Dams are enormous constructions, so solid foundations are needed to manage the heavy loads from the concrete dams and the volume of stored water. This means weaker sedimentary rocks are avoided due to the risk of structural failure. Harder rocks like granites are preferred.
Loss of water through seepage needs to be considered. Lake Powell loses 300 km3 annually. Impermeable and non-porous rocks are favoured to reduce seepage. There are many sedimentary rocks along the course of the Colorado, so in some locations concrete has been used to line the valley sides.
Dams must be built on geologically stable land, with little risk of earthquakes.
There is a need for a narrow cross section as this reduces dam length and therefore building costs.
A large and deep valley behind the dam is also necessary and means that a greater volume of water can be stored for use.
Temperatures are high along much of the course of the Colorado, so evapotranspiration rates are high. Lake Powell for example, loses 0.74 km3 annually.
Deeper valleys are better for building dams as there is a reduced surface area, therefore limiting water loss.
Rainfall patterns must be taken into account as some areas have higher rates of precipitation than others. Dams should be built in areas with reliable rainfall.
Reliable tributaries are also needed to feed the river.
There must be sufficient water flowing through the river, to ensure enough is available for storage.
Man-made lakes on the Colorado are huge. Lake Powell took 17 years to fill, finally reaching capacity in 1980. The lake has a storage capacity of 33 km3