The Ucayali River is generally considered to be the main headwater tributary of the Amazon River. The Ucayali is approximately 2,700 km long and its headwaters are approximately 6,400–6,800 km from the mouth of the Amazon River. Occupying approximately 353,000 km2, the Ucayali Basin is relatively smaller in area than basins of other long tributaries of the Amazon River.
All of the Ucayali is in Peru and its drainage basin is divided among 11 departments or regions as they are now called. More than 90 percent of the basin’s area is in the large departments of Loreto, Ucayali and Cusco. The Ucayali River generally flows southward between the high Andes to the west and low hills to the east. Rainforest similar to that of the lowlands occupies most of the Ucayali Basin up to approximately 1,000 m, where montane forest begins to dominate. Montane forest and cloud forest are common on the eastern slopes of the Basin to as far south as the confluence of the Urubamba and Vilcanota Rivers, approximately 300 km south of Cusco.
The Ucayali meanders over a broad valley bottom that generally lies below 300 m. The river’s two main headwater tributaries are the Urubamba and the Tambo-Ene-Apurimac, whose headwaters arise above 5,000 m. Three spectacular mountain ranges—the Cordillera Azul, the Cordillera Vilcabamba and the Cordillera Vilcanota—lie within the Ucayali Basin. The Cordillera Azul is in the lower Basin, west of Pucallpa. The Cordillera Vilcabamba and the Cordillera Vilcanota lie within the headwaters region of the Ucayali near Cusco.
Climatic conditions in the Ucayali Basin vary from arid highlands to wet lowlands. An average of approximately 550 mm of rain falls annually in the Vilcanota Basin near Cusco (3,000 m). In the lowlands, the average annual amount of rainfall is approximately 2,200 mm at Pucallpa (300 m). Some montane areas exposed to moisture-laden eastern or southern winds can receive more than 5,000 mm of precipitation annually.
There are few data available for average annual river-level fluctuation in the Ucayali Basin, but it appears to be approximately 8 m in the lowlands. The Ucayali River’s main flooding period is from December to May and the highest water levels near Pucallpa usually occur in March. Upland tributaries rise and fall rapidly and annual river-level fluctuations vary greatly due to narrow valleys.
The Ucayali River and its floodplain form a highly unstable network of levees, side channels, sandbars, mud bars, back swamps, lakes and islands. The oxbow lakes are usually less than 0.5 km wide, but some are more than 20 km long. Large beaches appear during the low-water period and often become wider than the main river channel. Although the Ucayali River is unstable, its floodplains in general last long enough for forest to become the dominant vegetation in areas not subject to deep permanent flooding. In areas of relatively permanent but shallow flooding, palm swamps dominated by aguaje (Mauritia flexuosa) are common. Extensive mats of floating herbaceous vegetation are found in floodplain water bodies reached by nutrient-rich Ucayali floodwaters.
There are few functionally large protected areas in the Ucayali Basin. With the exception of Pacaya-Samiria National Reserve, which is mostly in the Marañón Basin, most of the protected areas in the Ucayali Basin are relatively small and confined largely to the cordilleras.
The Cordillera Azul National Park protects one of the lower headwater areas of the Ucayali Basin. It is a mountainous area with sheer rock escarpments, lowland valleys, high-altitude lakes and marshlands. The Cordillera Azul lies west of Pucallpa and separates the Ucayali and Huallaga basins. Another important protected area in the Ucayali basin is the Otishi National Park, which is sparsely populated and designed to protect part of the area of the Tambo-Ene-Apurímac River, one of the two main tributaries of the Ucayali River.
Uses & Impacts
Human activities in the past 1,000 years have heavily shaped the landscape of the Ucayali Basin’s upper watershed. The upper watershed lies within the Peruvian departments of Cusco and Apurimac, where are also located the most visited great Inca ruins. The most famous headwater region of the Ucayali is the Sacred Valley, where such spectacular Inca ruins as Ollantaytambo and Machu Picchu are located or or found nearby at approximately 2,500-3,000 m. The Incas farmed the Sacred Basin intensively and grazed animals on the floodplains and slopes where they also built canals for irrigation and flood and erosion control.
At present deforestation of hillsides without terracing is recognized as a problem and there are many government reforestation projects. Pine or eucalyptus are typically planted, and later also used for timber and firewood. Local pollution of streams near urban centers has been acknowledged as a serious problem, such as with the Huatanay River, along which the city of Cusco is located and which flows into the Vilcanota at the beginning of the Sacred Valley.
There has been considerable recent highway and road expansion in the Ucayali Basin, including connections with the Interoceanic Highway from Brazil. The Cusco-Quillabamba railway, which trains began to use in the 1950s, has been replaced by a dirt road that now extends into the Urubamba Basin to about the Pongo de Mainique Gorge. Agricultural settlers from the high Andes have now transformed the lower Vilcanota and upper Urubamba areas into a major agricultural frontier. These valleys have become important agricultural areas for tea, citrus, coca, coffee and bananas, among other crops. Within the next few decades, a large part of these areas between approximately 800–1,500 m will likely be extensively deforested.
The Lima-Pucallpa highway is the primary road to the middle Ucayali Basin and although the highway has existed since the 1940s, Pucallpa did not start to grow rapidly until the mid-1980s. Both upland and floodplain deforestation is extensive near Pucallpa. Uplands have been deforested mostly for cattle ranching, whereas floodplain forests have been cleared to grow a wide variety of crops. The recent expansion of rice farms has also increased floodplain deforestation. Upland and floodplain forests are also being extensively logged for commercial timber.
Mining is widespread in the Ucayali Basin, especially in the higher regions, such as in the Mantaro Basin, where is found Peru’s most famous and also infamous mining area because of dangerous pollution levels from heavy metals and other elements. The Mantaro Basin also contains the Mantaro Hydroelectric Complex, which is Peru’s main source of hydroelectric energy. Numerous dams have been proposed or considered on various tributaries of the Ucayali River but challenges from indigenous groups and environmentalists and lack of financing have these on hold at the moment.