Figure 1.
Relation curve for estimating inflow to Chesapeake Bay at section B.
by Conrad D. Bue
Prepared in cooperation with the States of Maryland, Pennsylvania, and Virginia
U.S. Geological Survey Open-File Report
Arlington, Va.
October 1968
The following document has been converted to electronic form by retyping the original paper publication and scanning original illustrations. Some illustrations have been replaced with better-quality images showing equivalent information. The image quality of the scanned illustrations is similar to that in the original publication.
Note: A facsimile of the monthly inflow report was included in the original report as an appendix, but has not been reproduced here. It has been replaced by an on-line monthly report at http://md.water.usgs.gov/monthly/.
This report presents a convenient and rapid means of estimating, on either a monthly or a yearly basis, the inflow from surface streams to Chesapeake Bay. The method was developed as a working base for the release entitled "Estimated stream discharge entering Chesapeake Bay" prepared by the U.S. Geological Survey in cooperation with the States of Maryland, Pennsylvania, and Virginia. This release, issued monthly beginning December 1967, is directed to the various groups who have need for such data in their studies of the environment and resources of the Bay. A copy of that release is included as an appendix to this report.
In addition to the methodology used in making estimates of inflow, the report presents considerable data on drainage basins and on streamflow patterns. The report thus serves as a reference for those receiving the monthly release on current conditions. No account is taken of ground-water inflow or of rainfall on and evaporation from the water surface of the Bay. The years referred to herein are calendar years unless water years are specified.
Current stream discharge into Chesapeake Bay, whether for monthly or longer periods, is estimated using current records at the most downstream gaging stations on the three principal rivers discharging into the Bay: Susquehanna River at Marietta, Pa.; Potomac River near Washington, D.C., adjusted for diversions in the Washington metropolitan area that return to the Potomac downstream from the gaging station; and James River near Richmond, Va., adjusted for diversions by the James River and Kanawha Canal, which returns to the James downstream from the Richmond gaging station. These estimates of inflow to the Bay are derived from graphical relations between the three gaging stations and the total discharge into the Bay as calculated for the 10-year period 1951-60 (water years).
Relation curves were prepared by plotting the discharge at these gaging stations by months and years for the period 1951-60 against the corresponding total discharge into the Bay (the Bay was divided into several segments). Figure 1, which shows the discharge of the Susquehanna River at Marietta plotted against the discharge into the Bay above cross-section B, is an example of the relation curves. Total discharge into the Bay had been calculated in a previous study by months and years for the 10 water years 1951-60 (referred to hereinafter as the basic computations) using all available streamflow records, and estimating the ungaged discharge on basis of streamflow records from nearby streams. A list of the gaging-station records used in the basic computations, and their drainage areas, is presented in table 1.
Figure 1.
Relation curve for estimating inflow to Chesapeake Bay at
section B.
Table 1.--Streamflow records used in calculating flow into Chesapeake Bay for period 1951-1960
-----------------------------------------------------------------
Drainage Area
Gaging station (sq.mi.)
-----------------------------------------------------------------
Part 1-B
4850. Pocomoke R nr Willards, Md.---------------------- 60.5
4855. Nassawango C nr Show Hill, Md.------------------- 44.9
4860. Manokin Br nr Princess Anne, Md.----------------- a 5.8
4865. Beaverdam C nr Salisbury, Md.-------------------- 19.5
4870. Nanticoke R nr Bridgeville, Del.----------------- 75.4
4875. Trap Pond Outlet nr Laurel, Del.----------------- 16.7
4885. Marshy Hope C nr Adamsville, Del.---------------- 44.8
4890. Faulkner Br at Federalsburg. Md.----------------- 7.10
4895. Rewastico C nr Hebron, Md.----------------------- 12.2
4900. Chicamacomico R nr Salem, Md.-------------------- 15.0
4910. Choptank R nr Greensboro, Md.-------------------- 113
4920. Beaverdam Br at Matthews, Md.-------------------- 5.85
4930. Unicorn Br nr Millington, Md.-------------------- 22.3
4935. Morgan C near Kennedyville, Md.------------------ 10.5
4950. Big Elk C at Elk Mills, Md.---------------------- 52.6
