RATE OF DEGRADATION.
The amount of mechanical sediment which the Mississippi River carries to the Gulf of Mexico is estimated to represent a rate of degradation for the Mississippi basin of about one foot in 5000 years. But the mechanical sediment carried to the Gulf does not really represent the total degradation of the basin, for the water which sinks beneath the surface is dissolving more or less rock substance, especially lime carbonate. This material is carried to the sea in solution, and does not appear in the sediment on which the above estimate is based. Taking into account the matter dissolved by the water and carried to the sea in solution, the average rate of degradation for the Mississippi basin is estimated at one foot in 3000 to 4000 years.
It is not to be inferred that this rate is uniform, or even that erosion at any rate whatsoever is taking place in all parts of the basin. Such is not the fact. On the whole the rate of erosion is doubtless greatest toward the margins of the basins where the land is in its topographic youth or early maturity. It is notably less in the middle courses of the valleys, and erosion is locally exceeded by deposition along the lower courses of the Mississippi and some of its main tributaries.
The average elevation of North America is not accurately known, but it is probably not far from 2000 feet. If the present rate of degradation, say one foot in 3500 years, were to continue, it would take something like 7,000,000 years to bring the continent to sea-level. But this rate of degradation could not continue to the end, for as the continent became lower streams would become sluggish and erosion less rapid. Long before the continent reached base-level the rate of degradation, so far as dependent on mechanical erosion, would become so slow that the time necessary to bring the continent to sea-level would be almost inconceivably prolonged. Furthermore, it is quite possible that the land is suffering, or is liable to suffer, uplift, relative or absolute. If the rate of rise were equal to the rate of degradation the average height of the continent would of course not be affected.
The amount of sediment carried by streams in suspension varies notably according to the stage of the water. During a year when the stream was under careful study the Mississippi at Carrollton (Miss.) was found to carry ¹⁄₆₈₁ of its weight of sediment during the high-water stage of June, and ¹⁄₆₃₈₃ during the low-water of October, the average for the year being ¹⁄₁₈₀₈. The average of a greater number of records gives about ¹⁄₁₅₀₀ as the average ratio between the weight of the sediment and the weight of the water. This corresponds to about ¹⁄₂₉₀₀ by volume, the average specific gravity being about 1.9. The amount of material carried in the upper part of the water was notably less than that carried at greater depths, but that carried midway between top and bottom was about the same as that carried at the bottom.[32]
The discharge of the Mississippi River is about 19,500,000,000,000 cubic feet of water per year, and the sediment it carries in suspension is estimated to weigh about 812,500,000,000 pounds. This is equivalent to about 6,714,694,400 cubic feet. It is estimated that about 750,000,000 cubic feet of sediment is rolled along the bottom, giving a total of 7,468,694,400 cubic feet as the aggregate annual load carried to the Gulf by the river. This would be adequate to cover an area one square mile in extent to the depth of 268 feet per year.
PLATE XI.
U. S. Geol. Surv.
Scale, 2+ mile per inch.
PART OF THE CATSKILLS, NEW YORK.
PLATE XII.
U. S. Geol. Surv.
Scale, 2+ mile per inch.
Fig. 1. NEW MEXICO.
U. S. Geol. Surv.
Scale, 1+ mile per inch.
Fig. 2. VIRGINIA, WEST VIRGINIA AND MARYLAND.
