The Hollands Diep (historically spelled Hollandsch Diep) is a large, open estuary in the southwestern Netherlands, formed by the confluence of the Bergse Maas and the Nieuwe Merwede near Lage Zwaluwe. It functions as a major estuary of the Rhine and Meuse rivers, connecting through the Scheldt–Rhine Canal to the Scheldt river and Antwerp. Near Numansdorp, it splits into the Haringvliet and the Volkerak. The waterway is designated as a Ramsar Wetland site, recognized in 2000, reflecting its ecological significance.

In terms of physical character, the Hollands Diep is a wide, open waterway featuring deep channels, hard structural edges, shell beds, and broad shallow plateaus. This varied bottom topography creates a dynamic aquatic environment. The water is relatively clear compared to some nearby systems. The scale and openness of the estuary mean paddlers should expect exposure to wind, tidal influence, and significant commercial and recreational vessel traffic. The Moerdijk bridges cross the eastern section, carrying road and rail traffic, while the Haringvliet bridge and Hellegatsdam cross near the western split point.

Paddlers should be aware that the Hollands Diep carries substantial boat traffic, including commercial shipping, and the open expanse makes it susceptible to wind-driven chop. The tidal range in the broader Zeeland delta region is approximately two meters, which will affect currents and water levels on the estuary. Recreational use of the water is well established — canoes, sloops, and stand-up paddleboards are all seen on the water during warmer months, and the area is popular for summer recreation. Willemstad is one of the access points for the waterway, with boat ramps and free parking available nearby.

The Hollands Diep was historically formed by the catastrophic St. Elizabeth's flood of 1421, which connected an earlier saltwater inlet to the Merwede, shaping the delta landscape that exists today. The surrounding delta region includes connections to the Biesbosch, Haringvliet, and Volkerak, offering paddlers the possibility of extended routes through the southwestern delta system. A fishing license (VISpas) is required for angling on the water, and paddlers should familiarize themselves with any local closed zones or regulations before launching.