4955. Little Elk C at Childs, Md.---------------------- 26.8
4960. Northeast C at Leslie, Md.----------------------- 24.3
5760. Susquehanna R at Marietta, Pa.------------------- 25,990
5765. Conestoga C at Lancaster, Pa.-------------------- 324
5800. Deer C at Rocks, Md.----------------------------- 94.4
5815. Bynum Run at Bel Air, Md.------------------------ 8.52
5840. Gunpowder Falls nr Carney, Md.------------------- 314
5845. Little Gunpowder Falls at Laurel Brook, Md.------ 36.1
5890. Patapsco R at Hollofield, Md.-------------------- 285
5900. North R nr Annapolis, Md.------------------------ a 8.5
5925. Patuxent R nr Laurel, Md.------------------------ 132
5940. Little Patuxent R at Savage, Md.----------------- 94.4
5944. Dorsey Run nr Jessup, Md.------------------------ 11.6
5945. Western Br nr Largo, Md.------------------------- 30.2
6465. Potomac R nr Washington, D.C.-------------------- 11,560
6470. Little Falls Br nr Bethesda, Md.----------------- a 4.1
6480. Rock C at Sherrill Dr., Washington, D.C.--------- 62.2
6495. Northeast Br Anacostia R at Riverdale, Md.------- 72.8
6510. Northwest Br Anacostia R nr Hyattsville, Md.----- 49.4
6525. Fourmile Run at Alexandria, Va.------------------ 14.4
6535. Henson C at Oxon Hill, Md.----------------------- 16.7
6550. Accotink C nr Accotink Station, Va.-------------- 37.0
6575. Occoquan C nr Occoquan, Va.---------------------- 570
6580. Mattawoman C nr Pomonkey, Md.-------------------- 57.7
6585. South Fork Quantico C nr Independent Hill, Va.--- 7.5
6610. Chaptico C at Chaptico, Md.---------------------- 10.7
6615. St. Marys R at Great Mills, Md.------------------ 24.0
6680. Rappahannock R nr Fredericksburg, Va.------------ 1,599
6685. Cat Point C nr Montross, Va.--------------------- 45
6695. Dragon Run nr Church View, Va.------------------- 86
6700. Beaverdam Swamp nr Ark, Va.---------------------- 7.1
6730. Pamunkey R nr Hanover, Va.----------------------- 1,072
6735. Totopotomoy C nr Atlee, Va.---------------------- 6.0
6745. Mattaponi R nr Beulahville, Va.------------------ 619
Part 2-A
375. James R nr Richmond, Va.------------------------- 6,757
385. Falling C nr Drewrys Bluff, Va.------------------ 54
415. Appomattox R nr Petersburg, Va.------------------ 1,335
425. Chickamominy R nr Providence Forge, Va.---------- 249
Total drainage area gaged------------------------ 52,398.57
-----------------------------------------------------------------
a Approximately.
The drainage area of Chesapeake Bay is 65,476 square miles. The gaged area, i.e., the sum of the drainage areas of the drainage areas of the gaging stations in table 1, is 52,399 square miles, or 80 percent of the drainage area of the Bay. The greater part of the highland area is gaged; the deficiency in streamflow records is largely in the Coastal Plain. For the purpose of appraising the distribution of gaged areas, the five largest gaging stations--those on the Susquehanna, Potomac, James, Rappahannock, and Appomattox Rivers, total drainage area 47,221 square miles --are considered as measuring flow from the highlands. The remaining gaging stations, then--drainage area 5,152 square miles--measure flow from the Coastal Plain. The Coastal Plain is considered as occupying about one fourth of the drainage area of the Bay, or about 16,370 square miles. Accordingly, about 96 percent of the highland area is gaged, but only about one-third of the Coastal Plain area is gaged.
The inflow to Chesapeake Bay is estimated at five cross-sections in the Bay (fig. 2): A, mouth of the Susquehanna River; B, just above the mouth of the Potomac River; C, just below the mouth of the Potomac River; D, just above the mouth of the James River; and E, the mouth of the Chesapeake Bay (a line between Cape Charles ad Cape Henry).
Figure 2.
Map of Chesapeake Bay showing sections at which inflow is
estimated.
Inflow at these sections is estimated from tables 2-6, which were derived from the relation curves discussed in the preceding section of this report. Inflow at cross-sections A and B is estimated by entering tables 2 and 3, respectively, with the discharge at the Marietta gaging station. The increment of inflow between cross-sections B and C is estimated by entering table 4 with the adjusted discharge at the Potomac River near Washington, D.C., gaging station. The increments of inflow between cross-sections C and D and cross-sections D and E are estimated by entering tables 5 and 6, respectively, with the adjusted discharge at the Richmond gaging station.
Tables 2-6.
Table 2.--Relation table for section A (Susquehanna R at mouth)
-----------------------------------------------------------------
Susquehanna R | Susquehanna R
------------------- Tabular | ------------------- Tabular
at at diff. | at at diff.
