The first step in planning the construction of a reservoir with the help of a dam is for the decision makers to be sure of the needs and purposes for which the reservoir is going  to be built together with the known constraints (including financial), desired benefits. There may be social constraints, for examples people's activism may not allow a reservoir to be built up to the desired level or even the submergence of good agricultural level may be a constraint. 

Some times, the construction of a dam may be done that is labour intensive and using local materials, which helps the community for whom the dam is being built. This sort of work is quite common in the minor irrigation departments of various steps, especially in the drought prone areas. The Food-forWork schemes  can be utilised in creating small reservoirs that helps to serve the community. In a larger scale, similar strategy was adopted for the construction of the Nagarjuna Sagar Dam on the River Krishna, which was built entirely of coursed rubble masonry and using manual labour in thousands. 

The second step is the assembly of all relevant existing information, 

Which includes the following: 

• Reports of any previous investigations and studies, if any. 

• Reports on projects similar to that proposed which have already  been constructed in the region. 

• A geographical information system (GIS) for the area of interest may be   created using a base survey map of the region. 

• Topographical data in the form of maps and satellite pictures, which may be integrated within the GIS. 

• Geological data in the form of maps and borehole logs, along with the values of relevant parameters. 

• Seismic activity data of the region that includes recorded peak accelerations or ground motion record. 

• Meteorological and hydrological data - of available parameters like rainfall, atmospheric and water temperatures, evaporation, humidity, wind speed, hours  of sunshine, river flows, river levels, sediment concentration in rivers, etc. 

• For water supply projects, data on population and future population growth based on some acceptable forecast method, industrial water requirement and probable future industrial development. 

• For irrigation projects, data on soils in the project area and on the crops already grown, including water requirement for the crops. 

• For hydropower projects, data on past demand and forecasts of future public and industrial demand for power and energy; data on existing transmission systems, including transmission voltage and capacity. 

• Data on flora and fauna in the project and on the fish in the rivers and lakes, including data on their migratory and breeding habits. 

• Data on tourism and recreational use of rivers and lakes and how this may be encouraged on completion of the proposed reservoir. 

As may be noted, some of the data mentioned above would be needed to design and construct the dam and its appurtenant structures which would help to store water behind the reservoir.

However, there are other data that decides the following: 

1. How large the reservoir should be and, consequently, what should be the dam height? 

2. What should be the size of the spillway and at what elevation the crest level of the spillway be located? 

3. How many and at what levels sluices be provided and they should be of what sizes?