There are no official definitions for generic terms as applied to geographic features. Any existing definitions derive from the needs and applications of organizations using those geographic features. The Geographic Names Information System (GNIS) database utilizes 63 broad categories of feature types defined solely to facilitate retrieval of entries with similar characteristics from the database.
These categories generally match dictionary definitions, but not always. The differences are thematic and highly subjective. For example, a lake is classified in the GNIS as a "natural body of inland water”, which is a feature description that can also apply to a reservoir, a pond, or a pool. All "linear flowing bodies of water" are classified as streams in the GNIS. At least 121 other generic terms fit this broad category, including creeks and rivers. Some might contend that a creek must flow into a river, but such hierarchies do not exist in the nation's namescape.
The U.S. Board on Geographic Names once stated that the difference between a hill and a mountain was 1,000 feet of local relief, but this was abandoned in the early 1970s. Broad agreement on such questions is essentially impossible, which is why there are no official feature classification standards.