| Name of river | Bear | Croton | Cumberland | Delaware | Hudson, N. Y. | James |
|---|---|---|---|---|---|---|
| Collected at | Evanston, Wy. | Reservoir, New York City | Reservoir at Nashville, Tenn. | Reservoir at Trenton, N. J. | ............ | Richmond Water Works, Va. |
| Date | Dec., 1873 | 1881 | ............ | ............ | ............ | Oct. 24. 1876, after light rain |
| Analyst | F. W. Clarke | E. Waller | N. T. Lupton | H. Wurtz | C. F. Chandler | W. H. Taylor |
| Reference | Bulletin No. 9, U. S. Geol. Surv., p. 30 | Water supply of New York City, 1881 | Am. Chemist, July 16, 1876, p. 16 | Geol. of N. J., 1868, p. 702 | Public Health Papers, Vol. I, Am. Pub. Health Ass. | Ann. Rept. Board of Health, Richmond, Va., 1876 |
| Sodium, Na | .0082 | [34].00298 | .01032 | .00072 | .00244 | .00234 |
| Potassium, K | ...... | .00154 | .00050 | .00178 | .00058 | .00251 |
| Calcium, Ca | .0432 | .00905 | .02987 | .01104 | .02220 | .01284 |
| Magnesium, Mg | .0125 | .00336 | .00280 | .00435 | .00465 | .00377 |
| Chlorine, Cl | .0049 | .00213 | .00299 | .00121 | .00581 | .00105 |
| Carbonic acid, CO2 | [35].0982 | [34].02248 | .05727 | .02552 | .07278 | .02954 |
| Sulphuric acid, SO3 | .0105 | .00441 | .00563 | .00175 | .01257 | .00363 |
| Phosphoric acid, H3PO | ...... | .00172 | Trace | |||
| Nitric acid, HNO3 | ...... | ...... | .00511 | ...... | ...... | .00231 |
| Silica, SiO2 | .0070 | .03360 | Trace | .00852 | .00698 | .01024 |
| Alumina, Al2O3 | .00047 | .00041 | ||||
| Sesquioxide of iron, Fe2O3 | ...... | ...... | ...... | |||
| Sesquioxides of iron and alumina, Fe2O3 and Al2O3 | ...... | .00078 | .00671 | ...... | .00120 | ...... |
| Sesquioxides of iron and manganese, Fe2O3 and Mn2O3 | .00072 | |||||
| Carbonates of iron and manganese, FeCO3 and MnCO3 | ||||||
| Oxide of iron, FeO | ...... | |||||
| Oxide of manganese, MnO | Trace | ...... | ||||
| Hydrogen in bicarbonates, H | .00121 | |||||
| Chloride and sulphate of sodium, NaCl, and Na2SO4 | ...... | |||||
| Ammonia, NH4 | ...... | ...... | .01087 | ...... | .00001 | |
| Organic matter | .00400 | .01666 | .01197 | .00299 | ||
| Carbonates and sulphates of Na, K, and Mg | ...... | ...... | ...... | ...... | ...... | ...... |
| .1845 | .08433 | .13786 | .06795 | .14238 | .07246 |
| Name of river | Los Angeles | Maumee, O. | Mississippi | Ottawa | Passaic | Rio Grande del Norte | Sacramento |
|---|---|---|---|---|---|---|---|
| Collected at | Hydrant, City ater Works, New Orleans, La. | St. Ann’s Lock, Montreal, Can. | 4 miles above Newark, N. J. | Fort Craig, New Mexico | Hydrant, Sacramento, Cal. | Hydrant at Los Angeles, Cal. | ............ |
| Date | Sept. 8, 1878 | ............ | ............ | Mar. 9, 1854 | 1851 | 1873 | Sept., 1878 |
| Analyst | W. J. Jones | C. F. Chandler | W. J. Jones | T. S. Hunt | E. N. Horsford | O. Loew | W. J. Jones |
