The Rio Negro

As the largest tributary of the Amazon and one of the 10 largest rivers in the world, the Rio Negro is an enormously important body of water home to an estimated 800 species of freshwater fish. From its headwaters in Colombia to its juncture with the Amazon (Solimoes) near the city of Manaus, the Rio Negro is surrounded by thousands of square miles of largely pristine tropical rainforest.

A true blackwater river (and the largest of its kind in the world), the water of the Negro is characteristically dark brown in color. This is primarily due to the breakdown of organic material (wood and dead leaves) prevalent in the Rio Negro and its tributaries. The water is extremely soft, with a low conductivity, and highly acidic.

Like many other Amazon tributaries, the Rio Negro has a characteristic annual flood cycle. Beginning around February, the river floods its banks and overflows into the surrounding forests. At its peak, this flooding adds up to 10 meters of depth to the river and inundates an area of almost 300,000km2. During this time, many fish species enter the flooded forest (an abundant source of food) to spawn. Fish like the cardinal tetra produce billions of offspring in the Rio Negro's floodplain, most of which will be predated or trapped in gradually drying ponds as the floodwaters recede. This is one reason why ornamental fish collection, which occurs as the water levels drop, tends to have a low impact on wild populations. During the low water seasons, entire stretches of river become impossible to navigate and once-submerged rocks turn to dangerous rapids and falls.

Aside from the major population center of Manaus, a city of about 2 million, the rest of the Rio Negro drainage is sparsely populated. Small cities like Barcelos and Santa Isabel and tiny comunidades (towns) are home to most of the region's inhabitants, and almost all transportation is by boat.