Inca Hydraulic Engineering and Terraces

Inca hydraulic engineering and terraces constitute the agronomic and water-management infrastructure of the imperial state that ruled the Andes from the early fifteenth century until the Spanish conquest of 1532. The category includes the design and construction of irrigation canals, aqueducts, fountains, drainage channels, and agricultural terraces, all of which were organized at a scale and with a sophistication that supported a population of perhaps ten to twelve million people across the empire. Inca water management treated water as a sacred substance and integrated the distribution of water with the cosmological order, with the agricultural terraces (known in Quechua as andenes) tied to the astronomical calendar and to the ritual cycle of planting and harvest. The terraces and canals of the Inca are still in use in many parts of the Andes, and they have been the subject of intensive modern study both for their engineering qualities and for their contribution to the resilience of Andean agriculture and food systems.

The imperial hydraulic program was directed by the state, but it was executed by the local population through the mit’a system, a rotating form of corvée labor by which communities provided workers to the state in exchange for the protection of the Inca and the right to participate in the imperial cult. The hydraulic infrastructure was conceived as a public good, and the construction of a canal or terrace complex was often a collaborative project involving multiple ayllus, the basic kin-based Andean social units. The state stored the harvest in qollqas, distributed seed in years of poor harvest, and managed the redistribution of water and food through a complex bureaucratic system that included the camayoc, the specialist engineers, and the quipucamayos, the keepers of the knotted-cord records.

The principles of Inca water management

Inca water management was based on three principles: the protection of water sources as huacas (sacred places), the engineering of canals and aqueducts to deliver water to agricultural and ceremonial sites, and the integration of water distribution with the agricultural calendar. Water sources, including springs, rivers, lakes, and glaciers, were identified, protected, and venerated, and offerings were made to them at the beginning and end of the agricultural season. The canals that brought water from these sources to fields and cities were designed to minimize erosion, to use the natural gradient, and to deliver water at a consistent rate to multiple users.

The Cusco water system is among the best-studied examples of Inca water management. The city was supplied by a network of canals that brought water from surrounding springs, including the Pumacurco aqueduct, which carried water from a spring on the hillside of Pumacurco to the Coricancha, and the Antarumi canal, which delivered water to the markets and to a fountain in the Huanaypata plaza. The canals were typically cut into bedrock or lined with polygonal stones, and they used the natural topography to maintain a constant gradient. They are still in use today in some areas, although they have been modified and extended by Spanish and modern interventions.

The ceremonial use of water was as important as its practical use. The Coricancha included a series of fountains and channels that brought water from the Pumacurco aqueduct, and the water was used in ritual ablutions and offerings. The Temple of the Sun at Ollantaytambo included a system of channels and fountains that directed water through the ceremonial precinct, and the water was used to feed the agricultural terraces below. At Machu Picchu, a series of 16 fountains and channels distributed water from a spring on the northern slope of Machu Picchu mountain to the urban sector, and the system is still functioning today.

Agricultural terraces (andenes)

The Inca built more than 1 million hectares of agricultural terraces across the empire, particularly in the highlands of Peru, Bolivia, and Ecuador. Terraces transformed steep, often arid slopes into arable land by creating flat platforms backed by retaining walls and filled with layers of stones, gravel, and topsoil. The Inca developed several distinct types of terrace: simple field terraces for level ground, sloped or inclined terraces on gentle slopes, and steeply walled or pisqa terraces on near-vertical slopes. Each was used in a different environmental context.

The internal structure of Inca terraces was based on a series of layers, each with a specific function. The retaining wall, which was typically built of polygonal stone, was followed by a layer of large stones (the piedra brava), which provided drainage, a layer of q’illpa stones, which retained moisture, a layer of sandy soil, and finally a layer of topsoil, which was enriched with the deliberate incorporation of gravel and organic matter. The piedra brava layer, which is the largest of the three subsoil layers, can be more than 2 meters thick in large terraces. The q’illpa layer, often composed of volcanic or glacial deposits, retains moisture and prevents the topsoil from washing down to the drainage layer. Modern archaeological and soil studies have shown that this layered structure could support intensive cultivation for many decades.

