When looking for under water features, either relief or objects, the sonar is the most usual technology to use. The sonar can be compared with radar but it uses sound instead of electromagnetic waves. At high frequencies a sonogram (the result of the sonar scan) can have the detail of a photo. Physic characteristics of the features looked up can be easily determined.
The method was successfully used in rivers and lakes after and before dragging for determining the level of alluviums deposited and estimating the reserves of useful material.