3. Underground Reservoir An underground reservoir near Annis Hill

4. Al-Hoota cave under ground reservoir

5. Surface Reservoir
The volume of reservoir is reduced by deposition within it of river sediment and land slide debris.
Reservoir sedimentation is filling of the reservoir behind a dam with sediment carried into the reservoir by streams.
The flow of water from the catchment upstream of a reservoir is capable of eroding the catchment area and of depositing material either upstream of the reservoir, or in the still water of the reservoir.
The nature of the material in the catchment area and the slope of the catchment area and the inlet streams are a factor, as is the nature of the ground cover.
Heavy rainfall falling on erodible material on a steep slope with little ground cover resulting from overgrazing or wildfire is a recipe for substantial sediment transport and significant reservoir sedimentation.

6. Reservoir sedimentation

7. The rate of reservoir sedimentation depends mainly on the size of a reservoir relative to the amount of sediment flowing into it: a small reservoir on an extremely muddy river will rapidly lose capacity; a large reservoir on a very clear river may take centuries to lose an appreciable amount of storage.
Most modern dams are designed so that they can afford to lose some storage capacity without their performance being impaired – the part of a reservoir known as "dead storage" which lies beneath the elevation of the dam’s lowest outlet. However sediments do not build up evenly along a horizontal plane, so that some "live storage" is usually lost long before the dead storage is filled.

8. Mitigating Geology
"Watershed management" – including afforestation and the promotion of farming practices which reduce soil erosion – is frequently advocated as the best way of cutting sediment deposition in reservoirs. While these schemes may be recommended in project plans, they are rarely implemented: dam–building agencies are usually more interested in putting their funds toward building dams than planting trees and digging field terraces.
Overall, building a dam in a valley is much more likely to increase erosion than reduce it: dams open up remote areas to road–builders, developers, loggers, farmers and miners, accelerating deforestation and soil loss. When insufficient resettlement land is made available, oustee farming families may have no choice but to clear land further up the valley or hillside.