Marietta mouth | Marietta mouth
-----------------------------------------------------------------
1,500 1,900 | 20,000 21,400
550 | 5,100
2,000 2,450 | 25,000 26,500
1,100 | 5,000
3,000 3,500 | 30,000 31,500
1,100 | 5,500
4,000 4,650 | 35,000 37,000
1,050 | 5,500
5,000 5,700 | 40,000 42,500
1,050 | 10,000
6,000 6,750 | 50,000 52,500
1,050 | 10,000
7,000 7,800 | 60,000 62,500
1,050 | 10,500
8,000 8,850 | 70,000 73,000
1,050 | 10,500
9,000 9,900 | 80,000 83,500
1,100 | 10,500
10,000 11,000 | 90,000 94,000
1,100 | 10,000
11,000 12,100 | 100,000 104,000
1,000 | 21,000
12,000 13,100 | 120,000 125,000
2,000 | 20,000
14,000 15,100 | 140,000 145,000
2,100 | 20,000
16,000 17,200 | 160,000 165,000
2,100 |
18,000 19,300 |
2,100 |
-----------------------------------------------------------------
1 Includes water diverted to Baltimore and to
Chester, Pa.
Table 3.--Relation table for section B
-----------------------------------------------------------------
Susquehanna Inflow Tabular | Susquehanna Inflow Tabular
River to Bay diff. | River to Bay diff.
-----------------------------------------------------------------
1,500 3,500 | 20,000 26,000
800 | 5,500
2,000 4,300 | 25,000 31,500
1,500 | 5,500
3,000 5,800 | 30,000 37,000
1,400 | 5,500
4,000 7,200 | 35,000 42,500
1,400 | 5,500
5,000 8,600 | 40,000 48,000
1,300 | 12,000
6,000 9,900 | 50,000 60,000
1,200 | 12,000
7,000 11,000 | 60,000 72,000
1,200 | 12,000
8,000 12,300 | 70,000 84,000
1,200 | 11,000
9,000 13,500 | 80,000 95,000
1,200 | 11,000
10,000 14,700 | 90,000 106,000
1,200 | 11,000
11,000 15,900 | 100,000 117,000
1,200 | 23,000
12,000 17,100 | 120,000 140,000
2,300 | 22,000
14,000 19,400 | 140,000 162,000
2,200 | 22,000
16,000 21,600 | 160,000 184,000
2,200 |
18,000 23,800 |
2,200 |
-----------------------------------------------------------------
1 At Marietta, Pa.
Table 4.--Relation table for section B-C
-----------------------------------------------------------------
Potomac Inflow Tabular | Potomac Inflow Tabular
River between diff. | River between diff.
B & C | B & C
-----------------------------------------------------------------
| 5,000 6,900
| 1,250
500 800 | 6,000 8,150
150 | 1,250
600 950 | 7,000 9,400
150 | 1,200
700 1,100 | 8,000 10,600
150 | 1,200
800 1,250 | 9,000 11,800
150 | 1,200
900 1,400 | 10,000 13,000
150 | 2,400
1,000 1,550 | 12,000 15,400
280 | 2,400
1,200 1,830 | 14,000 17,800
280 | 2,400
1,400 2,110 | 16,000 20,200
270 | 2,400
1,600 2,380 | 18,000 22,600
280 | 2,400
1,800 2,660 | 20,000 25,000
280 | 5,800
2,000 2,940 | 25,000 30,800
670 | 5,700
2,500 3,610 | 30,000 36,500
670 | 11,000
3,000 4,280 | 40,000 47,500
1,320 | 11,500
4,000 5,600 | 50,000 59,000
1,300 |
-----------------------------------------------------------------
1 Near Washington, D.C., adjusted for diversions.
Table 5.--Relation table for section C-D
-----------------------------------------------------------------
James Inflow Tabular | James Inflow Tabular
River between diff. | River between diff.
C & D | C & D
-----------------------------------------------------------------
| 5,000 4,500
| 950
| 6,000 5,450
| 950
600 340 | 7,000 6,400
80 | 900
700 420 | 8,000 7,300
80 | 900
800 500 | 9,000 8,200
80 | 800
900 580 | 10,000 9,000
80 | 800
1,000 660 | 11,000 9,800
170 | 700
1,200 830 | 12,000 10,500
180 | 1,400
1,400 1,010 | 14,000 11,900
180 | 1,400
1,600 1,190 | 16,000 13,300
180 | 1,400
1,800 1,370 | 18,000 14,700
180 | 1,400
2,000 1,550 | 20,000 16,100
500 | 3,300
2,500 2,050 | 25,000 19,400
500 |
3,000 2,550 |
950 |
4,000 3,500 |
1,000 |
-----------------------------------------------------------------
1 Near Richmond, Va., includes flow of James River & Kanawha Canal.
Table 6.--Relation table for section D-E
-----------------------------------------------------------------
James Inflow Tabular | James Inflow Tabular
River between diff. | River between diff.
