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SCIENCE. 



[Vol. XXI. No. 542 



SCIENCE: 



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THE SEDIMENT OF THE POTOMAC RIVER. 



BY CYEUS O. BABB, U. S. GEOLOGICAL SURVEy'. 



The United States Geological Survey in May, 1891, established 

 a gauging station on the Potomac River at Chain Bridge, D.C., 

 for the measurement of the discharge of the river at that place. 

 From that date to the present time daily readings of the height of 

 the river have been maintained, which, taken in connection with 

 the measurements of discharge, makes it possible to compute the 

 daily discharges of the river at this place. A detailed account of 

 the methods and results of this branch of the work may be found 

 in a paper by the writer in the Transactions of the American 

 Society of Civil Engineers, No. 537, Vol. 27, and entitled "The 

 Hydrography of the Potomac River Basin." This article deals 

 with the discharge of the river and its relation to the rainfall in 

 the basin. It is also stated that measurements of the amount of 

 sediment transported by the river were being made. The results 

 are now available and are here given for publication. 



Daily heights of the Potomac River at Great Falls, about sixteen 

 miles above the City of Washington, have been kept since 1878 by 

 the officials of the Washington aqueduct, together with a daily 

 record termed "condition of water." Owing to the fact, however, 

 that the dam across the river at Great Falls was not completed 

 until 1886, the two sets of records previous to this latter date are 

 valueless for discussion. 



The results of this article are based in part upon the records of ' 'con- 

 dition of water," which are made as follows : A horizontal metallic 

 tube, 36 inches long and with glass ends, is filled with water, and the 

 distance at which a ball immersed therein can be seen from one end 

 is noted. The observations vary from 1 inch in very muddy water 

 to 36 inches, which is considered as clear. Samples of the river 

 water at Great Falls were collected and were sent in to the main 

 office in Washington, where they were analyzed quantitatively in 

 so far as the determination of the ratio of the weight of the con- 

 tained sediment to the total weight of the sample. At the same 

 time the " condition of water" was recorded. Fifty-five samples 

 were analyzed, with condition of water ranging from 1 to 36 

 inches. These quantities were plotted on cross-section paper, 

 with condition of water as abscissae and ratios of sediment to 

 water as ordinates. Through the points thus obtained a smooth 

 curve was drawn, from which a table was constructed, giving for 

 each inch of condition of water the corresponding ratio of sedi- 

 ment. In order to obtain the total amount of sediment trans- 

 ported by the river for any length of time the discharge of the 

 river for that period must be multiplied by the average ratio of 

 the sediment to the water for the same period. 



Simultaneous gauge readings of the height of the river at Great 

 Falls and at Chain Bridge were maintained for a year and a half. 

 From these observations a table was constructed, giving for the 

 gauge height at one place the corresponding gauge height at the 

 other. From the fact that no large tributary enters the river 



between these two points, the daily discharges at Great Falls 

 may be computed from the table of gauge relations and from the 

 daily discharges of the river at Chain Bridge. The daily ratio of 

 sediment to water was found from the daily record of condition 

 of water and the rating table of ratios and condition of water. 

 Knowing then the daily discharge of the river in cubic feet per 

 second and the daily ratio, it is simply a matter of multiplication 

 of second-feet times the ratio limes the weight of one cubic foot 

 of water to obtain the weight of the total amount of sediment 

 passing down the river per second. In this way the daily amounts 

 of sediment from 1886 to 1891, inclusive, have been computed. 



In considering the value of these figures, it would seem at first 

 sight that the above method of measurement for condition of 

 water was crude to base scientific results upon. The observations 

 are not made for that purpose, but are more for the benefit of the 

 fishermen in the vicinity of Washington. They have the advan- 

 tage of being simple and inexpensive and can be maintained by 

 an inexperienced observer. Another very important fact in their 

 favor for this river is that, owing to the absence of lakes and ex- 

 tensive swamps throughout the basin, such as are found in the 

 glacial region further north or the swampy regions of the extreme 

 south, the coloring matter of this river is almost wholly due to 

 mineral sediments and very little to vegetable deposits. It would 

 be more accurate if daily samples of the water could be analyzed, 

 but it would be expensive and would require a long time-interval 

 before the results would be of value. There is a six years' record 

 of condition of water, or over 3,000 observations. From a series 

 of measurements certain average values for this record have been 

 computed. Any one observation may depart greatly from this 

 average, but when considered in connection with the total num- 

 ber of observations the effect of its departure from the mean is 

 inappreciable. 



The lowest record is 36 inches. In some cases the ball is just 

 able to be seen at this mark ; in others distant objects are plainly 

 visible. There is here an arbitrary limit for the curve, which 

 ought to extend conssiderably below this point, but taken in con- 

 nection with the rest of the range, and especially with the upper 

 part, where the ratios are large, the weight of this lower end is 

 small. Errors will also arise depending upon the cloudiness of 

 the day. However, errors due to this method of sediment 

 measurement are not cumulative, but may be either plus or minus, 

 and in a large number of observations tend to equalize each other. 

 It is therefore considered that the results are sufficiently ac- 

 curate for all ordinary purposes. 



The following facts are brought out. The average annual dis- 

 charge of the Potomac River from a drainage area of 11, 043 square 

 miles is 30,160 second-feet, varying from 2,000 second- feet in time 

 of low water up to 470,000 second-feet during the great flood of 

 1889. The total annual amount of sediment transported is 5,557,350 

 tons, or 353 pounds per second, and distributed through the six 

 years from 1886 to 1891 as follows: 1886, 4,383,000 tons; 1887, 

 2,373,800 tons; 1888, 4,996,800 tons; 1889, 10,143,600 tons; 1890, 

 5,994,000 tons; and 1891, 5,544,300 tons. The average daily 

 amount varied from 1 pound to 21,900 pounds per second. It is 

 found from these figures that the average annual amount of sedi- 

 ment is to the weight of the annual discharge of water as 1 to 

 3,575. Assuming that one cubic foot of sediment weighs lOO 

 pounds, this average amount of sediment would cover one square 

 mile 3.98 feet in depth , and if spread over the drainage area would 

 cover it 0.0043 inches in depth. At this latter rate it would take 

 the river 2,770 years to erode one foot from the drainage area. 



These results appear in the following table, together with similar 

 data compiled for several other large rivers. The first column 

 gives the name of the river; second, its drainage area in square 

 miles; third, the average annual discharge of the river in cubic 

 feet per second. The fourth column gives the total amount of 

 sediment, in tons, annually transported by the river; fifth, the 

 ratio of the weight of this sediment to the weight of the water 

 annually discharged ; the sixth, the height of a column in feet 

 having a base of one square mile that the sediment would cover; 

 the seventh, the depth in inches that the drainage area would be 

 covered if this total amount of sediment should be spread over it; 

 and the last column the authority for the data. The discharge 



