MEGAFLOODS ON THE SANTA CRUZ RIVER, SOUTHERN ARGENTINA

The Santa Cruz River basin of Patagonia in southern Argentina displays numerous landforms that together indicate a landscape severely impacted by more than one Pleistocene megaflood. In the upper drainage basin (longitude 71.6 to 71.3w), paleolacustrine landforms (raised beaches, bars, spits, lagoons and deltas) and stratigraphy (widespread silt and clay deposits) demonstrate that “Paleolago Argentino” was a large, 75-m-deep, glacial lake at 250 m elevation above present sea level. Lacustrine landforms are juxtaposed upon Arroyo Verde moraines that acted as the dam, or likely, multiple dams for great glacial lakes. Breached Arroyo Verde 2 terminal moraine (longitude 71.2w) is bounded downstream by a spillway that is 2.5 km wide and over 90 m deep entrenched in glacial and proglacial deposits (longitude 71.11w). Eastward, within the widening spillway, are paired terraces covered by “gravel hills” (longitude 71.1 to 70.7w). These hills are composed of 0.3-m-diameter boulders displaying dune bedforms with lengths of more than 10 m and heights of up to 3 m. Andrea Pacifici (2009) described large fluvial dunes beneath Condor Cliff (longitude 70.7w) comparing them to dunes on Mars. Larger dunelike bedforms occupy higher terraces within the lower Santa Cruz River Valley (longitude 70.1 to 69.0w). One megaflood overtopped the eastward-diminishing basalt cliff on the north side of the channel at Rincon Grande (longitude 70.1w), depositing cobbles on top of the cliff. These spillover cobbles indicate a megaflood more than 100-m deep and 10-km wide. Cross-sectional parameters, channel roughness and slope allow us to estimate this megaflood's discharge at 5 million m³/sec. Downstream at the town of Piedra Buena near the modern Santa Cruz River delta, dunes up to 20 m long and 2 m high occur on top of a terrace 40 m above the present river level (longitude 69.4 to 69.3w). The biggest boulders up to 1.7 m diameter on the high Piedra Buena terrace were likely ice-rafted during a megaflood. Multiple terrace levels with coarse boulders and/or dune or antidune bedforms favor multiple megafloods. Depositional landforms containing boulders indicate megafloods with enormous bed shear stresses, Froude numbers of 0.8 to 1.2, and velocities exceeding 5 m/sec.