DRAIXAGE. 55 



to the present estuaries from the depths of the deepest portions of the 

 channels. The greatest depths of the lower Chesapeake Bay a re about 150 

 feet in the vicinity of Cape Charles City. Thus we feel confident that when 

 the depressions of the estuaries now filled with the tidal waters were ex- 

 cavated, the country must have been at least 150 feet higher than at present 

 and probably much higher since the old channel has been traced eastward of 

 the capes in water of considerably greater depth. 



The water in the lower portions of all these estuaries is almost as saline 

 as the sea water but in ascending the streams the water becomes brackish 

 and near the head of tide is essentially fresh. The terms river and creek as 

 applied to these estuaries seem to be misnomers since throughout most of 

 their courses the upward-moving tidal flood-current is almost as pronounced 

 as the downward current which is a combination of the normal flow of 

 the stream and the ebb tide. A difference is noted, liowever, as one ascends 

 the streams in the relative duration of flood and ebb tide. For example, at 

 the mouth of the Eappahannock Eiver the mean duration of flood tide is 

 5 hours and 50 minutes and of ebb tide 6 hours and 35 minutes while at 

 Fredericksburg in the upper part of the estuary the current moves down 

 stream almost all the time. 



The slow currents in the estuaries are not strong enough to transport the 

 materials brought in by the side tributaries and as a result the estuaries are 

 being rapidly filled. Every heavy rain causes an immense amount of mud 

 to be carried down by the side streams, nearly all of which is dropped before 

 the water passes into the ocean. Sometimes for weeks at a time the water 

 in the Potomac, Eappahannock, and James rivers will have a distinctly 

 muddy color because of the great quantity of fine particles held in suspen- 

 sion, a large part of which is deposited before the waters enter Chesapeake 

 Bay. In the early days many of the streams were navigable many miles 

 above the points where vessels are now able to go, while in other streams the 

 channels can only be kept open by dredging. The width of the estuaries 

 is al^o decreasing due to the encroachment of the marsh areas which so com- 

 monly border these streams. 



Marshes or terraces do not everywhere border the estuaries, for in many 

 places there are steep, almost vertical bluffs of considerable height which 

 afford excellent exposures of the underlying rocks. These are formed where 

 the meandering streams have cut away the lower-lying terraces and opposite 

 them are almost invariably extensive marsh areas. In Westmoreland County 

 the Nomini Cliffs, which extend for several miles along the Potomac Eiver 

 and reach a niaAimum elevation of 200 feet, are the highest bluffs in the 



