Stream alignments represent the stream network in the watershed. The alignment shows flow direction and indicates where confluences and bifurcations occur. It can also provide a sense of distance and scale. Stream alignments are composed of a series of stream elements, stream nodes, and stream junctions (Figure 1). A stream element is a segment of the stream alignment. A stream element typically begins or ends at a confluence, bifurcation, or at the boundary of the river system. It is composed of vertex points, with the beginning and ending vertex points being stream nodes. Stream junctions are also stream nodes but are located at points where two stream elements meet.
Stream elements are drawn in segments defined by vertex points. Each stream element must have at least two stream nodes, defining the beginning and end of the stream element. Stream elements, stream nodes, and stream junctions, are illustrated in Figure 2.

HEC-RTS creates and labels stream stations (or river stations if the user defines the stationing as such) along each stream element. Stream stationing is a reference system for locating features with distance along a stream. The station at the downstream end of an element is Station 0. The stationing increases upstream along the element according to the linear distance between vertices on the stream element in the watershed coordinate system. You can override the default stationing by editing the station values of the stream nodes at the upstream and downstream ends of the stream element and by adding additional nodes along the stream element.

Figure 1 Parts of a Stream Alignment
Figure J.1 Parts of a Stream Alignment
Figure 2 Relationships of Stream Elements, Stream Nodes, and Stream Junctions

Stream stationing is based on the geographic extents that are defined when a stream element is drawn. If the geo-extents are based on a map layer, the units for the stream stationing are the same as the map layer (that is, feet or meters). There is no indication of what the units are. A stream alignment created without an underlying map layer will use the default coordinate system of X-Y. This default system does not have any units associated with it. Setting Up the Coordinate System describes how to set up coordinate systems.

The primary tools used to define the stream alignment are the stream alignment tool and stream node tool (Figure 3). The zoom tool (Setup Module) is also useful. Use it to magnify the stream network. This makes defining the vertices along the stream easier. The three tools are located in the map window toolbar of the setup module.

Figure 3 Setup Map Window - Displaying Stream Alignment

If the stream alignment and stream node tools are not displayed in the map window tool bar, click the Setup tab. Expand the Stream Alignments layer in the layer tree. Double-click the Stream Alignment node.