D & E | D & E
-----------------------------------------------------------------
| 5,000 7,600
| 1,500
| 6,000 9,100
| 1,400
600 800 | 7,000 10,500
150 | 1,400
700 950 | 8,000 11,900
150 | 1,400
800 1,100 | 9,000 13,300
150 | 1,400
900 1,250 | 10,000 14,700
160 | 1,400
1,000 1,410 | 11,000 16,100
310 | 1,400
1,200 1,720 | 12,000 17,500
310 | 2,500
1,400 2,030 | 14,000 20,000
320 | 2,500
1,600 2,350 | 16,000 22,500
310 | 2,500
1,800 2,660 | 18,000 25,000
320 | 2,500
2,000 2,980 | 20,000 27,500
770 | 6,500
2,500 3,750 | 25,000 34,900
800 |
3,000 4,550 |
1,550 |
4,000 6,100 |
1,500 |
-----------------------------------------------------------------
1 Near Richmond, Va., includes flow of James River & Kanawha Canal.
As already stated, the discharge at the mouth of the Susquehanna River (section A) is obtained from table 2. Discharges at the mouths of the Potomac and James Rivers are not needed to calculate the inflow to the Bay, but if desired they can be estimated from tables 4 and 6, as 98 percent of the inflow between sections B and C, and between sections D and E, respectively. Of the 14,897 sq. mi. of drainage basin that contribute to Chesapeake Bay between sections B and C, 14,670 sq. mi. (98.5 percent) are in the Potomac River basin; therefore about 98 percent of the total inflow between sections B and C may be considered an estimate of the flow of the Potomac River at its mouth. Similarly, 98 percent of the total inflow between sections D and E may be considered and estimate of the flow of the James River at its mouth.
The land drainage area of Chesapeake Bay is 65,480 sq mi, of which 80 percent is gaged by stream-gaging stations on rivers and streams entering the Bay. The combined drainage area above the three reference gaging stations--Susquehanna River at Marietta, Potomac River near Washington D.C., and James River near Richmond--constitutes 68 percent of the entire drainage area of the Bay exclusive of the water surface of the Bay. The combined drainage area of the three principal river basins at their mouth constitutes nearly 80 percent of the land drainage area of the Bay--the Susquehanna, 42 percent; the Potomac, 22.4 percent; and the James, 15.3 percent (fig.3). During the 10 water years 1951-60 the unit discharge of the Susquehanna River at mouth was 1.47 cubic feet per second (cfs) per square mile of drainage basin, the Potomac, 0.96, and the James, 1.00. The average discharge at the mouths of the eight largest river basins and their drainage areas are given in table 8.
Table 7.--Drainage areas at points indicated
-----------------------------------------------------------------
Point Square miles
-----------------------------------------------------------------
Susquehanna River at Marietta gaging station---- 25,990
Susquehanna River at mouth, section A----------- 27,469
Increment between sections A and B------------ 6,015
Section B--------------------------------------- 33,484
Potomac River at D.C. gaging station------------ 11,560
Potomac River at mouth-------------------------- 14,670
Increment between sections B and C 1---------- 14,897
Section C--------------------------------------- 48,381
Increment between sections C and D------------ 6,843
Section D--------------------------------------- 55,224
James River at Richmond gaging station---------- 6,757
James River at mouth---------------------------- 10,002
Increment between sections D and E 2---------- 10,252
Section E, mouth of Chesapeake Bay-------------- 65,476
-----------------------------------------------------------------
1 Includes 227 square miles on eastern shore of Bay opposite
mouth of Potomac River.
2 Includes 250 square miles south of James River Basin that
contributes to Chesapeake Bay.
Table 8.--Average discharge into Chesapeake Bay, and average discharge of
the principal tributaries at mouth, 1951-1967 water years
-------------------------------------------------------------------------
Stream | Discharge | Drainage area
|-----------------------------------------------
| Cubic feet | Percent | Square | Percent
| per second | of total | miles | of total
-------------------------------------------------------------------------
Chesapeake Bay | 78,210 | 100 | 65,476 | 100
Susquehanna River | 40,290 | 52 | 27,469 | 42
Potomac River | 14,040 | 18 | 14,670 | 22
James River | 10,030 | 13 | 10,002 | 15
Total, three rivers | 64,360 | 82 | 52,141 | 80
| | | |
Rappahannock River | 2,480 | 3.2 | 2,720 | 4.2
York River | 2,420 | 3.1 | 2,660 | 4.1
Total, five rivers | 69,260 | 89 | 57,521 | 88
| | | |
Choptank River | 949 | 1.2 | 795 | 1.2
Patuxent River | 943 | 1.2 | 932 | 1.4
Nanticoke River | 934 | 1.2 | 815 | 1.2
Total, eight rivers | 72,086 | 92 | 60,063 | 92
------------------------------------------------------------------------
Note: Discharges shown in this table were calculated by using all
available streamflow records, which accounted for 80 percent of
the Chesapeake Bay drainage basin; discharge from the remaining
20 percent was estimated on basis of nearby gaged streams.
