The Sado River rises at an altitude of 230 meters, in the Serra da Vigia, in Ourique, and runs through the Alentejo and Setúbal Peninsula, covering a distance of about 180 kilometers until it flows into the Atlantic Ocean near Setúbal. Along its course, it passes through Panóias, Alvalade, and Alcácer do Sal, with its mouth in front of Setúbal. From downstream of Alcácer do Sal to the mouth, a wide estuary develops, separated from the ocean by the Troia Peninsula. It is one of the few Portuguese rivers that flows from south to north.

A large part of the municipality of Santiago do Cacém belongs to the Sado River basin. It crosses its eastern part in the parishes of Alvalade and Ermidas, passing one kilometer east of the village of Alvalade, heading from south to north.

On the left bank, 2 km downstream of the mentioned village, it receives an important tributary, the Ribeira de Campilhas, which rises in the Serra do Cercal do Alentejo, northeast of Vila Nova de Milfontes. On the right bank, 5 km downstream of Alvalade, it receives the Ribeira do Roxo, which rises near Messejana and passes north of Aljustrel, with a lower flow than that of Campilhas. It also receives another important tributary, the Ribeira de São Domingos.

Along the banks of the Sado River and its tributaries, ash trees, willows, and a large number of shrubs grow spontaneously, and since ancient times, there has been talk of the ornamental oleanders on the banks of the Roxo.

The Alvalade area, according to some researchers, may have been chosen by groups of humans, during various periods of history, for gathering favorable conditions to settle here, such as the proximity of the river (which had a much higher flow at the time), sandy and fertile soils, fresh water, and, at the time, a lot of forest. The history of the territory can be observed and proven at the Alvalade Archaeological Museum through numerous archaeological finds, through paleontology, by observing some fossils at the mouth of the Ribeira de Campilhas, where it delivers its waters to the Sado River.

The Medieval Bridge of Alvalade is located on the old bed of the Ribeira de Campilhas, in the parish of Alvalade, municipality of Santiago do Cacém. Its origin is probably Roman, receiving major works in the 16th century.

Tributaries
On the left bank, the Ribeira de Campilhas and the Ribeira de Corona.
On the right bank, the Ribeira do Roxo.

FLORA
Willows (Salix sp.), poplars (Populus), oleander (Nerium oleander), bulrush (Thypha domingensis); bramble (Rubus ulmifolius); rush (Juncus effusus), nut sedge (Cyperus esculentus); daphne (Daphne gnidium); various shrubs.

FAUNA
Fish – carp (Cyprinus Carpio), barbel (Barbus barbus), Iberian chub (Chondrostoma lusitanicum), bordalo (Squalius alburnoides) appears in spring, pumpkinseed (Lepomis gibbosus), pike (Esox Lucius), catfish or wels catfish (Silurus glanis), largemouth bass (Micropterus salmoides), and bleak or shiner (Alburnus alburnos), these three species may have been introduced). Spanish toothcarp (Anaecypris hispânica); European eel (Anguilla anguilla), and mullet (Mugilidae) – (they go up the river to near Moinho da Gamita, east of Vale da Eira, in the parish of Ermidas-Sado). The effects of the tide are felt up to Santa Margarida do Sado. The migration of mullets is certainly not unrelated to this fact).

Fishing
Fishing was done with nets (gillnets and traps), with rods, lines, and hooks (this type of fishing is still practiced today).

Bivalves
Cockle (Cerastoderma edule) and oyster (Ostrea edulis).

Birds
Kingfisher (Alcedinidae), reed warbler (Acrocephalus scirpaceus), common buzzard (Buteo búteo), sparrowhawk (Accipiter nisus), kestrel (Falco tinnunculus), mallard (Anas platyrhynchos), blackbird (Turdus merula), European bee-eater (Merops apiaster), chiffchaff (Phylloscopus collybita), golden oriole (Oriolus oriolus), moorhen (Gallinula chloropus), white stork (Ciconia ciconia), black stork (Ciconia nigra), raven (Corvus), barn owl (Tyto alba), tawny owl (Strix aluco)

Other animals
Otter (Lutra lutra), fox (Vulpes vulpes), badger (Meles meles), Egyptian mongoose (Herpestes ichneumon), grass snake (Natrix maura and Natrix natrix), ocellated lizard (Lacerta lepida); fire salamander (Salamandra salamandra).

Other common animals that can be found are the common genet (genetta genetta), wild boar (Sus scrofa), Cetti's warbler (Cettia cetti), Eurasian blue tit (Cyanistes caeruleus), blackbird (Turdus merula).

The Louisiana crayfish (Procambarus clarkii) was introduced here in the 1980s. It is a very harmful species to fish and amphibian communities because it feeds on tadpoles and juvenile fish.

Mediterranean turtle (Mauremys leprosa) – is the most common turtle in freshwater systems south of the Tagus.

European hedgehog (Erinaceus europaeus) – Small mammal belonging to the insectivorous order and the Erinaceidae family. It is easily identifiable by the fact that it is the only mammal in the Portuguese fauna that has a body covered with modified hairs in the form of spines. The areas of the body that do not have spines have a yellow-brown color.

Distribution: Throughout the national territory and most of the European continent

Curiosities: The European hedgehog is a species that hibernates throughout the winter, for which it has internal adaptations. This species has an omnivorous diet, consisting mainly of insects such as beetles, spiders, and ants. It can also feed on small rodents and birds and some fruits and mushrooms.

White-toothed shrew (Crocidura russula) – One of the typical micromammal species of cork oak montado areas. Shrews differ from other micromammals due to their carnivorous diet (insects), and their teeth are adapted for this purpose (very sharp to pierce the shells of insects). As the name indicates, this species is distinguished from other shrews by its white-based dentition. It has a pointed nose and very small eyes due to its nocturnal habits, with smell being its main sense of orientation.

Curiosity: the offspring accompany the mothers in hunting by following them in a line (each one grabbing the tail of the next).

Egyptian mongoose (Herpestes ichneumon) - Probably introduced to the Iberian Peninsula by the Arabs, the Egyptian mongoose is the only diurnal mammal in our fauna. Of medium size, with dark brown fur and elongated body, it is easily identifiable by its long tail with hair only at the end. Egyptian mongooses form family groups, and it is common to see more than one individual.

European otter (Lutra lutra) - otters have evolved to thrive in aquatic systems. Therefore, they have interdigital membranes between their toes to aid in swimming and a hydrophobic coat. They form family groups and are usually seen swimming in groups.

Distribution: Throughout the national territory and most of the European continent

Curiosities: The otter has no known natural predators, constituting the top of the food chain.