Ucayali-Urubamba Piedmont

Geography & hydrology

The Ucayali is usually counted as the main headwater stem of the Amazon: about 2,435 mi long, draining roughly 353,0 mi² entirely within Peru, with its source waters some 6,400–6,497 mi from the river's mouth. The Piedmont ecoregion is the slice of that basin caught between the Andean front and the lowland floodplain — FEOW puts it at 820–2461 ft elevation and folds in the lower and upper Ucayali plus the Aguaytía, Pachitea, Tamaya, Urubamba, Perené, Ene, Tambo and Apurímac. Three cordilleras frame it: the Cordillera Azul west of Pucallpa, and the Cordillera Vilcabamba and Vilcanota in the headwaters near Cusco. The two great forks that build the Ucayali — the Urubamba and the Tambo-Ene-Apurímac — both rise above 5,0 ft, so the water reaching the piedmont has already fallen several kilometers (Aguas Amazónicas/WCS; FEOW Ecoregion 317).

The defining piece of geography is the Pongo de Mainique, the gorge where the upper Urubamba slices through the Cordillera Vilcabamba before spilling onto the Amazon plain. It is short and violent — roughly 148 ft wide and 1–2 mi long, walled by cliffs that climb over a kilometer — and it splits the river into an upper Urubamba of Andean torrents and a lower Urubamba of broad, navigable whitewater. Above and around it sits the Megantoni National Sanctuary (215,868 ha). Downstream the river loses its grade quickly: rainforest takes over below about 1,0 ft, and by the time the Ucayali is meandering past Pucallpa the valley floor lies under 984 ft, a shifting network of levees, side channels, sandbars and oxbow lakes (Aguas Amazónicas/WCS; Megantoni/SERNANP).

Underneath this drainage lies the Fitzcarrald Arch, a low upland thrown up by Late Miocene–Pliocene uplift around 4 million years ago that separates the headwaters of the Juruá, Ucayali, Purús and Madre de Dios. The arch matters biologically because it isolated those headwater basins from one another, leaving a piedmont fauna that several surveys read as relatively ancient — older than the uplift that fragmented its rivers (Albert et al., 2011; Carvalho et al., 2011). Climate splits sharply with altitude: near Cusco at 3,0 ft only about 21.5 in of rain falls a year, while Pucallpa in the lowlands gets roughly 2,8 in and exposed montane slopes can take more than 5,0.5 in. The flood pulse runs December to May and peaks in March, with lowland river levels swinging about 26 ft; upland tributaries in their narrow valleys rise and fall fast and unpredictably (Aguas Amazónicas/WCS).

Water & habitat

FEOW classifies the Ucayali-Urubamba Piedmont as montane freshwaters, and its signature water is Andean whitewater — turbid, sediment-charged rivers swollen by Andean rains and snowmelt that, in FEOW's words, "carry large sediment loads to the extensive várzea floodplains." This is the muddy, nutrient-rich water Harald Sioli called whitewater (água branca): pale café-au-lait in flood, near-neutral in pH, mineral-rich because it drains young, erodible Andean rock rather than old lowland shield. Those suspended sediments are the engine of the downstream floodplain's fertility, and the same Andean whitewater rivers feed the productive fisheries of the central Amazon (FEOW Ecoregion 317; International Rivers, 2011).

But the piedmont is not one water type. It is a coarse mosaic, and that is the point. The Fitzcarrald-region surveys around Sepahua and Breu collected in three broad habitat classes: the channels and flooded beaches of large rivers more than 131 ft wide; small clearwater rivers and forest streams; and floodplain oxbow lakes. The large turbid rivers carry the Andean sediment load; the small terra-firme streams draining the uplands themselves run clear or tea-stained, soft and acidic; and the cut-off oxbows are warm, still and seasonally low in oxygen. Surveys measured the basics — water temperature, pH and conductivity with handheld meters — and found the chemistry tracking the substrate, each sub-catchment producing its own signature (Carvalho et al., 2011; Albert et al., 2011).

