Quicker or slower, the water that fills it will wash in sand and mud, and year by year this process will go on till ultimately the whole reservoir is filled up. The embankment is raised, and raised again, but at last it is better to abandon it and make a new tank elsewhere, for it would never pay to dig out the silt by manual labour.
On the other hand, the partial or total failure of smaller dams of this description, to retain the reservoir water, has been much more common in the past than is generally supposed.
Rivers and innumerable torrents, and at flood-time serves as a reservoir for the Mekong, with which it is connected by a channel some 70 m.
The actual size of the reservoir which would certainly yield the assumed supply throughout the driest periods has therefore been largely a matter of judgment.
Those which are only partially submerged bear stomata on their exposed portions, so that their environment approximates towards that of a terrestrial plant, but the communication even in their case is much less easy and complete, so that they need amuchlarger reservoir of air in their interior.
A noteworthy feature of the metropolitan public water service was begun in 1896 in the Wachusett lake reservoir at Clinton, on the Nashua river.
Of the city in a valley of the Serra do Mar, where a large reservoir has been created by building a dam across the Rio das Lages.
By a tunnel inserted at the rear, this reservoir is kept replenished with water as fast as it evaporates.
There are many structures at present in existence bearing considerably greater loads than this, and the granite ashlar masonry of at least one, the Bear Valley dam in California, is subject to compressive stresses, reaching, when the reservoir is full, at least 40 to 50 tons per sq.
Thirlmere itself was raised in level, and adapted by means of a dam at the north end, as a reservoir for the watersupply of Manchester in 1890-1894.