Input layers

The Input layers group includes the following parameters:

• The Input surface raster is the elevation raster that will be used for calculation. It can be a digital elevation model (DEM) with no prior sinks filled or a hydroconditioned DEM.

  The tool is not sensitive to errors in the surface raster that can act as depressions or sinks where flow terminates; filling sinks or depressions is not necessary.

  If the input surface raster contains real depressions, the locations of the depressions must be specified in the Input depressions raster or features parameter to be considered cells where water can flow into but not out of.

  NoData cells in the input surface raster do not have an associated value. These cells are ignored when identifying the direction of the least steep uphill neighbor, as well as the determination of flow direction and accumulation.

The Optional layers group includes the following parameters:

• Input depressions raster or features is the dataset that defines real depressions or sinks, where water can flow into but not out of.

  You can choose a layer using the Layer button or use the Draw input features button to create a sketch layer to use as the input.

  If the input is a raster layer, the depression cells must have a valid value, including zero, and the areas that are not depressions must be assigned NoData.

  If the input is a feature layer, it can be point, polyline, or polygon. Feature input will be converted to a raster internally before performing the analysis.

• The Input accumulation weight raster parameter defines the fraction of flow that contributes to flow accumulation at each cell.

  The weight is only applied to the accumulation of flow.

  If a weight raster is applied, specify an appropriate flow accumulation threshold for the Accumulation threshold parameter.

  If no accumulation weight raster is provided, a default weight of 1 will be applied to each cell.

Analysis settings

The Analysis settings group includes the following parameters:

• The Accumulation threshold parameter specifies the threshold for determining whether a given cell is part of a stream in terms of the total area that flows into such cell.

  By default, the tool calculates an area threshold based on the Input surface raster size (0.2 percent of the total number of cells).

  If Input depressions raster or features data, Input accumulation weight rasterdata, or data to apply environment settings is used, the default Accumulation threshold value will be recalculated based on the area of intersection between the inputs. However, once you have specified a value for the Accumulation threshold, it will no longer be recalculated based on changes in input selection.

  When specifying an Accumulation threshold value, use a value that reflects the complexity of the terrain in the study area or that matches the size of a contributing area of your choice. For example, if the threshold is equal to 20 hectares, only cells with 20 or more hectares of upstream flow will define a stream raster.

• The Stream designation method parameter specifies the unique value or order of the stream segments in the output.

  • Constant—The output cell values will all equal 1. This is the default.

  • Unique—Each stream will have a unique ID between intersections in the output.

  • Strahler—The Strahler method, in which stream order only increases when streams of the same order intersect, will be used. The intersection of a first-order and second-order link will remain a second-order link, rather than creating a third-order link.

  • Shreve—The Shreve method, in which stream order is assigned by magnitude, will be used. All links with no tributaries are assigned a magnitude (order) of one. Magnitudes are additive downslope. When two links intersect, their magnitudes are added and assigned to the downslope link.

  • Hack—The Hack method, in which each stream segment is assigned an order greater than the stream or river to which it discharges, will be used. For example, the main river channel is assigned an order of 1, all stream segments discharging to it are assigned an order of 2, any stream discharging to an order 2 stream is assigned an order of 3, and so on.

• The Force edge cells to flow outwards parameter specifies whether edge cells will always flow outward or follow normal flow rules.

  • Unchecked—The flow direction will be toward the inner cell with the steepest drop in z-value. However, if the drop is less than or equal to zero, the cell will flow outward from the surface raster. This will be the same for all cells across the raster. This is the default.
  • Checked—The flow direction at edge cells will always flow outward from the surface raster.

Result layer

The Result layer group includes the following parameters:

• The Output stream raster name parameter is the name of the output raster representing stream locations.

  The name must be unique. If a layer with the same name already exists in your organization, the tool will fail and you will be prompted to use a different name.

• Save in folder specifies the name of a folder in My Content where the result will be saved.