Terraces in the Sacred Valley of the Urubamba are particularly well preserved. The Pisac terraces, built on a steep mountainside above the town of Pisac, are among the most extensive in the region and include a sequence of more than 30 levels. The terraces are still in use and produce maize, potatoes, quinoa, and a range of other crops. The Chinchero terraces, on the plateau above the town of Chinchero, are also in use and are part of an integrated system that includes irrigation canals, the so-called labyrinth terrace, and a series of ritual structures. The terraced system of Moray, a few kilometers from the town of Maras, is a unique group of four circular depressions, each of which contains concentric circular terraces. The depressions are natural, but the Inca built the circular terraces to a depth of more than 30 meters in the largest case, and the temperature difference between the top and bottom of the largest depression can be as much as 4–5°C.

Sacred Valley irrigation

The Sacred Valley irrigation system is one of the most extensive and best-documented of the imperial hydraulic works. The system includes a network of canals, aqueducts, and distribution channels that brought water from the glaciers and high-altitude lakes of the Cordillera Vilcanota to the agricultural terraces and urban centers of the upper Urubamba valley. The principal canals include the Pampamarca canal, the Chinchero canal, the Yanacocha canal, and the Taray canal, each of which was cut into bedrock or lined with polygonal stones and used the natural gradient of the valley to maintain a constant flow of water.

The canals were designed to deliver water to multiple users, and the system included a complex arrangement of intake structures, drop structures, and distribution gates. The intake structures, often located at the high-altitude sources, included stone-lined channels that directed water into the main canal, while the drop structures, which were used to manage the gradient, included small dams and chutes that controlled the velocity of the water. The distribution gates, often made of stone, allowed water to be diverted to individual fields or terraces.

The Sacred Valley irrigation system is still in use in many areas, although it has been modified and extended by Spanish and modern interventions. The canals of the Pampamarca and Chinchero systems, for example, have been repaired and extended in the twentieth and twenty-first centuries, and they continue to deliver water to the agricultural terraces of the Sacred Valley. The system is recognized as a significant achievement of imperial engineering and as a continuing element of the Andean agricultural landscape.

Drainage systems and flood control

The drainage systems of imperial sites were designed to handle heavy rainfall, to prevent flooding, and to direct water away from buildings and terraces. The drainage system of Machu Picchu is particularly well studied: it includes 129 separate drainage channels, a series of stone-lined drains, and a system of agricultural terraces that help stabilize the slopes against erosion. The drainage channels are typically cut into the stone walls and the bedrock, and they use the natural gradient of the site to direct water to the agricultural terraces and to the river below. The system has performed well over the centuries, and it has been the subject of a long series of studies by the Peruvian Ministry of Culture and by international projects.

The drainage systems of imperial sites were integrated with the agricultural terraces, the canals, and the urban fabric in a coordinated way. At Tipón, a site in the Lucre basin southeast of Cusco, the drainage system includes a sequence of channels and fountains that bring water from an underground spring to a series of pools and outlets, and the system is designed to control erosion, distribute water, and produce a visually striking display of the camayuc, the life-giving water. The hydraulic infrastructure of Tipón was studied in detail by the American engineer Kenneth Wright and his team, who demonstrated that it includes an upstream drainage system, a main canal, drop structures, and a sequence of waterfalls, fountains, and pools.

Tipón and the imperial hydraulic laboratory

Tipón, in the Lucre basin southeast of Cusco, is the best-preserved example of Inca hydraulic engineering. The site includes a series of agricultural terraces, ceremonial buildings, and a complex of channels and fountains that bring water from an underground spring to a sequence of pools and outlets. The hydraulic system of Tipón was studied in detail by the American engineer Kenneth Wright and his team, who published a comprehensive account in 1997. The system includes an upstream drainage system, a main canal, drop structures, and a sequence of waterfalls, fountains, and pools, and it is designed to control erosion, distribute water, and produce a visually striking display of the camayuc, the life-giving water.

The system at Tipón is more than a hydraulic work. It is also a ceremonial center, and the channels and pools are arranged in a way that creates a visual and acoustic effect: the water cascades from pool to pool, and the sound of falling water is amplified by the stone walls. The site is interpreted as a ceremonial complex as well as a functional irrigation system, and the combination of hydraulic engineering and ritual function is characteristic of the imperial Inca approach to water. The system was used for irrigation, for ceremonial ablutions, and for the display of imperial power.