NHD is a digital vector dataset used by geographic information systems (GIS). It contains vector and polygon features such as rivers and lakes, and point features such as stream gages and water quality stations. The NHD allows scientific data such as water flow rates or water quality measurements to be linked by way of a linear referencing system. Links to external scientific data are known in the NHD as events. Because the NHD is designed as a connected flow network, events make many kinds of scientific analysis possible, such as how a source of poor water quality upstream might affect a fish population downstream, or how dams affect water temperatures downstream. The USGS is currently adding point events to the NHD including steam gages, water quality stations, dams, and diversions. Linking scientific information as events to the NHD improves its utility. At the same time, the linked scientific information becomes more useful because it is now connected to other events by way of the NHD network. Events connected in a flow network create a synergy between the data and the network. By adding scientific information to the NHD as events, the USGS is improving the utility of the NHD for customers and scientists.