| Constituents. | Tons in a Cubic Mile. |
|---|---|
| Calcium carbonate (CaCO3) | 326,710 |
| Magnesium carbonate (MgCO3) | 112,870 |
| Calcium phosphate (Ca3P2O8) | 2,913 |
| Calcium sulphate (CaSO4) | 34,361 |
| Sodium sulphate (Na2SO4) | 31,805 |
| Potassium sulphate (K2SO4) | 20,358 |
| Sodium nitrate (NaNO3) | 26,800 |
| Sodium chloride (NaCl) | 16,657 |
| Lithium chloride (LiCl) | 2,462 |
| Ammonium chloride (NH4Cl) | 1,030 |
| Silica (SiO2) | 74,577 |
| Ferric oxide (Fe2O3) | 13,006 |
| Alumina (Al2O3) | 14,315 |
| Manganese oxide (Mn2O3) | 5,703 |
| Organic matter | 79,020 |
| Total dissolved matter | 762,587 |
Murray also estimates that the aggregate amount of water flowing into the sea annually is about 6528 cubic miles, which, on the above basis, would carry about 4,975,000,000 tons of mineral matter in solution.
A large number of analyses of waters of rivers from the United States and Canada give an average of about .15,044 part in a thousand of mineral matter in solution, more than one-third being CaCO3. The average amount of mineral matter in solution in 48 European streams cited by Bischoff[40] is .2127 part in a thousand, of which CaCO3 is rather more than half. The average mineral matter in solution in 36 rivers cited by Roth[41] (including some of those tabulated by Bischoff) is .2033 part in a thousand, of which CaCO3 is slightly less than one-half.
An average for American and European rivers, so far as determinable from data at hand, is about .1888 part in a thousand in solution, of which CaCO3 is slightly less than one-half. These last figures are probably not very far from an average for river water in general.
The following table shows the total amount of solids carried in solution by the rivers indicated:[42]
| Rhine | 5,816,805 | tons per year. |
| Rhone | 8,290,464 | “ “ “ |
| Danube | 22,521,434 | “ “ “ |
| Thames | 613,930 | “ “ “ |
| Nile | 16,950,000 | “ “ “ |
| Croton | 66,795 | “ “ “ |
| Hudson | 438,000 | “ “ “ |
| Mississippi | 112,832,171 | “ “ “ |
ECONOMIC CONSIDERATIONS.
Certain considerations of human interest in connection with river erosion are worthy of note. When a drainage system has reached its mature stage its basin has the roughest topography which it will have at any time during that cycle of erosion. At that stage, therefore, road construction is relatively difficult. If the relief be great, roads must follow the valleys, or the crests of the ridges between them, if they would avoid heavy grades. In such regions roads are usually few and crooked.
The stage of development of valleys has an influence on the navigability of their streams. Streams well advanced in life are much more readily navigable than young ones, because their grades are lower and their volumes of water greater. Old streams, on the other hand, are sometimes depositing sand or silt along their lower courses to such an extent as to interfere with navigation.
At certain stages of their development the power of streams is more easily utilized than at others. Young streams, depending as they do for their supply on the rainfall of a limited area, are likely to be fitful in their flow, and therefore unreliable as a source of power. This is especially true where the precipitation is unequally distributed, and where the slopes are steep and free from forests. Because of their great volume, old and large streams, though sluggish, have great power, but it is less easily controlled. Where streams are large enough to be navigable industrial considerations often prevent the utilization of their power, the streams being more serviceable as highways than as sources of power. Other things being equal, it follows that streams are most available for water-power when they are large enough to have a moderately steady flow, and not so large as to be beyond ready control, or to be valuable for purposes of navigation.