In addition to what the river carries down mechanically in suspension or sweeps along the bottom, there is a third class of material; namely, that which is dissolved in the waters of the stream. Dissolved matters are always present in greater or less quantity, and are the same in kind as those which we have already found to occur in spring-waters, whence they are, for the most part, derived by the rivers. River-water is, however, usually more dilute than that of springs, because of the rain which falls into it, or pours in from the banks. In very dry regions, where this additional rain supply is at a minimum, and where the streams are concentrated by continual evaporation, they are frequently undrinkable, on account of the quantity of matter in solution which they contain. Examples of this are the salt and so-called "alkali" (a very comprehensive term) streams of the arid West, which contain a great variety of dissolved minerals.

The quantity of material which rivers are continually sweeping into the sea is enormously great. Every year the Mississippi carries into the Gulf of Mexico nearly 7,500,000,000 cubic feet of solid sediment, either in suspension or pushed along the bottom, an amount sufficient to cover one square mile to a depth of 268 feet. In addition to this is the quantity brought down in solution, which is estimated at 2,850,000,000 cubic feet annually.

Different rivers vary much in the proportion of suspended and dissolved materials which they carry and discharge into the sea; a roughly approximate average makes the amount of material removed equal to about 11,400 cubic feet (600 tons) of annual waste for every square mile of the land surface of the globe, that is, under existing conditions of slope, temperature, rainfall, etc. How great a difference in the result a change in these factors may produce will be seen from a comparison of the Mississippi and the Ganges. The amount of suspended matter discharged by the former represents a lowering of the surface of the entire drainage area at the rate of one foot in 4920 years, while in the case of the Ganges it is one foot in 1880 years, or more than twice as fast. The amount of material carried by the Amazon has not been determined, but there can be little doubt that it is far greater than that discharged by the Mississippi. The area drained by the Amazon is less than twice as large as the drainage basin of the Mississippi, and yet it brings to the sea five times as much water as does the great river of North America.