| Reference | Rept. Cal. State Board of Health, 1878 | Report of Toledo Water Works, 1881 | Rept. La. State Board of Health, 1882, p. 370 | Geol. of Canada, 1863, p. 567 | Geol. of N. J., 1868, p. 708 | U. S. Geog. Surv. west of 100th M., Vol. III. p. 576 | Rept. Cal. State Board of Health, 1878 |
| Sodium, Na | .02968 | .00162 | .0310 | .00239 | .02357 | .03220 | .00200 |
| Potassium, K | ...... | .00309 | ...... | .00139 | .00163 | .00063 | ...... |
| Calcium, Ca | .01750 | .02645 | .0372 | .00992 | .01459 | .01633 | .01279 |
| Magnesium, Mg | .02097 | .00443 | ...... | .00161 | .00404 | .00123 | .00121 |
| Chlorine, Cl | .01044 | .00250 | .0480 | .00076 | .03192 | .03604 | ...... |
| Carbonic acid, CO2 | .05635 | .04438 | .0383 | .02255 | .02634 | .01025 | .00887 |
| Sulphuric acid, SO3 | .05724 | .01401 | .00194 | .01716 | .04700 | .00397 | |
| Phosphoric acid, H3PO | .02638 | Trace | Faint trace | .0179 | |||
| Nitric acid, HNO3 | ...... | ...... | ...... | Trace | ...... | ||
| Silica, SiO2 | .02005 | .00724 | .02060 | .01342 | Trace | .03167 | |
| Alumina, Al2O3 | .00171 | ...... | Trace | Trace | .00120 | Trace | |
| Sesquioxide of iron, Fe2O3 | .00100 | ||||||
| Sesquioxides of iron and alumina, Fe2O3 and Al2O3 | ...... | ...... | |||||
| Sesquioxides of iron and manganese, Fe2O3 and Mn2O3 | ...... | ...... | |||||
| Carbonates of iron and manganese, FeCO3 and MnCO3 | .00443 | ...... | ...... | .01088 | |||
| Oxide of iron, FeO | Trace | Trace | |||||
| Oxide of manganese, MnO | Trace | ||||||
| Hydrogen in bicarbonates, H | ...... | ||||||
| Chloride and sulphate of sodium, NaCl, and Na2SO4 | ...... | .02431 | |||||
| Ammonia, NH4 | ...... | Trace | |||||
| Organic matter | .00499 | ...... | .01392 | ||||
| Carbonates and sulphates of Na, K, and Mg | ...... | ...... | .0154 | ...... | ...... | ...... | ...... |
| .24475 | .10971 | .1699 | .06116 | .13287 | .15760 | .11484 |
| Name of river | St. Lawrence | Humboldt | Truckee | Walker | Jordan | Mohawk | Genesee |
|---|---|---|---|---|---|---|---|
| South side Point des Cascades | Collected at | Battle Mt., Nev. | Lake Tahoe, Nev. | Mason Valley, Nev. | Utah Lake | Utica, N. Y. | Rochester, N. Y. |
| Date | Mar. 30, 1863 | Dec., 1872 | Oct., 1872 | Oct., 1872 | Nov., 1873 | ||
| Analyst | T. S. Hunt | T. M. Chatard | F. W. Clarke | F. W. Clarke | F. W. Clarke | C. F. Chandler | C. F. Chandler |
| Reference | Geol. of Canada, 1863, p. 567 | U. S. Geol. Surv., Monograph XI, p. 41 | U. S. Geol. Surv., Monograph XI, p. 42 | U. S. Geol. Surv., Monograph XI, p. 40 | Bulletin No. 9, U. S. Geol. Surv., p. 29 | Johnson’s Cyclopedia, Vol. IV | Johnson’s Cyclopedia, Vol. IV |
| Sodium, Na | .00513 | .0467 | .0073 | .0318 | .0178 | .0036 | .0044 |
| Potassium, K | .00115 | .0100 | .0033 | Trace | ...... | .0009 | .0023 |
| Calcium, Ca | .03233 | .0489 | .0093 | .0228 | .0558 | .0318 | .0417 |
| Magnesium, Mg | .00585 | .0124 | .0030 | .0038 | .0186 | .0069 | .00896 |
| Chlorine, Cl | .00242 | .0075 | .0023 | .0131 | .0124 | .0023 | .0024 |
| Carbonic acid, CO2 | .06836 | [35].1544 | [35].0287 | [35].0576 | .0608 | .0569 | .0646 |
