Stream Network Navigation
StreamStats has the ability, known as “stream-network navigation” or “network tracing”, to analyze the stream network upstream and downstream from a user-selected point and to identify and provide information for other points of interest that are located along the network. An explanation of stream networks, focusing on the National Hydrography Dataset (NHD), is needed before this functionality can be fully understood and utilized.
A stream network is a digital representation of streams and constructed channels for a given area as would be seen on an aerial photograph or on a topographic map. As with photographs and maps, stream networks can be shown at various scales, with smaller scales having less detail than larger scales. In a digital stream network, connecting lines are drawn through lakes, wetlands, bridges, culverts, and any other features where the stream channel is not visible from maps or photographs, thus eliminating any breaks in the network.
In a stream network, reaches are segments of surface water with similar hydrologic characteristics. Reaches are commonly defined by a length of stream between two confluences, or a lake or pond. Each reach is assigned a unique reach code, flow direction, reach length, reach type, and other characteristics that are stored as reach attributes. Points of interest along a network, such as the locations of dams, streamgaging stations, and effluent discharges, can be assigned a reach address, which consists of a reach code and the percentage of the distance along the reach from the downstream end. These points of interest are called point events. In addition, activities or designations that affect an entire reach, a part of a reach, or multiple reaches, such as an impaired water-quality designation, are called linear events. This stream network data structure allows users to navigate upstream or downstream from a user-selected point on a stream to locate and obtain information for the associated reaches, point events, and linear events.
The NHD was developed cooperatively by the U.S. Environmental Protection Agency (USEPA) and the USGS. Medium-resolution and high-resolution versions of the NHD are available nationally. The medium-resolution NHD was developed at a scale of 1:100,000. The high-resolution NHD was developed mostly at a scale of 1:24,000, but finer resolution is available for some areas. Detailed information on the NHD and access to available data can be found on the Web at http://nhd.usgs.gov. In addition, an NHDPlus dataset has been created, which incorporates numerous drainage-area and flow-line attributes; such as drainage areas, flow-line minimum and maximum elevations and slopes, and flow volumes and velocities. Navigation also can be accomplished along the NHDPlus stream network and attributes from the dataset can be obtained through StreamStats for states where this functionality has been implemented. Detailed information on the NHDPlus and access to available data can be found on the Web at http://www.horizon-systems.com/nhdplus/index.php.
StreamStats can navigate upstream, downstream, or in both directions from a user-selected site. The network may be a version of the NHD or any other stream network that is available in StreamStats, as different events may be associated with different stream networks. The Config tool is used to specify the network upon which tracing (searching upstream or downstream) will be done, the direction of the trace, and the map layers of point events that will participate in the trace. Tracing can be done either by first delineating a drainage basin for a selected site and then using the Config tool and the Trace From Outlet tool in sequence, or by first using the Config tool and then using the Ad Hoc Trace tool and selecting any point on a stream to begin the trace. Directions for using these tools are provided on the User Instructions page. When the process is completed, a window will appear that contains a table that lists the reaches and events found from the trace. Items in the table usually will contain links from the items to additional information about them. In addition, the map frame will redraw, with the stream reaches identified in the trace highlighted.
The figure at right is a view of the StreamStats Map Frame that illustrates the results of an upstream trace for a user-selected point on a stream. A user has selected a point (dark blue dot with red cross behind it) and used the Delineate tool to delineate the drainage basin (shaded magenta area) for the point. The user then used the Config button to select the NHD Flowline data layer and the upstream direction for tracing. As a result, StreamStats has highlighted the reaches (in light blue) that were found in the trace. These reaches include the reach on which the selected point is located and all upstream reaches. A streamgaging station, represented as red diamond, and three dams, represented as dark blue pentagons, were also found in the trace. The report produced as a result of the trace (not shown) provides information for each of the reaches and the point events identified in the trace.
The primary benefit of the trace functionality is to understand how the flow at a particular site may be affected by upstream activities or how downstream flow may be affected by existing or proposed activities at the selected site. The USEPA and many state and local agencies have worked to associate their water-related data to the NHD, and many similar efforts are underway. Dams and USGS streamgaging stations have also been associated to the NHD. The layers that are associated to stream networks in StreamStats will vary by state and with time.
