likely to occur along these planes. Alluvial fan sedimentology and past floods The alluvial fans that spread out into the sea from valley mouths were built by previous flood and debris flow events. The modern channel systems of these alluvial fan deltas are incised into previously deposited debris flow and flood material. Scientists at the US Geological Survey measured these old deposits. They found that they are thicker than the December 1999 ones and contain larger boulders. This means that previous debris flows were even larger than those in December 1999 and reached higher velocities. On the Caraballeda fan, the extent of the 1951 event paled in comparison to the 1999 event. Much of the deposits that constitute the Caraballeda fan are of a thickness similar to those produced in the 1999 event and contain boulders of a size similar to those observed in 1999. The USGS geologists found paleosols with organic material above and below a thick layer of debris flow deposits. The bottom paleosol was radiocarbon dated to 4267 ±38 years Before Present (BP), and the top one was dated to 3720±50 years BP. This means that, at least in this area, the bed aggraded in 550 years, for an average rate of about per year (though the aggradation occurs only during short-lived events). The scientists were not able to tell whether the deposits were from a single debris flow or multiple events. Surficial geology and geomorphology Alluvial fan deltas in this region have shallow slopes. They are poorly channelized because sediment is being added to them from upstream (infilling the channels) at a rate equal to or greater than the rate at which it can be removed. Hillslopes are steepened past the angle of repose for noncohesive materials. This oversteepening is more than could be provided for by the frictional resistance of the sandy soils. Internal soil cohesion, negative pore pressure ("soil suction"), soil structure, and/or tree root reinforcement may be responsible for this. Neotectonics Terraces containing previous debris flow deposits are now perched above the modern stream channels. Erosion from the 1999 flood exposed bedrock benches to above the present channel. These abandoned high surfaces suggest recent and continuing tectonic uplift of the Venezuelan coast and corresponding river channel incision. In spite of the fact that most onshore faults active in this region during the Quaternary are mapped as right-lateral strike-slip, it is possible that there