Figure 3.--
Chesapeake Bay drainage basin showing outline of Susquehanna,
Potomac, and James River basins.
The total estimated inflow to the Bay by months for the calendar years 1951-67 is given in table 9. During this 17-year period the inflow ranged from 7,800 cfs in September 1964 to 230,700 cfs in April 1960. For the two months of highest mean flow, March and April, the range was less, percentagewise, than for the other months. August shows the greatest percentage range because of the extremely wet hurricane month in 1955. The highest October and November were also in 1955, reflecting the two-hurricanes in October 1955 and carryover of high runoff into November. The data in table 9 are shown graphically by short horizontal lines in the chart on the first page of the appendix. The estimated mean monthly inflow at each of the five sections for the period 1951-67 is shown in figure 4.
Table 9.--Estimated monthly mean inflow, in cubic feet per second, into Chesapeake Bay, 1951-67, based on three reference gaging stations.
Note: The following table is reproduced exactly as shown in the original publication. Several very minor arithmetic errors in the annual means have not been corrected. A corrected and updated table with values through the present can be found here.
-------------------------------------------------------------------------------------------------------------------------- Year Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. Mean -------------------------------------------------------------------------------------------------------------------------- 1951 119,400 175,400 148,100 179,100 66,000 87,900 42,100 22,700 16,300 13,600 41,600 82,200 82,100 1952 173,500 123,300 182,100 180,100 142,100 47,000 33,500 30,400 38,800 16,800 68,300 97,100 94,300 1953 136,000 111,100 170,700 129,000 123,600 74,400 23,700 17,000 12,700 10,800 17,900 48,000 72,800 1954 39,700 71,800 135,100 95,200 99,900 45,400 19,100 14,000 13,600 41,600 51,100 78,300 58,700 1955 83,300 79,400 208,800 90,300 46,300 44,900 19,100 93,400 26,800 79,700 74,000 33,300 73,400 1956 27,400 107,900 161,400 161,500 82,800 45,000 49,900 39,500 30,700 36,400 69,400 101,900 76,000 1957 76,400 109,900 114,800 183,800 62,700 37,500 19,500 11,900 17,900 19,700 30,600 93,000 64,400 1958 89,100 72,900 160,900 216,300 154,400 51,500 43,000 40,400 24,900 25,400 37,400 37,500 79,500 1959 72,800 71,900 96,700 138,200 69,800 46,100 20,600 18,900 19,100 55,400 70,500 117,700 66,400 1960 95,500 118,100 84,000 230,700 145,700 92,900 32,100 26,100 42,600 22,100 24,300 20,100 77,300 1961 30,000 144,300 181,400 202,900 111,000 55,700 31,700 29,200 23,200 38,000 31,500 63,800 78,000 1962 78,800 71,800 207,200 195,300 61,000 38,800 21,900 16,800 13,700 31,500 60,500 41,700 69,800 1963 65,800 43,200 228,600 86,400 55,700 40,600 17,200 12,200 10,600 8,600 18,800 38,200 52,400 1964 103,400 80,600 222,700 127,300 88,700 23,600 16,300 11,400 7,800 13,000 14,000 33,200 61,900 1965 65,200 110,300 118,000 112,900 59,300 23,900 13,000 12,000 11,700 21,300 20,500 25,500 49,000 1966 29,600 110,200 130,100 66,500 105,800 30,700 10,500 9,300 23,600 35,000 30,500 61,400 53,300 1967 61,000 67,000 205,100 101,300 120,900 38,700 30,600 47,800 27,500 51,000 67,000 104,600 77,200 Mean 79,200 98,200 162,100 146,900 93,900 48,500 26,100 26,600 21,300 30,600 42,800 63,400 69,800 --------------------------------------------------------------------------------------------------------------------------
Figure 4.
Estimated cumulative mean monthly inflow to Chesapeake Bay at
five sections, 1951-67.
Low-lying streams in the upper Bay basin tend to contribute less, proportionately, at high discharges and more at low discharges than do the Susquehanna and Potomac Rivers. This is illustrated graphically in figures 5 and 6 by the Conestoga Creek and the South Branch Patapsco River. Other streams north of the Potomac, particularly those on the west side of the Bay, display this same tendency. Low-lying streams in Virginia, when compared with the James River near Richmond, do not display this tendency, but decline at a rate more nearly parallel to that of the James, or even more rapidly.
Figure 5.
Unit discharge of Susquehanna River and Conestoga Creek,
April-August 1965.
Figure 6.
Unit discharge of Potomac and South Branch Patapsco Rivers,
April-August 1965.
