WATER SUPPLY 
237 
from Thirlmere in the Lake District, and Liverpool and Bir- 
mingham their supplies from artificial lakes in Wales. 
Reservoir SiTes—The site of a storage reservoir needs 
careful geological survey to avoid loss by percolation and to 
secure safe foundations for the dam. Sandstone and lime- 
stone are unsuitable, for their permeability may lead to serious 
leakage. Loss of water through a thin porous bed can be 
prevented by a puddle trench—a deep trench beside the dam 
filled with packed clay or concrete. Faults may lead to loss 
of water and render the foundation of the dam insecure. 
The reservoir must also be provided with an adequate 
overflow channel to discharge any sudden inflow of water as 
from a cloudburst. Few catastrophies reek such complete 
devastation as the rush of water through a broken dam.