GEOLOGY 



and 20^ in the Lea basin, giving a total average yield for the two basins 

 of about 132 million gallons of water per diem. Also that in three 

 successive years the average annual supply from these two catchment- 

 basins may be from 35 to 40 per cent, less than this average, and in six 

 successive years about 25 per cent. less. In the same paper the inference 

 is drawn that too much water is being taken by the New River and East 

 London Water Companies from the basin of the Lea for the welfare of 

 our county, and that the same would be the case in the basin of the 

 Colne if water were supplied to London from near Harefield as has been 

 proposed. That the plane of saturation in the valley of the Lea is being 

 unduly lowered artificially, which can only be by excessive pumping 

 from the deep wells of these companies, is shown by the following table 

 which gives the average rainfall in Hertfordshire and average flow of the 

 Chadwell Spring near Ware for twenty-four years in six-yearly periods, 

 with ratios to the rainfall of 1842 to 1899 (April to March), and to 

 3,600,000 gallons per diem as the accepted mean flow of the spring up 

 to at least the year 1874. The last column shows how the flow is 

 rapidly decreasing in relation to the annual rainfall. 



RAINFALL IN HERTFORDSHIRE AND FLOW OF THE CHADWELL 



SPRING COMPARED 



Some of our rivers vary greatly throughout the year, and from one 

 year to another, in the position of their source. After a winter of heavy 

 rainfall the inclination of the plane of saturation in the Chalk is raised, 

 and cuts the bed of the river near the head of its valley ; but after a 

 winter of small rainfall, and in the summer and to a greater extent in the 

 autumn, the inclination of this plane is less, and cuts the bed of the 

 river some distance down its valley ; and wherever the plane of satura- 

 tion rises into or cuts a river-bed on the permeable Chalk, there is the 

 source of the river for the time being. The Ver is a good example of 

 these variable rivers, in some years rising above Markyate Street, and in 

 others below Redbourn. 



Rivers which are called ' bournes ' are merely extreme instances of 

 this phenomenon. Such rivers only flow occasionally after a very heavy 

 rainfall, and especially when a wet winter is preceded by a prolonged 

 wet period. They usually begin to flow in the spring, as soon as the 

 rain has had time to percolate through the Chalk to its plane of satura- 



29 



