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Synthetic river derivation

NetMap Synthetic ("Smart") River Networks

NetMap's synthetic river networks (stream layers) are derived directly from digital elevation models, using flow routing algorithms; they are referred to as "synthetic" to distinguish it from stream layers that are primarily based on optical imagery (referred to as "cartographic"), such as the National Hydrography Dataset (NHD/NHDPlus) and others worldwide. A synthetic stream layer has numerous attributes and functions (like terrestrial-riverine connectivity and routing) required for resource management and conservation plannning. However, NetMap's synthetic networks can also match other stream layers, such as the NHD/NHDPlus or other state GIS layers.
NetMap's synthetic river network is designed to be customizable and flexible to meet the diverse needs of analysis and decision support.
Using NetMap tools, stream networks can also be "trimmed" thereby matching channel density to actual field conditions.

How is a synthetic river network (stream layer) built?

NetMap uses several criteria to determine the headward extent of channels within a basin including: 1) contributing areas, 2) critical drainage area (drainage area per unit contour length), 3) plan curvature, hillslope gradient and 5) minimum flow length. NetMap’s delineation of channel networks includes a calibration component to match channel extent and density to specific landscapes. 
In addition, NetMap's synthetic river networks within virtual watersheds allow for other types of flexibility and customization.

Critical drainage area is drainage area per unit contour length and various thresholds can be used to identify provisional locations of channel heads or channel extent (Figure 1).  Critical area is equal to area multiplied by slope gradient-squared (AS2). Threshold values vary with steep and less steep topography, for example, less than 25%, greater than 25% etc.  Plan curvature is the curvature of the surface on a cell-by-cell basis, as fitted through that cell and its eight surrounding neighbors. Curvature is the second derivative of the surface, or the slope-of-the-slope. The plan curvature is perpendicular to the direction of the maximum slope (Figure 2). Algorithms for flow direction and channel delineation are described by Clarke et al. 2008 but see latest Technical Help.

Figure 1. NetMap’s channel delineation method requires calculating the drainage area per unit contour length, distinct for both steep and low gradient areas. Example is from northern California.

Figure 2. The plan curvature is shown for a basin defined as the second derivative of the surface, or the slope-of-the-slope. The plan curvature is perpendicular to the direction of the maximum slope. The plan curvature is combined with the specific drainage area (Figure 1) and a minimum channel length threshold to define channels and the upward extent of channels.