Rapids
Rapids are found on rivers. Rapids are formed by one or more of the following:  constriction, obstruction, gradient, and flow rate.  For instance, when water is flowing swiftly over a rocky river bed, rapids can be formed.  Rapids are typically found in or near the mountains, due to the element of gradient.  Formally, the word "rapids" is always plural, (like "scissors" and "trousers"), but is often used in the singular when speaking of a specific rapid or when it would be grammatically awkward; e.g., "Sweet's Falls Rapids vs. Sweet's Falls Rapid).  The word "rapid" means "very fast."