The Mondego estuary (located about 200 km northern of Lisbon) is the smallest estuary, assessed in this study. It has a narrow inlet and two channels in the lower part separated by a large sediment island. The southern channel is very shallow, especially in the upper part, where communication with the northern channel is negligible due to engineering works. The communication between the channels has been artificially reduced to a minimum in order to increase the discharge through the northern channel to minimize the maintenance costs of the navigation channel.

The Pranto River is a small tributary of the southern channel, which importance for the ecological conditions of that channel has increased after the artificial closing of the upper communication between the channels. The average discharge of Pranto River is much, and episodic due to artificial control of the water in the basin but can generate episodic reductions of salinity in the upper part of the channel.