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 Hertfordshire Mean Rainfall Spring Period Flow of to Chadwell Spring Rain- Summer Winter Year fall ins. ratio ins. ratio ins. ratio gals, per diem ratio ratio 1875-81 16-85 129 H'57 I II 31-42 I2O 3,640,000 101 84 1881-87 12-77 9 8 14-71 112 27-48 105 3,073,000 85 81 1887-93 13-10 IOO 12-07 92 25-17 9 6 2,644,000 73 76 1893-99 11-15 85 13-52 103 24-67 94 2,056,000 57 61 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
