What is "ijssel"

The River IJssel , sometimes called Gelderse IJssel (" Gueldern IJssel") to avoid confusion with the Hollandse IJssel, is the branch of the Rhine in the Dutch provinces of Gelderland and Overijssel. The Romans knew the river as Isala. The IJssel flows from Westervoort, east of the city of Arnhem, until it discharges into the IJsselmeer ("Lake IJssel", known prior to the 1932 completion of the Afsluitdijk as the Zuiderzee, a North Sea inlet). The River IJssel is one of the three major distributary branches (along with the rivers Nederrijn and Waal) into which the Rhine divides shortly after crossing the German-Dutch border.

The name includes the digraph ij, which behaves like a single letter in Dutch orthography - this explains why both letters appear capitalized (compare: IJmuiden and IJsselmeer).