232 
TYPHOID FEVER IN DISTRICT OF COLUMBIA. 
The South Branch rises by two “forks” in the northern part of 
Highland County, Va. These flow in a northeasterl}^ direction and 
unite about 6 miles west of Petersburg, W. Va. The South Branch, 
thus formed, continues in a northeasterly direction, and, as above 
stated, unites with the North Branch to form the mam stream. 
At Moorefleld the South Branch is joined by the Moorefield River, 
a beautiful stream which rises in the northeastern corner of Highland 
County, Va. 
At Harpers Ferry, 45 miles above Great Falls, the Potomac is 
joined by the Shenandoah River, which drains an area of about 3,000 
square miles of the valley of Virginia, exceeding somewhat in its drain- 
age area the combined areas drained by the North and South branches 
of the Potomac River. The Shenandoah is formed at Riverton, 
W. Va., by the confluence of the North with the South Fork. The 
South Fork of the Shenandoah rises by three “rivers,” North, Middle, 
and South, in Augusta County, Va., which unite at Port Republic. 
The North Fork of the Shenandoah rises in the northwestern corner 
of Rockingham County, Va. The two forks, separated by Massa- 
nutten Mountain, take a general northeasterly direction, and, as has 
been stated, unite at Riverton to form the main stream, which con- 
tinues in the same general direction to Harpers Ferry. There are 
several dams, besides shoals and rapids, which obstruct the smooth 
flow of this important stream. 
The North Branch of the Potomac, before it is joined by the South 
Branch, receives several tributaries, the most important of which are 
New and Patterson creeks from the south, and Savage River, Georges 
Creek, and Wills Creek from the north. The main stream in its course, 
before it is joined by the Shenandoah, receives the Great Cacapon 
River, Back Creek, and Opequon Creek from the south, and Cono- 
cocheague and Antietam creeks from the north. 
Below Harpers Ferry and above Great Falls the Potomac is joined 
by Catoctin Creek (Virginia) and Goose Creek from the south, and by 
Catoctin Creek (Maryland), Monocacy River, Seneca Creek, Muddy 
Branch and Watts Branch from the north. 
The Potomac, from Fairfax Stone to Great Falls, is 256 miles long, 
and it averages a fall of 12 feet a mile. As a whole it drains narrow 
fertile valleys without any lakes to retard the flow, making it subject 
to frequent floods. Besides the natural shoals and ledges which 
obstruct the stream, the Chesapeake and Ohio Canal Company has 
erected dams in order to get water to feed the canal. 
There are seven of these dams, each of which is numbered. Dam 
No. 7 is at Cumberland, No. 6 just above Great Cacapon, No. 5 about 
6 miles above Williamsport, Md., No. 4 about in a line from Williams- 
port to SI herdstown and about 67 miles above Great Falls, No. 3 
at Harper "erry. No. 2 half a mile east of Seneca Creek, and No. 1 
