Regardless of the type of dam, it is necessary to de-water the site for final geological inspection, for foundation improvement and prepartation, and for the first stage of dam construction. The magnitude, method and cost of river diversion works will depend upon the cross-section of the valley, the bed material in the river, the type of dam, the expected hydrological conditions during the time required for this phase of the work, and finally upon the consequences of failure of any part of the temporary works.

At most sites it will be necessary to move the river whilst part of the dam is constructed; this part will incorporate either permanent or temporary openings through which the river will be diverted in the second stage. If the first diversion is not large enough the initial stages of construction will be inundated, if the second stage outlets are too small, the whole works will be flooded.

At some sites there is a distinct seasonal pattern of river flows and advantage can be taken of such conditions but noting that Nature is random.

Construction of the Hendrik Verwoerd Dam, South Africa required a sophisticated arrangement of cofferdams. An approach was developed based on the frequency and distribution of floods that could occur over a five year period of construction. The following is an extract of the original detailed specification:

First Stage (A) -

Second stage (B) & (C) -
Third stage (C) & (D) & (E) -
Diversion can also be achieved by means of a tunnel, which depends on the nature of the rock and depth of weathering and should be far away from the dam itself to not interferre with the foundations. The tunnel also should be large enough to avoid the possibility of job jams.
(C) Thomas, Henry H. The Engineering of Large Dams