On a yearly basis the net rainfall on the Bay constitutes only a small part of the total streamflow into the Bay. The average annual rainfall on the Bay is in the range 32-48 inches (Reference 1), and the average annual evaporation is in the range 36-40 inches (Reference 2). If the average annual rainfall is assumed to be 40 inches and the average annual evaporation 38 inches, the net rainfall is only 2 inches, which on the 2,800 square miles of water surface to the Bay is equivalent to about 400 cfs. A net annual rainfall of even as much as 10 inches would be equivalent to only 2,000 cfs, which is less than 3 percent of the average annual inflow to the Bay.
Rainfall on and evaporation from the water surface of the Bay might be significant items in the water budget during months of very low streamflow, but would not be during months of high streamflow. If, during a March that was wet and cool, rainfall exceeded evaporation by 4 in., the net contribution by rainfall on the surface of the Bay would be about 10,000 cfs, which would be negligible in months such as March 1963 and 1964 when the streamflow into the Bay was more than 220,000 cfs. But in dry month such as September 1964 when the streamflow to the Bay was only 7,800 cfs, a net evaporation of as much as 3 in. might reduce the monthly outflow of the Bay almost to zero if ground-water inflow is neglected.
Accurate figures of monthly rainfall on and evaporation from the water surface of the Bay are not readily available. It may be, however, that in many summer months the evaporation is largely offset by rainfall. July and August 1966, for example, were consecutive months of low inflow--the inflow was the lowest for those months since at least 1951--but in each of these months Weather Bureau records from the Eastern Shore of Maryland and Tidewater Virginia, although showing exceedingly variable amounts of rainfall and evaporation, indicate that net evaporation probably was not significant.
There are several large diversions from streams draining into Chesapeake Bay, but most of the diverted water is returned to the Bay as effluent from sewage treatment plants or by other means. The largest diversions, those from the Potomac River, have been considered in developing the procedures given in this report, and so do not affect the accuracy of the monthly estimates of inflow. A greater part of the wastage at the Back River treatment plant of the city of Baltimore is likewise considered. The diversions and wastage not adjusted for are relatively small and have little effect of the accuracy of the estimated inflow.
A large diversion from the Bay itself, which is not adjusted for, is the Chesapeake and Delaware Canal, a sea-level navigation canal. The water may move in either direction, depending on the tide, but the Corps of Engineers has found that there is a net movement of water eastward from Chesapeake Bay to the Delaware River. This canal is discussed in greater detail in a following section of this report.
A source of error in the estimated inflow--if not adjusted for--might be regulation of the monthly flow at reference stations, if the regulation were comparatively large. If, for example, the flow of the Potomac River near Washington, D.C., adjusted for diversion, were 1,000 cfs, the estimated inflow between sections B and C given by table 4 is 1,550 cfs, which indicates that 550 cfs would be contributed from the drainage area of the Potomac River downstream from Washington. Assume, then, that during the month an average of 100 cfs had been released from storage somewhere upstream so that the natural flow at Washington were only 900 cfs. Table 4 would then show 1,400 cfs inflow between sections B and C, or 500 cfs from the drainage area downstream from Washington. Thus, the estimated inflow below Washington would be in error by 50 cfs. Unless the regulation were much greater than that used in this example, however, the effect on the estimated inflow would be negligible.
Although flow records used in estimating monthly stream discharge into Chesapeake Bay are subject to some correction because of diversions above and below measuring stations, not all such corrections have been made because of their small magnitude and because most of the diverted waters enter the Bay not too far from the natural routes. The amounts not adjusted for are well within the probable limits of accuracy of the estimates of flow into the Bay. For example, even during very low months diversions from the Susquehanna River that are not adjusted for are only about 1 percent of the flow of the Susquehanna River at its mouth, and wastage into the Potomac River that is not adjusted for is only about 2 percent of the flow of the Potomac River at its mouth. Practically all the water diverted from the James River below the Richmond gaging station is wasted back into the river.
The principal diversions and wastage on upper Chesapeake Bay not adjusted for in the monthly release are as follows (fig. 7): (1) diversion from Chesapeake Bay to the Chesapeake and Delaware Canal, which averages about 1,000 cfs; (2) diversion from the Susquehanna River basin to the Chester, Pa., area, which in 1967 averaged 40 cfs; (3) diversion from the Susquehanna River to the city of Baltimore, which in 1967 averaged 39 cfs; (4) effluent of 371 cfs by the city of Baltimore at the Back River treatment plant, less diversions of 318 cfs previously adjusted for, netted an average of 53 cfs wastage during 1967; and (5) wastage by the Washington Suburban Sanitary District into the Potomac River at the D.C. treatment plant, which in 1967 averaged 77 cfs. These diversions and wastage, in greater detail, including those on the Potomac River, which are adjusted for, are as follows:
Figure 7.
Schematic diagram showing routes of water diverted from major
streams flowing into upper and middle Chesapeake Bay.
