The Oude Rijn ("Old Rhine") is a branch of the Rhine delta running through the Dutch provinces of Utrecht and South Holland. It originates near Harmelen, west of Utrecht, and flows approximately 52 kilometres westward before discharging into the North Sea at Katwijk via a mechanical pumping station. The river passes through or near several significant towns, including Woerden, Alphen aan den Rijn, and Leiden, making it a well-connected waterway through a densely populated and historically rich region of the Netherlands.

The river sits at roughly -0.4 metres elevation, reflecting the characteristic flatness of the Dutch lowland landscape. The terrain along the entire length is essentially level, with no significant gradient changes for paddlers to contend with. The river has three tributaries and three side channels along its course.

Historically, the Oude Rijn was once the main lower channel of the Rhine, wide and tidal, forming part of the northern frontier of the Roman Empire — a boundary known as the Roman Limes. In medieval times it silted up as the Lek became the Rhine's primary outlet, and the river subsequently served as a drainage channel, a route for the clay industry, and a transport corridor. A towpath, the Oude Rijn Jaagpad, runs alongside much of the river. Originally constructed in the 17th century to allow horses and people to tow trekschuiten (tow barges) upstream between Leiden and Utrecht, this path now functions as a cycling and walking route. Its presence along the bank is a notable feature of the paddling environment, offering a sense of the river's long commercial history.

The river flows through a mix of urban and rural settings as it connects the cities and towns of the western Netherlands. The surrounding landscape is typical of the Dutch polder region — low-lying, open, and largely agricultural between the urban centres. The mechanical pumping station at Katwijk controls the river's outflow to the sea, which is a relevant consideration for paddlers approaching the western end of the route.