Boundaries

This ecoregion encompasses the drainage basins of the Río Uruguay above Yucumã Falls. This includes the Río Uruguay and all of its tributaries, including the two forming rivers, the Canoas and the Pelotas.

Topography

The headwaters of the Uruguay rise in the Serra do Mar and Serra Geral, which are coastal ranges in southeastern Brazil. The basin lies on the Serra Geral Formation, which represents the largest area of continental magmatism in the world (volcanic activity occurred between 120-130 mya). The bedrock includes sedimentary and basalt rocks with soils that are shallow with high clay content (Filho & Schulz 2003). Elevations in the ecoregion range from 150 m to over 1700 m asl (Hijmans et al. 2004).

Freshwater habitats

The main source of the Uruguay, the Pelotas, rises in the Serra Geral 64 km from the Atlantic Ocean. It then flows inland until it meets the Rio Canoas, where it forms the main stem of the Uruguay (Di Persia & Neiff 1986). The upper Uruguay flows over steep and rocky terrain marked by rapids and falls. One of the longest is Yucumã Falls, which divides the upper and middle Uruguay. Tributaries are generally short and also disrupted by waterfalls (Filho & Shulz 2003).

The Uruguay’s maximum average flow rate is 9,387 m³/s, although its historical peak flow has exceeded 23,000 m³/s. The physical and chemical characteristics in the upper Uruguay include a pH that varies between 6.7-7.4, conductivity between 26.1-73.1 μS/cm, dissolved oxygen between 9.3-10.1 mg/lm, temperature between 11.5-30 ºC, and alkalinity between 10.7-24.6 mgCaCO₃/l (Filho & Schulz 2003).  

High turbidity, low nutrients, and high flow contribute to low phytoplankton productivity along the Uruguay. Similarly, variable water levels, rapid flow, steep banks, and rocky substrates limit permanent aquatic vegetation along the main stem. Aquatic vegetation does occur in permanent and seasonal wetlands along tributaries, with species such as Scirpus californicus, Cabomba australis, and Panicum prionitis. In general, however, extensive wetlands are lacking (Di Persia & Neiff 1986).

Compared to the Paraná, the upper Uruguay lacks a well-defined dry season since rainfall occurs throughout the year. Flooding tends to be brief, with waters flowing quickly through deep valleys with no marginal lakes or floodplains to absorb rising waters (Filho & Schulz 2003). 

Terrestrial habitats

The subtropical Araucaria moist forests cover the higher elevations in this ecoregion in a complex of trees including emergent Brazilian Araucaria (Araucaria angustifolia), Brazilian sassafras (Ocotea pretiosa) O. catharinense, gabiroba (Campomanesia xanthocarpa), and angico (Parapiptadenia rigida).  These forests are a relict of a once widespread ecosystem of mixed coniferous and broad-leafed trees. Lower elevations are covered by Atlantic semi-deciduous forests comprised of trees dominated by the families Lauraceae, Apocynaceae, and Leguminosae. The southwestern part of the ecoregion extends into Uruguayan savanna, which encompasses palm savannas, gallery forests, and outcrops of submontane forests (WWF 2001).

Description of endemic fishes

There are presently 27 known endemic species and no endemic genera in the upper Uruguay. More than 40% of these are loricariids in the Hemiancistrus, Pareiorhaphis, Pogonopoma, and Rineloricaria genera. Endemic cichlids are represented by four Crenicichla species. Other endemics include three characids (Astyanax ojiara, Bryconamericus patriciae, Hypobrycon leptorhynchus), one auchenipterid (Tatia boemia), one pseudopimelodid (Microglanis eurystoma), and one rivulid (Austrolebias varzeae).

Other noteworthy fishes

The most abundant species by biomass in the Uruguay is the streaked prochilid or curimbatá (Prochilodus lineatus) (23%), followed by the voga (Schizodon nasutus) (4%). Other species of commercial importance are the piava (Leporinus obtusidens) and dorado (Salminus brasiliensis) (Filho & Schulz 2003). 

Ecological phenomena

Migratory species are generally restricted to the main stem of the Uruguay and lower tributaries due to the presence of waterfalls. With the absence of floodplain lakes in this ecoregion, species have adapted by using the mouths of the lower tributaries as nursery areas for larval and juvenile rearing. Notable migratory species along the main stem include the dorado (Salminus brasiliensis) and curimbatá (Prochilodus lineatus). Some that migrate laterally into tributaries for spawning include the pintado (Pimelodus maculatus) and South American catfish (Rhamdia quelen) (Filho & Schulz 2003).

Justification for delineation

The Río Uruguay falls within Ringuelet’s Alto Paraná and Parano-Platense icthyographic provinces (1975). It is separated from the lower Uruguay by its steep topography and hydrological characteristics.

Level of taxonomic exploration

Fair