Which term best describes a watershed used for excess flow storage?

The term that best describes a watershed used for excess flow storage is "Detention Watershed." This concept refers to an area designed to temporarily store stormwater or excess flow during periods of heavy rainfall or runoff, allowing it to be released slowly over time. By detaining water, such systems help reduce peak flow rates into nearby rivers or streams, minimizing flooding risk and improving water quality.

In contrast, a controlled watershed would generally refer to an area where flow or other elements are regulated to achieve specific management objectives, which may not necessarily focus exclusively on excess flow storage. A flow-through watershed indicates that water may quickly pass through without significant storage, which does not align with the primary function of detaining excess flow. Lastly, a retention watershed is designed to permanently hold water, typically including a permanent pool, making it distinct from a detention watershed focused on temporary storage. Thus, the characteristics of a detention watershed make it the most appropriate term in this context.