We tried several approaches to derived stream gradient. The stream gradient data layer that was produced is a computer-generated representation of stream gradient: it is meant to be used as a study area wide surrogate for stream gradient information until a better representation of stream gradient can be produced. A good use of this data layer would be to guide field teams and to field verify where the layer correctly represents stream gradient and where it does not.

Central to both approaches of developing a stream gradient GIS layer were the digital elevation model (DEM) data acquired from the CLAMS group at Oregon State University. DEMs were mosaicked as previously described. Recall that the elevation value assigned to a DEM cell represents the elevation of a 100 m^2 area as a single value. For relatively flat areas that extend for 100's of meters, the DEM value is probably a pretty good representation of elevation, at least within the vertical error associate with the original data. However, in areas with variable terrain on a spatial scale of a few meters, DEM values may not capture topographic variability.

Ideally, a 10 X 10 m grid, such as the DEM, would produce a stream gradient value for every cell based on the elevation change between the cell immediately above and below it. However, most of the slope calculation routines in GIS generate a slope value from the elevation change measured within a 1 grid cell (in our case, 10 m) buffer around the cell. In other words, the slope of a single cell is determined by the maximum elevation change from any two of the 9 cells immediately surrounding each cell. This has the effect of smoothing out the terrain. However, in deeply incised valleys, such as those found in the study area, stream gradient may be over estimated by this procedure because steep valley walls may be included in the determination of stream channel cells. In addition, stream widths may be only a few meters across compared to the relatively coarse grid cell size of 10 m.

There are at least two potential sources of error in using GIS to calculate stream gradients: the possible incorporation valley wall elevations into the stream channel slope calculations and the relatively coarse DEM grid compared to the relatively narrow stream widths.

We used the slope function in ArcView to create a slope grid of the entire study area from the 10 m DEMs. The slope function in ArcView identifies the maximum rate of change in elevation within a 3 X 3 cell neighborhood. This means that for each cell, ARCView examines the elevation of the surrounding cells, selects the cells with the maximum and minimum values, calculates the difference in elevation and then calculates the slope as the change in elevation (rise) over horizontal distance (run). ArcView returns the slope in degrees, which has to be converted to percent slope. We used the following conversion to go from degrees to percent slope.

Percent slope * 100 = tan (degree slope)

One of the drawbacks in using the slope function in ArcView Spatial Analyst is that slope values are returned as integers. Salmon biologists are interested in stream gradient classes that occur in 1 percent slope intervals (i.e., 1%, 2%, etc.), especially in low gradient streams. Calculating whole number values for stream gradient in degrees of slope, it was impossible to separate low gradient stream reaches. Therefore, we expressed stream gradient as 0-2%, 2-4%, etc.

ArcView was then used to intersect the slope grid with the ST1400 derived stream layer. This transferred the gradient attributes onto each stream segment. The resulting ST1400 stream layer, over 440,000 stream segment records, containing the derived stream gradient information was then merged with the 6th field watershed [a-6th_field] coverage to produce a coverage that could be used to summarize the stream gradient classes for each 6th field.

Please address questions or comments to:

Dr. Ralph Garono
Principal Investigator
Earth Design Consultants, Inc.
rgarono@earthdesign.com

Laura Brophy
Principal Investigator
Green Point Consulting
brophyl@peak.org | homepage