330 ADJUSTMENT OF RIVERS 



tion of a system often radically different from the original one. 

 Even after a river system has become maturely adjusted, a reeleva- 

 tion of the country may produce a new and entirely different adjust- 

 ment, by changing the relation of the folds and outcrops of hard 

 and soft strata to the base-level. A region of great antiquity which 

 has repeatedly been worn down and reelevated will have experienced 

 many revolutions of its drainage systems. In the difficult work of 

 deciphering these complex histories all the indications of abandoned 

 stream channels must be carefully examined. Some of these, like 

 gravel deposits or wind gaps, are easy to find and to trace out, but 

 subsequent denudation will frequently have removed the gravels 

 and otherwise masked the old stream courses. In rocky regions 

 a welcome indication of ancient stream courses is often given by pot 

 holes, which are deep, circular, well-like openings excavated in the 

 rock, and wherever they occur was once the bed of a stream. A 

 pot hole is made by the gyration of stones which are whirled around 

 by an eddy in the current or at the foot of a waterfall The con- 

 ditions must remain constant for some time for the hole to be cut 

 to any depth, some of them being as much as twenty feet deep. 



Accidents to Rivers. — This term is employed to express the 

 interruptions which hinder or prevent the normal development of 

 a river system. The diastrophic changes and their effects we have 

 already considered, but there are others which should be men- 

 tioned. A change of climate from moist to arid greatly interferes 

 with the development and adjustment of a river system. Many 

 stream channels are abandoned and others are occupied only after 

 rains, while the reduced flow in the permanent streams diminishes 

 their erosive powers. Large areas, like the Great Basin region, 

 may have no outlet to the sea, because the mountain streams all 

 lose themselves in the desert sands. Lake Bonneville (see p. 146) 

 had an outlet until the increasing dryness of the climate so lowered 

 its waters that the outlet could no longer be reached, evaporation 

 exceeding influx. Great lava flows may obliterate the drainage 

 system of a region and compel the establishment of an entirely 

 new one, as has happened in southern Idaho and southeastern 

 Oregon, a region of exceedingly immature topography and drain- 



