A temporary type used to construct other structures is a coffer.
One of the largest Roman example of these constructions was built by Nero at Subiaco, while Shapur I used  Roman prisoners of wars to construct one of these called the Band-i-Kaisar. Depending on who you ask, the  oldest known one of these is either located at Jawa in Jordan or at Sadd el- Kafara. John Frederick LaTrobe  Bateman used these objects to regulate a series of five related constructs as a part of his Longdendale project.  They are not buildings, but they can be reinforced by buttresses and less material is generally required to form  the curved arch type. A temporary type used to construct other structures is a coffer. Large amounts of concrete is  need for the construction of the gravity type of these devices used in flood control. For 10 points, name these  constructions, one of which on the Colorado River is named for Herbert Hoover.
One of the largest Roman example of these constructions was built by Nero at Subiaco, while Shapur I used  Roman prisoners of wars to construct one of these called the Band-i-Kaisar.
dam [prompt on reservoir]