HYDROLOGY OF NEW YORK 



687 



Minimum Elevation 



roiui 



Area of flow (cubic of overflow 

 surface feet In 24 above tide 

 (acres) hours) (feet) 



Jamaica . 

 Brookfield 



Clear stream 

 Valley stream 

 Rockville .... 

 Hempstead . . 



40.00 419,315 7.90 



8.75 265,098 15.40 



1.07 100,448 11.50 



17.78 325,291 12.80 



8.00 353,388 12.60 



32.52 1,054,713 10.60 



The same streams were measured in October and November, 

 1851, and the aggregate result then was 3,137,500 cubic feet. With 

 the exception of Clear stream, they were again measured in 

 October, 1852, the result then being 2,606,300 cubic feet in 24 

 hours. 



According to a survey made by Theodore Weston in the fall 

 and winter of 1859, the catchment area of the streams originally 

 taken for the municipal supply of Brooklyn was found to measure 

 46.8 square miles, but subsequent measurements have placed it at 

 49.9, which is the figure now used. 1 



The drainage grounds lie mainly on the Hempstead plains, 

 although a small portion may be considered as lying on the 

 southern slope of the central ridge. The ridge slopes are com- 

 posed of clay and alluvial earth, with little power of retaining 

 water. Hempstead plain, on the other hand, consists of a very 

 uniform deposit of sand and gravel with occasional thin veins 

 of clay ; hence Hempstead plain is largely receptive and retentive 

 of water. The sand and gravel on this plain serves two purposes 

 as regards the rainfall sinking into it: (1) It retains the water, 

 only gradually delivering it to the surface in the valleys of the 

 brooks or on or near the seashore in the form of springs; (2) it 

 filters and purifies it, the gravel and sand performing the func- 

 tion of a natural filter bed. It is considered that but a small 

 portion of the ground water of this gravel plain has been derived 

 from the rainfall of any single year. The greater portion of it 

 is considered to have collected during a series of years. Borings 

 and open wells show that this ground water has a nearly uniform 

 inclination toward the south shore of about 12 feet per mile. 



*As to the difficulty of determining just what the catchment area of any 

 one of these streams actually is, see De Varona's History and Description 

 of the Brooklyn Water Works, 1896. 



