Erland Nordenskiold was a pioneering Swedish anthropologist who undertook explorations in eastern Bolivia between 1901 and 1914. This often-cited article was originally published in German in 1916. He describes the seasonal flooding of the Mojos savannas and how prehistoric Indians adapted to inundation by constructing canals, dams, causeways, mounds, and raised fields, the first academic to do so. Other prehistoric hydraulic works and earthworks in lowland South America are briefly reported. Included is a letter from the anthropologist Koch-Griinberg suggesting that the section of the Casiquiare Canal which connects the Rio Orinoco with the Rio Negro and the Amazon is not natural but rather was dug by Indians.