The Oude Rijn is a branch of the Rhine delta in the Dutch provinces of Utrecht and South Holland, starting west of Utrecht, at Harmelen, and running by a mechanical pumping station into the North Sea at Katwijk. Its present-day length is 52 kilometres.

South Holland is a province of the Netherlands with a population of over 3.8 million as of January 2023 and a population density of about 1,410/km2 (3,700/sq mi), making it the country's most populous province and one of the world's most densely populated areas. Situated on the North Sea in the west of the Netherlands, South Holland covers an area of 3,308 km2 (1,277 sq mi), of which 609 km2 (235 sq mi) is water. It borders North Holland to the north, Utrecht and Gelderland to the east, and North Brabant and Zeeland to the south. The provincial capital is the Dutch seat of government The Hague, while its largest city is Rotterdam. The Rhine-Meuse-Scheldt delta drains through South Holland into the North Sea. Europe's busiest seaport, the Port of Rotterdam, is located in South Holland.