It's impossible to avoid talking of numbers when describing the Amazon: it's the largest drainage basin in the world (some 40% of South America), with tributaries pouring in from Venezuela, Ecuador, Peru, Bolivia, Colombia, and Brazil. It is as much as 30 miles wide in places at high water (approximately November to June), and nearly a mile wide in Iquitos. Yet for all its length it drops only about one inch per mile for some 4,000 miles. The water is thick and opaque with clay sediment, unlike the ''black water'' feeder rivers, clearer, yet darkened with tannin. A cone of sediment some 200 miles or so long coats the ocean at its outlet. A nightmare of navigation after dark, the river is replete with floating logs and branches that occasionally stick up like the remains of a rotting pier, and constantly shifting sandbanks. The entire river used to flow directly past Iquitos but shifted course a few years ago, and Iquitos now lies along the Rio Itaya. A goodly portion of the land through which the rivers flow is floodplain.