Ch. 16] FALL-LINE HARBORS 29o 



nant-water harbors except for small local stream flow, and the inter- 

 change of water between the river and the harbor. Examples of off- 

 river harbors are Greenville, Mississippi, and Freeport, Texas. 



These off-channel harbors generally have little difficulty with sand 

 and gravel, though the channel from the river to the harbor may be 

 subject to severe shoaling. Such harbors, however, do have difficulty 

 with silt and clay brought in by the interchange of the waters in the 

 harbor and the river. This silt and clay may settle in both the entrance 

 channel and the harbor proper. 



Solutions to the problems involved in the development and main- 

 tenance of such harbors and entrance channels have for the most part 

 been: 



(1) Dredging to remove shoal material from entrance channel and 

 harbor proper. 



(2) Construction of training walls and dikes at river entrance to 

 decrease rate of shoaling in entrance channel. 



(3) Construction of locks or floodgates that will decrease the inter- 

 change of water between the river and the harbor during time of 

 turbid flow in the river. 



Advances in the knowledge of the following phenomena would be 

 helpful in the development and maintenance of such harbors : (a) char- 

 acteristics of movement of bed load in a flowing stream to enable better 

 selection of entrance channel sites and design of protective work at 

 the river entrance; (6) intermingling or exchange of silt-laden river 

 waters with less turbid waters in the harbor as a step toward developing 

 means to lessen the rate of interchange. 



FALL-LINE HARBORS 



Economic considerations have frequently brought about the develop- 

 ment of harbors at the transition in a river course from a relatively 

 turbulent mountain river to a more or less placid tidal estuary. This 

 transition point, or fall line, is very well marked in certain areas, 

 particularly along the eastern seaboard of the United States, where the 

 Appalachian foothills meet the tidewater area. Examples of fall-line 

 harbors developed in such areas are: Troy, New York; Trenton, New 

 Jersey; Washington, D. C; and Richmond, Virginia. 



These fall-line harbors are generally troubled with shoaling due to 

 sand and gravel, or even small boulders, brought down by the relatively 

 high-velocity flood flows in the mountainous section into the harbor 



