Bedrock channel morphology of the Upper Uruguay River (South America). Bathymetric sonar data of the inner canyon

KRÖHLING, DANIELA; GÓMEZ, EDUARDO; RANIOLO, ARIEL

Berna

Congreso; XVIII International INQUA Congress; 2011

INQUA (International Union for Quaternary Research)

The upper Uruguay River is developed on the K-basaltic plateau of S Brazil and NE Argentina. The bedrock channel morphology is dominated by erosional processes, reflecting the spatial variability in hydraulics along the channel. Variations in channel width, depth and gradient produces differences in flow energy generated longitudinal grooves, knickpoints, inner channel, rapids and step-pools. At the largest scale, tectonic features and patterns of stream power dominate the erosive channel morphology. A persistent characteristic of the Uruguay bedrock is the narrow and sinuous, deep inner channel (canyon). It was detected along the Argentina-Brazil border and it is continuous and permanently submerged, except in a segment of ca. 3 km. There the river crosses an elevated tectonic block producing the emergence of the deep canyon (50 m width). It forms the Moconá, a system of longitudinal waterfalls of 10-12 m high in normal periods. A boat-based survey of the incised channel was made in a continuous segment of 55 km between the mouth of the Yabotí River and El Soberbio (Misiones province), using the Phase Measurement Bathymetric Sonar System. Digital maps (1 m gridded with decimeter vertical precision) were generated. The results revealed the 3D nature of the canyon; en general it occupies 1/3 to 1/10 of the channel width and it is incised between 6 to 22 m in the plane-bedrock channel (2 to 3 m depth). In segments of 1.5 to 4 km, the canyon depths are in the range of 24 to 32 m, 50 to 100 m wide. The section is well delineated with 40 to 80° flanks and flat to slightly concave bottom. Pools (100 to 350 m long and depths between 32 and 47 m) with symmetrical cross section and asymmetrical longitudinal profile were also detected, indicating the position of subfluvial cataracts. Conditions of macroturbulent and fine sediment-impoverished flood would have facilitated the preservation of erosive features and their enhancement during the Quaternary.