An Event in the NHD is a location on a route system such as a point along a stream network. Events are described by a route identifier and a measure along the route. An example of a route identifier in the NHD is a reach code. A measure provides a distance along the reach code. Essentially, Events in the NHD are used to attach information about that particular, point or section of a stream, or a certain area of a waterbody. Events can only be linked to flowlines when the flow direction is set. Events are linked to a particular location on a stream which allows for geographic analysis. This can help determine upstream and downstream cause and effect relationships.
There are many types of events associated with stream networks. An NHD user can use the Hydrography Event Management (HEM) tool to attach these events to the network. At this time however, only Dams, Diversions, Streamgages, and Water Quality Monitoring Stations are accepted back into the NHD National database. However, in addition to the events allowed back into the NHD, other event types may be generated for individual use.
Events provide powerful information about the water system. In some cases, events are hyperlinked to allow real-time access to readings or online documentation.