Lisbon, the capital of Portugal, is observable north of the Tagus River estuary in this photo taken from the Space Shuttle. The estuary of the Tagus River, the longest river in the Iberian Peninsula, is 19 km long and is considered one of the best harbors in Europe. Visible south of the Tagus River estuary are the cites of Almada, Barreiro, and Montijo; farther south are Setúbal Bay and the Sado River estuary.
An estuary is the typically funnel-shaped part of a river near its mouth where fresh- and seawater mix and which is affected by the tides. At ebb tide both tide and river current assist in the erosion of the estuary. Estuaries may also form by local subsidence of the coast. As well as the mouths of rivers, estuaries may include bays, salt marshes, and lagoons. These brackish water ecosystems shelter and feed marine life, birds, and wildlife, but are particularly vulnerable to damage as a result of human actions. Many estuaries provide important harbors.