48 J. HOPKINSOX — HERTFORDSHIEE EAIIS^FALL, 



augment the volume of water in our rivers, sometimes to such 

 an extent that they cause floods, but their effects soon cease. 



Even the estimated extra supply of 33 million gallons per diem 

 derived from rain running off impermeable beds should not all be 

 counted on as available for water-supply. It is essentially 

 a winter supply, and in winter less water is required than in 

 summer. We should therefore base our estimate of what we can 

 rely upon on the available summer supply, and this is almost 

 entirely derived from the rain which percolates through the Chalk 

 in winter. With a gentle rain, even if long continued, it is seldom 

 that any runs off the surface into our rivers in summer ; it is only 

 when there is a very heavy rain that it does so, and then only 

 on impermeable beds, except perhaps once a year or so when about 

 an inch falls in a few hours. The occasional flooding of the Lea 

 Yalley below Feilde's Weir, when very heavy rain falls in the 

 summer, is, doubtless, almost entirely due to the large Tertiary 

 area in the Valley of the Stort (two and a half times that of the 

 Chalk area). In the Colne Valley I have never seen any of 

 the tributaries of the Colne flooded in summer. Their valleys are 

 entirely cut out of the Chalk, except that they have about five 

 square miles of Tertiary outliers in them. But of the 70 square 

 miles in the Colne Valley alone {i.e., excluding its tributaries) 

 above Harefield, about half are occupied by Tertiary beds, and 

 when two or three inches of rain fall in as many days, which very 

 rarely happens, or when a long wet period culminates in a heavy, 

 though smaller, fall than this, whether in winter or summer, the 

 valley is flooded for a day or two about Watford and Rickmans- 

 worth. This water quickly runs off, and its presence is not an 

 indication that the Chalk is saturated up to it. The Chalk 

 doubtless receives some flood-water from impermeable beds, as in 

 swallow-holes, but I believe that there is only one district where 

 any very considerable quantity is thus absorbed, and that is the 

 upper valley of the Colne near North Mimms, which will be 

 referred to presently. 



It might be suggested that flood-water could be utilized by 

 making reservoirs where our valleys are subject to inundation, but 

 such water is wholly unsuitable for drinking purposes. Excessively 

 heavy rain carries off everything loose with which it comes in 

 contact, and as much of our land is heavily manured, a consider- 

 able quantity of manure must be carried off it. Nearly all our 

 Sewage Farms are on the banks of our rivers, necessarily on low 

 land and liable to be flooded, water at such times running off them 

 into the rivers. Water, therefore, should not be drawn from our 

 rivers in times of flood, much less should flood-water be stored for 

 future supply. 



The results given above are based on the average rainfall and 

 percolation for a long period. Eut we cannot rely upon the 

 average, or anything near the average, year by year, or for a 

 short series of years. Taking the average ratio of percolation 

 in the experiments on percolation through soil at Nash Mills, 



