General Dam Questions

What is a dam?

There are a number of technical and legal definitions of a dam but generally it is any structure that impounds or diverts water. In New Jersey, a dam is any artificial dike, levee or other barrier, together with appurtenant works, which is constructed for the purpose of impounding water on a permanent or temporary basis, that raises the water level five feet or more above the usual, mean, low water height when measured from the downstream toe-of-dam to the emergency spillway crest or, in the absence of an emergency spillway, the top-of-dam. (NJDS)

How do I measure dam height?

Height of dam is the vertical dimension when measured from either the invert of the outlet pipe or, in the absence of an outlet pipe, the lowest point in the stream bed or ground surface at the downstream toe of dam, whichever is lower, to the top-of-dam. See the graphical demonstration of the definition of a dam. (NJDS)

What benefits do dams provide?

Dams may provide a variety of benefits, including water supply, power generation, flood control, recreation, and irrigation. (AR)

How many dams are there in the United States?

The exact number of dams is not known. There are more than 90,000 (as per more current info below) dams in the US Army Corps of Engineers’ (Corps) National Inventory of Dams (NID), which is the most comprehensive inventory of dams nationwide. However, this inventory only covers dams that meet minimum height and impoundment requirements, so an unknown number of small dams are not included in the inventory. Of the 90,000 dams in the database, approximately 66,000 are located on rivers (the remainder impound water off-river). (AR)

What is the National Inventory of Dams (NID)?

The U.S. Army Corps of Engineers maintains the National Inventory of Dams (NID) to help compile national data on dams in the United States. The NID lists over 90,000 dams. State and Federal regulatory agencies submit data to the NID.

The NID gives national and state by state information on the current state of dams. To be a part of the NID, the dam must either be: of high- or significant-hazard potential classification, equal or exceed 25 feet in height and exceed 15 acre-feet in storage or equal or exceed 50 acre-feet storage and exceed 6 feet in height. To access the site visit: (ASDRSO)

What is the Infrastructure Report Card?

The American Society of Civil Engineers (ASCE) released a Report Card for America’s Infrastructure every 4 years. The report card grades each facet of America’s Infrastructure in order to understand the funding required to address each issue, as well as to understand the overall condition and maintenance each facet already receives. As part of the report, ASCE considers the condition of dams and the state of regulatory and owner oversight.

ASCE considers America’s dams to be in poor to fair condition and that dam rehabilitation and safety should be of significant concern.

The 2017 report card gave Infrastructure as a whole received a D+ while the conditions of dams received a D. Dams also received a D in 2013, the last time ASCE released a report.

To read the report on dams visit (ASDRSO)