structural:scholar_edition
Inverted delta

The Tagus river is one of the few rivers in the world to have an inverted delta. Its delta is wider at the beginning and narrows down as it approaches the sea, contrary to a typical delta. This is because it flows into the sea through a small opening in a valley. Although due to sedimentation, this delta is now only very partially inverted, with the valley now mostly filled with sediment. The delta is about 15 km (9 mi) wide and 25 km (16 mi) long, but its exit into the sea is only 2 km (1.2 mi) wide. It thus forms a large lagoon with large and very shallow sand banks which go uncovered during low tides. The delta used to be even bigger thousands of years ago.