one of the longest streams of the Iberian Peninsula, flowing generally westward through south-central Spain and southeastern Portugal to the Gulf of Cádiz in the Atlantic Ocean. The river has a drainage area of 23,455 square miles (60,748 square km), a length of 483 miles (778 km), and about 30 major tributaries. Its flow is relatively meagre—only about half that of the Tagus or the Douro—because its basin drains the low-rainfall areas of the Toledo Mountains, the Sierra Morena (Morena, Sierra), and the plains of La Mancha.

The Záncara (the true source of the upper Guadiana) and Gigüela rivers rise in the wetter mountains of Cuenca province of Spain. To the west of Daimiel these rivers form marshy lakes, known as Ojos del Guadiana (“Eyes of the Guadiana”), a noted wildfowl sanctuary. By contrast, the porous limestones found in other parts of the river's basin form a shallow water table, producing intermittent streams such as the Guadiana Alto, Azuer, and Cárcoles that disappear underground, though eventually feeding the Guadiana.

The Guadiana's waters are used for what is perhaps the most successful irrigation project in Spain. As it flows westward the Guadiana cuts a series of defiles through the ridges of the Toledo Mountains, and these defiles are now the sites of several dams that provide hydroelectric power and irrigation waters for Badajoz province. The four major dams have each created reservoirs more than 20 miles (32 km) long in the eastern part of the province. The Badajoz Plan, as this project was called, was launched in 1952 to develop one of the poorest and most arid regions of Spain. Upon the project's completion in the late 1960s, several hundred thousand acres of land had been put under irrigation, most of the towns and villages of Badajoz were provided with electricity, and dozens of new industrial enterprises had been established in the growing urban centres of the province.

Between Badajoz city, Spain, and Monsaraz, Port., and again downstream from Pomarão, Port., the Guadiana forms parts of the Spanish-Portuguese frontier. The rocky defiles and the sandbars at its mouth make the river suited to the navigation of small boats for only 42 miles (68 km) upstream to Mértola, Port. Larger vessels ply only between Pomarão and the coastal ports of Ayamonte, Spain, and Vila Real de Santo António, Port.