310 



FORESTRY AND IRRIGATION 



July 



from the weekly mean, which expres- 

 ses the regularity of precipitation, is 

 about 78 per cent. As far then as this 

 single factor is concerned, no differ- 

 ence between the two streams in reg- 

 ularity of flow may be expected. 



2. The same similarity holds in the 

 matter of temperature which has an 

 important bearing upon stream flow 

 through its influence upon evapora- 

 tion. The seasonal and annual means 

 are practically the same for the two 

 drainage areas and no difference in 

 stream flow can be attributed to this 

 source. 



The Wallkill basin is, in effect, a 

 broad, rolling valley of very moderate 

 slopes. This stream drains a region 

 of non-resistant shales and limestones 

 which have been eroded into a series 

 of level, winding valleys, separated by 

 low, gentle divides, 600 to 900 feet 

 in height. The valleys follow the soft- 

 er limestone areas, while the low 

 divides mark the outcrops of harder 

 shale. The average grade of slope of 

 the entire basin is 6.68 per cent, and 

 the average fall of the stream 10 feet 

 per mile, two facts which summarize 

 in themselves the undulating charac- 



The Upper Wallkill Basin 



3. With reference to the important 

 factor of topography, however, mark- 

 ed differences are found in the two 

 catchment areas. The Wallkill River 

 of southeastern New York drains the 

 valley region of the Appalachian belt. 

 a region of old geological formations, 

 worn down by prolonged erosion, and 

 of low, rolling topography. Esopus 

 Creek, on the other rnnd, drains the 

 Catskill Plateau, a region of much 

 more recent and less eroded geologi- 

 cal structure and of steep and rugged 

 topography. 



ter and gentle slopes of this water- 

 shed. 



The catchment area of Esopus 

 Creek, on the other hand, consists 

 very largely of steep mountain slopes. 

 The Catskill region, drained by this 

 stream, is composed in the main of 

 shales and sandstones of comparative- 

 ly recent origin which have been 

 sharply eroded into a series of high 

 ridges separated by dee]), gorge-like 

 valleys. On many of the slopes the 

 resistant rock is exposed as bare, out- 

 cropping ledges which often form a 



