249 



IM.AXATIO.X STKKA.M I'lKACV 



BY 



CrA'DE A. IMaloit. 



Tntrodnction. 



Definition of (Stream P i ra r i/. -i^trvam ])iriicy is not ati uncommon phase 

 of stream ndjiistment dnrins Ihc dovcloidiiont of tlie erosion cycle. Stream 

 piracy consists of the diversion of n jtorllon of a stream system usually hj 

 the encroachment of a portion of a more vigorous stream system. Thus 

 the drainage area of the more vigorous stream is enlarged at the expense 

 of the weaker stream. This encroahcment and divei-sioii take place slowly 

 liy stream adjustment during the development of the erosion cycle. Stream 

 piracy has nothing in common with stream derangement. Streams are fre- 

 ((uently thrown out of their normal courses by such proces.>-es as glaciation 

 vtdcanism, deposition of sand by the wind, etc.. 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 

 nicnt are very different physiographic processes. 



Types of ^trc(i)n Pinicii. — lii-oadly 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 vigoi'ous 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 

 .-ire Kaaterskill and Plaaterskill creeks on the frontal scarp of the fatskill 

 .Mountains. Here these streams have undercut the headwaters of Schoharie 

 (^reek on the back-slope of the cuesta. A notable case of this kind in south- 

 ern Indiana has recently been described by the writer.* Muddy Fork of 

 Silver Creek of Clark and AVashington counties lias taken some 35 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 ero.sion 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 

 ca.se of the Shenandoah liiver and Beaverdam Creek near Snickers Gap in 

 the Harpex's Ferry locality as described by Willis is a classical example of 

 this form of headward erosion stream piracy. 



A second type of stream i)iracy may be called the subterranean tyix?. 

 This type is fairly c(tmmon in limestone regions where subterranean drain- 

 age exists. A favorable condition is considerable relief of the limestone 



i"Some special Physiographic Features of the Knobstone Cuesta region of 

 southern Indiana — An Example of E.xplanafor.v Physiography," Proceedings of the 

 Indiana Academy of Science, 1919. 



