.
*Path Granularity - The estimate will be larger when the curves of the river [[Coastline paradox|are traced at a smaller scale]].
*Intermediate Bodies of Water - The length of a watercourse through a reservoir or lake is open to interpretation. For this table, when a source for the total length of a river system involving lakes is lacking, the shortest possible course through the lakes is used
*Path Changes - Over time, a river's length can change through canalization, the creation of reservoirs, and natural changes in the water course.

The catchment areas are more consistent between sources. However, in low relief the watershed is less obvious, while underground connections (especially in [[karst]] systems) further complicate area measurements. Unless excellent sources are available, the areas below 70°N latitude are taken from the HydroBASINS project.<ref>[http://www.hydrosheds.org/page/hydrobasins HydroBASINS] at the HydroSHEDs website</ref> The [http://www.riversnetwork.org River Networks] and [https://rsis.ramsar.org Ramsar Sites Information Service] websites provide convenient interfaces to assess the accuracy of many of the basins. Areas for rivers above 70°N are warned{{clarify|date=August 2019}} and found to be less reliable.

The listed multiyear mean discharges are even less reliable than the lengths. Underestimates are most common, as the gauging stations are often far above the mouth, so that only a fraction of the drainage basin is represented. On the other hand, the highest volume of a river may not be at the mouth due to water loss by human usage, diversion (e.g. through [[distributary|distributaries]]), evaporation, or underground drainage.