What sorts the fishes is less the chemistry than the structure and the flow. In the Sepahua survey the great majority of species were stenotopic — about 69% restricted to a single habitat type — and only around 5% turned up across all three. Flowing-water (lotic) habitats held far more species than the still lakes. Above the Pongo de Mainique the upper Urubamba is a high-gradient mountain river; below it the river slows, widens and warms, and the assemblage changes with it. Through the lowland reaches the seasonal flood does the rest of the organizing, connecting channels and floodplain lakes through the December–May high water and then stranding them as the river drops, so that the same oxbow can shift from river-connected to isolated and oxygen-poor within a single dry season (Carvalho et al., 2011; Aguas Amazónicas/WCS).

The cichlid fauna

The piedmont's cichlids are mostly small, and many are tightly local — which is what you would expect of a fauna carved up by the Fitzcarrald Arch and scattered across isolated headwater tributaries. Endemism overall is modest and under-sampled: the Sepahua-region work could point to only three confirmed Fitzcarrald endemics, and one of them is a cichlid, Bujurquina eurhinus. The roster's standout local form is Tahuantinsuyoa chipi, described by Sven Kullander in 1991 from the Pachitea drainage, a slim ~3 in cichlasomine known only from these piedmont waters and distinguished from its single congener by a simple dark spot behind the eye (Albert et al., 2011; Kullander, 1991).

The dwarf cichlids are the heart of the list. Several Apistogramma here are restricted to small, often blackwater tributaries of the Ucayali system: A. baenschi and A. eremnopyge from westerly Ucayali affluents, A. panduro, A. cruzi, A. eunotus, A. barlowi — a mouthbrooder — and A. nijsseni, the panda dwarf, tied to tea-stained creeks in the Río Carahuayte–Copal area. Wider-ranging species such as A. agassizii and A. bitaeniata reach into the lowland piedmont as well. Apistogrammoides pucallpaensis, the sole member of its genus and one of the smallest cichlids anywhere, takes its name from Pucallpa on the Ucayali, where it lives in slack, weedy margins. These are fishes of the quiet edges — leaf litter, undercut banks, slow blackwater — rather than the turbid main channels.

The larger and more current-tolerant cichlids occupy the rivers themselves. Pike cichlids are well represented: Crenicichla sedentaria, C. anthurus, C. cincta and C. lucius hunt along banks and over structure, with C. sedentaria among the most consistently collected species across the Sepahua habitats. The substrate-sifting eartheaters Geophagus proximus and the broadly distributed Cichlasoma amazonarum work sandy and muddy bottoms in the larger rivers; Astronotus ocellatus, the oscar, holds in floodplain lakes and slow backwaters; and the smaller Laetacara thayeri rounds out the slackwater guild. The genus Bujurquina — the most widespread and species-rich cichlid lineage of the western Amazon piedmont — threads through the whole region, the upland counterpart to the lowland acaras. Taken together the roster is a gradient in miniature: turbid-river generalists and pike cichlids in the main stems, a dense set of blackwater dwarfs along the edges, and a handful of genuinely piedmont-restricted forms like Tahuantinsuyoa and Bujurquina eurhinus that the Andean uplift left stranded in their own headwaters (Carvalho et al., 2011; Albert et al., 2011; FishBase).

Conservation

The piedmont's threats come from the same Andean topography that makes it distinctive: narrow, high-energy gorges are exactly what dam engineers want. In June 2010 Peru and Brazil signed an energy agreement opening the Peruvian Amazon to a series of large Brazilian-built dams, the power largely destined for export. The first five named — Inambari, Pakitzapango, Tambo 40, Tambo 60 and Mainique — sit squarely on or beside this ecoregion's rivers, with an estimated price around US$16 billion (≈ €15 billion) and financing expected from Brazil's BNDES (International Rivers, 2011). The Mainique dam would wall the Urubamba at the Pongo de Mainique itself — the gorge that defines the ecoregion — while the Pakitzapango dam, a 165-m wall across the Ene rated at 2,000 MW, would flood roughly 456 mi² and displace close to 10,000 Asháninka people whose food depends on the river's fish. Tambo 40, Tambo 60 and Sumabeni were slated for the same Ene-Tambo system (Amazon Watch/International Rivers, 2011).

