Nine stream channel characteristics (channel unit frequency, channel unit length, pool spacing, depth variability, width variability, large woody debris jam spacing, large woody debris volume, relative roughness, and average bank‐full width used as a scale) were measured in 12 reaches in old growth forests on Haida Gwaii and Vancouver Island. They are applied to calculate a Euclidean distance measure of dissimilarity between all possible reach pair combinations. Frequency distributions of the resulting dissimilarity values express the range of variability present in the streams analyzed and enable definition of ranges of favorable and unfavorable comparisons. Reach pairs exhibiting high dissimilarity values have significant differences in several key stream channel characteristics that vary between reach pairs. Those reaches consistently appearing in reach pairs with high dissimilarity values exhibit significant variance from the norm for the group. Dissimilarity distributions provide a basis for appraising the outcome of stream channel manipulation (for example, in channel “restoration” programs) and for selecting channel pairs that are sufficiently similar to act as treatment and control units in experimental manipulations.