What type of concrete reservoir is constructed much like a swimming pool, is rather shallow, and difficult to cover?

The correct choice pertains to a type of concrete reservoir known for its method of application and structural characteristics. Hydraulically applied concrete-lined reservoirs are constructed using a specialized process where a concrete mixture is sprayed onto a surface, similar to applying a coating. This technique allows for the creation of shallow reservoirs, akin to swimming pools, that can be difficult to cover due to their design and the method of application.

These reservoirs are often used because the hydraulic application can create a smooth, durable surface that can effectively contain water. The challenges associated with covering such reservoirs stem from their shallow depth and the potential for a significant amount of surface area that may complicate standard covering methods. This makes them distinct from other types of concrete reservoirs, which may have different construction methods and characteristics.

Understanding the unique construction and application methods of hydraulically applied concrete helps clarify why it matches the description provided in the question, highlighting its specific use cases and the challenges related to its structure.