| Sulphuric acid, SO3 | .00831 | .0477 | .0054 | .0284 | .1306 | .0187 | .0431 |
| Phosphoric acid, H3PO | Trace | ...... | |||||
| Nitric acid, HNO3 | ...... | ...... | ...... | ...... | ...... | ...... | |
| Silica, SiO2 | .03700 | .0326 | .0137 | 0.225 | .0100 | .0067 | .0014 |
| Alumina, Al2O3 | .0013 | ||||||
| Sesquioxide of iron, Fe2O3 | ...... | ...... | |||||
| Sesquioxides of iron and alumina, Fe2O3 and Al2O3 | .0013 | .0014 | |||||
| Sesquioxides of iron and manganese, Fe2O3 and Mn2O3 | |||||||
| Carbonates of iron and manganese, FeCO3 and MnCO3 | ...... | ||||||
| Oxide of iron, FeO | Trace | ...... | |||||
| Oxide of manganese, MnO | Trace | ...... | ...... | ||||
| Hydrogen in bicarbonates, H | ...... | ...... | ...... | ||||
| Chloride and sulphate of sodium, NaCl, and Na2SO4 | ...... | ...... | ...... | ||||
| Ammonia, NH4 | ...... | ...... | ...... | ||||
| Organic matter | ...... | ...... | .0234 | .0250 | |||
| Carbonates and sulphates of Na, K, and Mg | ...... | ...... | ...... | ...... | ...... | ...... | ...... |
| .16055 | .3615 | .0730 | .1800 | .3060 | .1525 | 019526 |
The following table[36] gives the percentage of material carried in suspension by various rivers:
| River. | Drainage Areas in Square Miles. | Mean Annual Discharge (in Cubic Feet.) per Second. | Total Tons Annually. | Ratio of Sediment to Water by Weight. | Height in Feet of Column of Sediment with a Base of One Square Mile. | Thickness of Sediment in Inches if Spread over Drainage Area. |
|---|---|---|---|---|---|---|
| Potomac | 11,043 | 20,160 | 5,557,250 | 1 : 3,575 | 4.0 | .00433 |
| Mississippi | 1,244,000 | 610,000 | 406,250,000 | 1 : 1,500 | 241.4 | .00223 |
| Rio Grande | 30,000 | 1,700 | 3,830,000 | 1 : 291 | 2.8 | .00116 |
| Uruguay | 150,000 | 150,000 | 14,782,500 | 1 : 10,000 | 10.6 | .00085 |
| Rhone | 34,800 | 65,850 | 36,000,000 | 1 : 1,775 | 31.1 | .01075 |
| Po | 27,100 | 62,200 | 67,000,000 | 1 : 900 | 59.0 | .01139 |
| Danube | 320,300 | 315,200 | 108,000,000 | 1 : 2,880 | 93.2 | .00354 |
| Nile | 1,100,000 | 113,000 | 54,000,000 | 1 : 2,050 | 38.8 | .00042 |
| Irrawaddy | 125,000 | 475,000 | 291,430,000 | 1 : 1,610 | 209.0 | .02005 |
| Mean | 334,693 | 201,468 | 109,649,972 | 1 : 2,731 | 76.65 | .00614 |
The composition of rain-water falling near London, as determined by analysis, was as follows:[37]
| Organic carbon | .99 | part in 1,000,000 of water. |
| Organic nitrogen | .22 | “ “ “ “ “ |
| Ammonia | .50 | “ “ “ “ “ |
| Nitrogen as nitrates and nitrites | .07 | “ “ “ “ “ |
| Chlorine | 6.30 | parts in“ “ “ |
| Total solids | 39.50 | “ “ “ “ “ |
A comparison of the composition of rain-water with that of springs and rivers gives some idea of the solvent work of water. From a study of the water of nineteen of the principal rivers of the world Murray has compiled the following table[38] showing the amount of mineral matter in average river water:
| 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 | “ “ “ |