The valley could be a U-shaped glacial valley, or a V-shaped river valley or a broader synclinal valley. Flatlands and plains cannot make convenient places for reservoirs.

Embankment dams include a variety of non-rigid structures which are built over wide valleys with varying foundation characteristics from easily available materials such as earth and rock fragments. These are generally of trapezoidal shape.

The single most important feature that must be known thoroughly at site and all around and below valley up to a reasonable depth is Lithology, i.e., types of rocks that make area.

An arch dam having a curvature both in horizontal and vertical alignment is often called a cupola dam.

The main advantage of embankment dams is that y can be constructed on weak foundations such as unconsolidated weak-river or glacial deposits.

Since reservoirs are essentially water storage basins, area should possess such geological characteristics that favour holding of water in basin so created. Such characters are: topographic suitability, ground water conditions, permeability and structural stability and also trend and rate of wearing.

The longest dam is Hirakud dam having a height of 59 m and length of about 5 km and built across Mahanadi in Orissa.

Both arch dams and embankment dams can be constructed on rock foundations which sufficiently strong enough, whereas, gravity dams are constructed only on strong rock foundations.

Reservoirs may broadly be defined as artificially created water storage basins with storage capacity that may range from a few thousand cubic meters to thousands of millions cubic meters.

The most unfavourable strike direction is one in which beds strike parallel to axis of dam and dip is downstream. It must be avoided as far as possible.