A publication by the Corps of Engineers, Committee on Tidal Hydraulics, dated August 1965 and entitled "Inland Waterway between Delaware River and Chesapeake Bay - Problem of Disposal of Material to be Removed from a Portion of Channel in the Chesapeake Bay" states the "under present conditions (27 x 250 foot channel), the Chesapeake and Delaware Canal carries approximately 43,000,000 cubic feet more flow eastbound than it does westbound per tide cycle of 12.42 hours during normal tides." The estimated 43,000,000 cubic feet in 12.42 hours is equivalent to an average of about 960 cfs, or about 30 billion cubic feet per year. A pamphlet issued by the Philadelphia District, Corps of Engineers, dated April 1967 and entitled "Inland Waterway, Delaware River to Chesapeake Bay - Historic Chesapeake and Delaware Canal" states that "The mean range (of tide) at the Delaware River end is approximately 5-1/2 feet while at the western end of the canal proper it is about 2 feet *** The mean level of the water surface at the western end is about 0.3 foot higher than mean river level in the Delaware at the eastern end." The canal is in the process of being enlarged from its present 27 x 250 foot channel to a 35 x 450 foot channel, which will more than double its cross-sectional area. When the enlargement has been completed the canal will likely carry proportionately more water from Chesapeake Bay than it does now (P. N. Walker, written commun., June 7, 1968).
An average of 40 cfs (26 mgd) was diverted from Octoraro Creek, tributary to the Susquehanna, to the Chester, Pa., area in 1967. The maximum monthly rate was 48 cfs (30.8 mgd) in June. As the point of diversion is downstream from the measuring point on the river, the 40 cfs should be subtracted from the flow at section A (the monthly release makes no adjustment). The waste is discharged into the Delaware River after being given primary treatment. The average diversion of 40 cfs was less than 0.1 percent of the average flow at section A. The maximum monthly diversion of 48 cfs in June was 0.2 percent of the flow at section A that month.
An average of 39 cfs (25 mgd) was diverted from the Susquehanna River. As the point of diversion is downstream from the measuring point on the river the diversion should be subtracted from the flow at section A (the monthly release makes no adjustment). This diversion was less then 0.1 percent of the average flow at section A. The maximum diversion was 155 cfs (100 mgd) in February, which was 0.5 percent of the flow at section A that month. Had the diversion of 155 cfs been made in September, the month of lowest streamflow, it would have been 1- 1/2 percent of the flow at section A. During six months in 1967-- April, May, and September through December--no water was diverted from the Susquehanna. The present pumping capacity for this diversion is 387 cfs (250 mgd). Had 387 cfs been diverted in September 1967 it would have amounted to 3-1/2 percent of the flow at section A.
An average of 222 cfs (143 mgd) was diverted from the Gunpowder and 110 cfs (71 mgd) from the Patapsco, or 332 cfs (214 mgd) from the two sources. The points of both of these diversions are upstream from the measuring points on the two rivers, so no adjustment is applicable to the records of inflow to the Bay. The sum of three diversions--from the Susquehanna, Gunpowder, and Patapsco--371 cfs (239 mgd), is wasted into the Bay at Baltimore's Back River treatment plant, and should be added to the inflow entering between sections A and B. However, the basic computations from which the monthly estimates are made include a flat adjustment of 318 cfs (205 mgd), which in 1967 left an average of only 53 cfs (34 mgd) unadjusted for. (Some of this waste is used by the Bethlehem Steel Company, which discharges its effluent at Sparrows Point, and whether or not all of it is subsequently returned to the Bay is not known.) The 53 cfs in 1967 was 0.1 percent of the average inflow above section B, and was 0.8 percent of the inflow to the Bay between sections A and B. During months of low streamflow these percentages could be somewhat greater, particularly if months of maximum diversion coincided with months of seasonal low streamflow.
The two diversions for the Susquehanna averaged 79 cfs in 1967, or slightly less them 0.2 percent of the average flow at section A. The diversion by the city of Baltimore is supplemental and variable, but the diversion to the Chester area appears to be more uniform. If the average diversion to Chester, 40 cfs, is added to the maximum monthly diversion to Baltimore, 155 cfs in February, the total is 195 cfs, which in February would have been 0.6 percent of the flow at section A.
An average of 77 cfs (50 mgd) was diverted from the Patuxent River to the Washington Suburban District in 1967. As the point of diversion is upstream from the measuring point on the river, the diversion is accounted for in the records of streamflow entering the Bay. The effluent is wasted into the Potomac River at the Washington, D.C. treatment plant and should be added to the inflow to the Bay between sections B and C (no adjustment is made in the monthly release). The 77 cfs was 0.1 percent of the average flow at section C, and 0.5 percent of the average inflow between sections B and C. During September, the month of lowest streamflow, the diversion of 79 cfs (51 mgd) was 0.3 percent of the flow at section C, and 0.9 percent of the inflow between sections B and C.
