Boundaries

The lower Río Uruguay basin encompasses an area including the lower portion of the Río Uruguay and its tributaries in western Rio Grande do Sul State of Brazil, northeastern Argentina, and western Uruguay. The Yucumã Falls that runs along a fault line is considered the boundary between the upper and lower Uruguay.  The ecoregion is limited to the east by the drainage divide with the Patos system, and to the west by the drainage divide with the Paraná basin.

Topography

Elevations of the lower Uruguay ecoregion extend from sea level at the confluence with the Río Paraná at the Río de la Plata estuary to more than 500 m in the foothills of the Serra Geral (Hijmans et al. 2004). Most of the ecoregion lies below 130 m asl and is characterized by plains and low ranges like the Cuchilla de Haedo, which is an area of elongated undulating hills that cut between the ríos Uruguay and Negro. The basin rests on sedimentary and basaltic rocks of the Paraná Plateau. Soils are shallow with high clay content (Filho & Shulz 2003).

Freshwater habitats

The Plata basin is the second largest in South America, occupying 3.1 million km². It comprises seven ecoregions [332, 333, 342, 343, 344, 345, 346], including the Paraná, Paraguay, and Uruguay, which is the smallest at roughly 365,000 km². This ecoregion includes the middle and lower Uruguay from Yucumã Fallsdownstream to its confluence with the Río Paraná, where it forms the estuary of the Río de la Plata.

Below the mouth of the Río Quaraí is the site of the former Salto Grande Falls, which was flooded after the construction of the Salto Grande Dam. The dam and reservoir are now the divide between the middle and lower Uruguay (Filho & Schulz 2003). The middle Uruguay up to Salto Grande has some rapids, and descends with an average drop around .16%, with a flow regime determined by rainfall from the upper catchments (Filho & Schulz 2003). In contrast, the lower Uruguay has a higher sediment load, an even lower gradient, and flow regime that is subject to tidal influences from the Rio de la Plata as well as influences from the Salto Grande Dam (Di Persia & Neiff 1986). Floodplains exist along the middle and lower Uruguay, although they are smaller than the vast floodplains of the Paraná. They are highly productive, however, accumulating nutrients in the shallow seasonal lakes and ponds. Water levels vary from an average of 2 m along the lower Uruguay to up to 10 m in the middle Uruguay, but historically exceeded 10 m in the lower Uruguay before the dam construction (Filho & Schulz 2003).

The physical and chemical characteristics in the middle and lower Uruguay include a pH that varies between 7.1-7.8, conductivity between 69.6-210 μS/cm, dissolved oxygen between 7.6-12.6 mg/lm, and temperature between 13-30 ºC (Filho & Schulz 2003).

Terrestrial habitats

Whereas the upper part of the ecoregion is dominated by the Alto Paraná Atlantic Forest ecoregion, drier savanna habitats dominate the remainder of the ecoregion, with the Río Uruguay dividing Southern Cone Mesopotamian savanna, espinal, and humid pampas off its left bank from Uruguayan savanna off its right bank. Uruguayan savanna covers a majority of the ecoregion and includes palm savannas, outcroppings of submontane forests, and gallery forests that line the Uruguay and its tributaries. Southern Cone Mesopotamian savanna, which lies to the northeast of Río Mirinãy, includes seasonally wet grasslands, marshes, woodland, and gallery forests. Espinal, which extends southwest of the Río Mirinãy, is characterized by deciduous xerophytic forests, palm groves, savanna, and steppe (WWF 2001).

Description of endemic fishes

Thirty-seven endemic species are presently recorded from this ecoregion. These include 13 loricariids, seven rivulids, six characids, two cichlids, as well as individual representatives from five other families. The genera Austrolebias and Rineloricaria represent over a third of the endemics, with seven and six species, respectively. However, no genera are restricted to the lower Uruguay ecoregion, nor are any restricted to the entire Uruguay basin.

Other noteworthy fishes

The streaked prochilid or curimbatá (Prochilodus lineatus) is the most abundant species in the Uruguay basin (23%), followed by the voga (Schizodon nasutus) and granulated catfish (Pterodoras granulosus) at 4% and 3%, respectively (Filho & Schulz 2003).  Other species of commercial or recreational importance include the boga (Leporinus obtusidens), dorado (Salminus brasiliensis), and patí (Luciopimelodus pati) (Filho & Schulz 2003).

Ecological phenomena

The dorado undergoes extensive migrations throughout the Uruguay basin. Although this species can be found across the extent of the Río Uruguay, it was blocked historically during low water periods by three rapids - Salto Grande, Salto do Yucumã, and the Augusto César Gorge. The prochilid or curimbatá (Prochilodus lineatus) is another migratory species that occurs throughout the basin, but migrates upriver along the Paraná and Uruguay to spawn, returning to the lower Uruguay to feed (Filho & Schulz 2003). Other migratory species include the spotted sorubim (Pseudoplatystoma corruscans) and granulated catfish (Pterodoras granulosus).

Justification for delineation

The Río Uruguay falls within Ringuelet’s Alto Paraná and Parano-Platense icthyographic provinces (1975). It is distinguished from the upper Uruguay by its topography and hydrological characteristics, as well as its affinities to the lower Paraná.

Level of taxonomic exploration

Good overall with small areas of sparse collections.