Channel units in small, high gradient streams on Vancouver Island, British Columbia

Descriptions and a classification are presented of channel units in very high gradient, first-order mountain channels. Bedrock cascades, boulder cascades, rapids, chutes, riffles, glides, and pools had distinct mean bed gradients of 0.49, 0.45, 0.20, 0.12, 0.09, 0.06, and 0.03, respectively. Multiple pairwise comparisons showed that these gradients are significantly different, except bedrock cascades were not different from boulder cascades and glides were not different from chutes, riffles, or pools. Channel width and entrenchment do not discriminate channel units in any consistent way, but characteristic substrates occur. Constituent substrates of bedrock and boulder cascades are nominally apparent, while chutes also are bedrock bound. Mixtures of boulders and cobbles constitute rapids, while mixtures of cobbles and gravel constitute riffles. Glides and pools generally are composed of boulders, cobbles, and gravel in various proportions. Log steps sometimes delimit units, particularly pools, but wood is not a dominant element in these channels. Gradients of channel units in these small, first-order, dominantly nonalluvial channels are significantly higher than correspondingly identified units in larger channels farther down the drainage system. The reason for this distinction is supposed to be related to channel scale, hence potential stream power.