Mira river basin is located in the Southwest coast of Portugal. The river rises in Caldeirão Mountain – about 470 m high - and flows Northward along about 145 Km long, to reach the estuary off Vila Nova de Milfontes, 30 Km Southward of Sines.

Mira estuary, represented in Figure 1, is a long (32 km) and narrow estuary (maximum width of 150 m). The average depth of the estuary is 6 m and its maximum depth, 16 m, is reached near the mouth. The estuary is oriented along the Northwest-Southeast direction, conferring it a favourable exposure to predominant northwest coastal winds, which generate surface waves and enhance vertical mixing. Discharge of the river into the estuary is controlled by Santa Clara reservoir, located ahead of Odemira, the main population aggregate in the upper estuary.

Figure 1 - River Mira  Estuary.

The lower estuary - zone 1 in Figure 1 - has clear marine properties and the upper estuary - zone 3 in the figure – is mostly fresh water. Major gradients are located in zone 2 where mixing of marine and fresh water occurs, mostly due to tidal oscillation. Small salt marshes can be found on the banks located along marine and brackish zones, which stimulate longitudinal shear diffusion in the estuary. The upper zone is quite narrow and shallow and has a small contribution for the tidal prism of the estuary, which is demonstrated by the results of the hydrodynamic model.