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PLANATION STREAM PIRACY. 
BY 
CrypE A. MALorTT. 
Introduction. 
Definition of Stream Piracy.—Stream piracy is not an uncommon phase 
of stream adjustment during the development of the erosion cycle. Stream 
piracy consists of the diversion of a portion of a stream system usually by 
the encroachment of a portion of a more yigorous stream system. Thus 
the drainage area of the more vigorous stream is enlarged at the expense 
of the weaker stream. This encroahecment and diversion take place slowly 
by stream adjustment during the development of the erosion cycle. Stream 
piracy has nothing in common with stream derangement. Streams are fre- 
quently thrown out of their normal courses by such processes as glaciation 
vuleanism, deposition of sand by the wind, ete., but such stream derange- 
ment is not considered as a phase of stream adjustment and does not 
come under the term stream piracy. Stream piracy and stream derange 
ment are very different physiographic processes. 
Types of Stream Piracy—PBroadly speaking there are three types of 
stream piracy which may accompany stream adjustment. The most com- 
mon, perhaps, is the headward erosion type. A condition favoring the devel- 
opment of this type is the cuesta where vigorous streams flow down the 
steep scarp. Such streams are frequently able to etch their way by 
headward erosion through the frontal scarp of the cuesta and divert the 
head-waters of more sluggish streams upon the back-slope. Examples of the 
headward erosion type have been described by Gilbert, Davis, Darton, and 
others. Probably the most famous examples of the headward erosion type 
are Kaaterskill and Plaaterskill creeks on the frontal scarp of the Catskill 
Mountains. Here these streams have undercut the headwaters of Schoharie 
Creek on the back-slope of the cuesta. A notable case of this kind in south- 
erm Indiana has recently been described by the writer... Muddy Fork of 
Silver Creek of Clark and Washington counties has taken some 385 square 
miles of the drainage basin of one of the forks of Blue River. In this case 
the pirate stream had its beginning on the steep slope of the Knobstone 
escarpment. This form of headward erosion piracy usually gives rise to a 
barbed drainage pattern. Another form of headward erosion piracy is 
common in such mountains as the Appalachians, where not doubt the trellis 
drainage patterns so prevalent have been brought about by stream adjust- 
ment in which headward erosion piracy has played the chief role. The 
case of the Shenandoah River and Beaverdam Creek near Snickers Gap in 
the Harpers Ferry locality as described by Willis is a classical example of 
this form of headward erosion stream piracy. 
A second type of stream piracy may be called the subterranean type. 
This type is fairly common in limestone regions where subterranean drain- 
age exists. A favorable condition is considerable relief of the limestone 
“Some special Physiographic Features of the Knobstone Cuesta region of 
southern Indiana—An Example of Explanatory Physiography,’’ Proceedings of the 
Indiana Academy of Science, 1919. 
