The Tapajós is one of three major clearwater rivers in the Amazon Basin (the others are Xingu and Tocantins; the latter arguably outside the Amazon).[2][6] Clearwater rivers share the low conductivity and relatively low levels of dissolved solids with blackwater rivers, but differ from these in having water that at most only is somewhat acidic (typical pH ~6.5)[2] and very clear with a greenish colour.[6] Although most of the tributaries in the Tapajós basin also are clearwater, there are exceptions, including the blackwater Braço Norte River (southeastern Serra do Cachimbo region).[7] About 325 fish species are known from the Tapajós River basin, including 65 endemics.[8] Many of these have only been discovered within the last decade, and a conservative estimate suggests more than 500 fish species eventually will be recognized in the river basin.[8]