The record of flow used for the Potomac River in preparing the monthly release represents the sum of: (1) the amounts of water diverted for public supply of Washington D.C., Washington Suburban Sanitary District, and Rockville; (2) the amounts released into the lower reaches of the Chesapeake and Ohio Canal; and (3) the amounts discharged into the head of the Potomac estuary just upstream from Chain Bridge. Item 3 includes water diverted to the Corps of Engineers' hydro plant, which reenters the river just below Little Falls (and below the gaging station). The flow values for the Potomac River used in preparing the monthly release are determined by adjusting the flows at the gaging station, Potomac River near D.C., for the diversions cited in items 1 and 2, and the diversion to the hydro plant. A portion of the water diverted to Washington, D.C. is treated and pumped to Virginia communities for public supply (a few cubic feet per second is diverted directly to Fairfax). Except for the water which passes through the hydro plant, virtually all the water diverted returns to the potomac River estuary either through the Washington, D.C. treatment plant or the treatment plant in metropolitan Virginia. The water diverted to the C & O Canal returns to the river at the terminus of the Canal at Georgetown.
The total diversion from the Potomac River in 1967 averaged 400 cfs, which was distributed about as follows: 230 cfs to Washington, D.C. water users; 70 cfs to the Washington Suburban Sanitary District; 50 cfs to communities in Virginia; 42 cfs to the hydro plant; and a few cubic feet per second each to Rockville and the C & O Canal.
The relation curves from which tables 2-6 are derived are well defined by the ten yearly points for 1951-60 (water years) throughout the range of those points. The curves as defined by the yearly points do not, however, cover the range required for monthly estimations of inflow. To extend the curves, at both the high and low ends, mean monthly inflow for the ten years was computed for the two high months March and April, and for the four low months, July to October. To further define the low ends of the curves, inflows for the ten individual Septembers--generally the lowest month--were computed. The monthly points scatter considerably but help define the low ends of the curves.
Streamflow records for the 10-year period show that the patterns of streamflow around the Bay can vary considerably from year to year. For example, a rise on the Susquehanna may have no counterpart on either the Potomac or the James. Even within a comparatively small area, in any given month the flow in one stream may be substantially less than in the same month of the preceding year, while in a nearby stream the flow in that month may be substantially greater than in the preceding year. If the pattern of flow from gaged areas is erratic it is safe to assume that the flow from nearby ungaged areas is equally erratic, and that estimates of flow from ungaged areas are equally erratic and subject to considerable error. These errors, however, are likely to be both plus and minus, and should to some extent tend to balance each other.
It is not possible to make a rigorous determination of the accuracy of the estimates obtained by use of tables 2-6, as the basic computations contain inherent errors owing to the fact that the inflow from 20 percent of the drainage area of the Bay was estimated. As nearly as can be determined, the standard error of the monthly estimates of total inflow to the Bay is about 20 percent, and that of yearly estimates about 10 percent.
The accuracy of the estimates could be improved by including gaging stations on one or more representative coastal streams as reference stations. The three reference stations now used measure streamflow that originates mainly in the highlands, so the accuracy of estimates based on those three stations is contingent, at least to some extent, on the uniformity in the pattern of streamflow throughout the Chesapeake Bay basin. Streamflow records show that the pattern of monthly streamflow can vary considerably, but that the yearly pattern is much more uniform. This is confirmed by the plotting of points on the relation curves: the ten yearly points plot very close to the average curve, but some individual months show considerable deviation, both above and below, from the average curve.
The fact is emphasized that the estimates of inflow at section E are estimates of total surface inflow to the Bay, which theoretically would equal the outflow to the ocean if adjustments were made for all diversions and wastage, for precipitation on and evaporation from the water surface of the Bay, and for ground-water inflow. During very low months when evaporation and precipitation might be significant items in the water budget, adjustments can be estimated on basis of climatic records collected by the Weather Bureau at points around the Bay.
Ground-water inflow is largely an unknown quantity, as no comprehensive estimate of it has ever been made. Ground-water inflow consists of two main components: (1) direct seepage from water-table aquifers along the shore, and (2) upward leakage into the Bay from artesian aquifers lying beneath it. The U.S. Geological Survey has estimated the upward leakage to be about 250 cfs, qualifying the estimate as possibly being in error by an order of magnitude but has made no estimate of the direct seepage along the shore (E. G. Otton, written comm., August 17, 1967).
The outflow of the Bay could be gaged by techniques now available, but the project would be extremely involved and costly. Even if inflow and outflow could be measured within say 1 percent, the difference would be relatively very small and subject to such large percentage errors as to be meaningless. Hence, for the purpose of isolating gains or losses in the Bay itself, gaging the outflow by mechanical means would not be practicable.