For the fishes the damage is structural. Sediment-laden Andean whitewater is what builds the downstream floodplain; impounding it traps that sediment behind the dam, starves the várzea, and converts fast piedmont river into still reservoir — wiping out the lotic habitat that held most of the region's species in the Fitzcarrald surveys. Walls across gorges like Pakitzapango and Mainique also sever migration routes for the large catfishes (piraíba, jaú) that climb into the piedmont to spawn. Indigenous and environmental opposition, plus financing problems, have kept these projects stalled rather than dead; one central-Amazon megaproject on the middle Ucayali system collapsed only after its developer withdrew, citing the vast forest area it would have flooded (WWF; International Rivers, 2011; FEOW Ecoregion 317).

Hydrocarbons are the second front, concentrated on the lower Urubamba where the Camisea gas fields sit inside the ecoregion. Since production ramped up in the early 2000s, communities along the lower Urubamba have reported sharp declines in fish, blaming constant company boat traffic, repeated gas-duct ruptures and the turbidity and noise that came with development; by the mid-2000s some river villages had shifted from fresh fish to canned tuna and packaged staples (ICT/Indian Country Today). Layered on top is fast-moving deforestation: oil and logging roads radiating out from Pucallpa and a road now pushed up the Urubamba to near the Pongo have opened uplands to cattle ranching, logging and an agricultural frontier of coffee, coca, citrus and tea between roughly 800 and 1,1640 ft, with both upland and floodplain forest cleared along the way. Protection is thin and patchy — Cordillera Azul and Otishi National Parks and the Megantoni Sanctuary guard fragments of the headwaters, but most of the piedmont's small, range-restricted cichlids live outside any reserve, in precisely the tributaries that roads and dams reach first (Aguas Amazónicas/WCS; NASA Earth Observatory; FEOW Ecoregion 317).

Sources

1. Ucayali - Urubamba Piedmont — Freshwater Ecoregions of the World (FEOW Ecoregion 317; geography, hydrology, endemism, threats)
2. Ucayali — Amazon Waters / Aguas Amazónicas (Goulding, Wildlife Conservation Society; basin geography, hydrology, Pongo de Mainique, deforestation, dams)
3. Carvalho, Espino, Máxime, Quispe, Rengifo, Ortega & Albert (2011) — Fishes from the Lower Urubamba river near Sepahua, Amazon Basin, Peru. Check List 7(4): 413–442
4. Albert, Carvalho, Petry, Holder, Maxime, Espino, Corahua, Quispe, Rengifo, Ortega & Reis (2011) — Aquatic Biodiversity in the Amazon: Habitat Specialization and Geographic Isolation Promote Species Richness (Fitzcarrald region). Animals 1(2): 205–241
5. Kullander, S.O. (1991) — Tahuantinsuyoa chipi, a new species of cichlid fish from the Río Pachitea drainage in Peru. Cybium 15(1): 3–13
6. Brazil Eyes the Peruvian Amazon: Wild Rivers and Indigenous Peoples at Risk (International Rivers, 2011; Peru-Brazil 2010 energy agreement, Pakitzapango/Tambo/Mainique dams)
7. Fact Sheet: Pakitzapango Dam — Wild Rivers and Indigenous Peoples at Risk (Amazon Watch / International Rivers; Ene River dam, 165 m, 2,000 MW, 734 km² flooded, Asháninka)
8. Company withdraws from hydroelectric megaproject in Peru's central Amazon (WWF; middle Ucayali / Tambo basin dam)
9. The Camisea chronicles: River people try to cope with a life without fish (Indian Country Today; lower Urubamba gas project, fish declines)
10. Deforestation Follows a Road Through Ucayali (NASA Earth Observatory; oil/logging road-driven forest clearing in the Ucayali piedmont)
11. Apistogrammoides pucallpaensis — FishBase (distribution: Pucallpa, Río Ucayali, Peru)