Channel geomorphic units as benthic macroinvertebrate habitat in small, high gradient streams on Vancouver Island, British Columbia
Creators: name: Halwas, Karen L.
Publisher: name: University of British Columbia
Year: 2009

description: Headwater streams typically have no fish, owing to steep gradients and impassible barriers; therefore, scientific research and protection measures have been focused on fish bearing streams. The Scientific Panel for Sustainable Forest Practices in Clayoquot Sound (CSP) developed a channel classification system which is pertinent to all streams, fishless and fish bearing alike, and upon which management prescriptions in Clayoquot Sound, Vancouver Island, British Columbia are based (CSP, 1995). The CSP classification delineates channels according to four physical criteria: bed material, gradient, entrenchment, and width. The current study was undertaken to determine the efficacy with which the CSP classification system delineates small, steep streams, on the basis of channel geomorphic units within them, and to examine the benthic macroinvertebrate habitat capability of these geomorphic units. Falls, bedrock cascades, boulder cascades, rapids, chutes, riffles, glides, and pools were described according to their bed slope and dominant channel-material type and organization. In addition, the area of each geomorphic unit was measured. Seventeen streams were grouped into four CSP channel classes which were compared with respect to the mean relative proportion of class area in geomorphic units. Stratified random benthic samples were extracted from geomorphic units in order to investigate and to compare their habitat capability. "Alluvial channels" in the study exhibited only weak, very infrequent fluvial transport; therefore, they were termed semi-alluvial. In general, high gradient geomorphic units (i.e. bedrock and boulder cascades) were dominant in steep, largely non-alluvial channels. Lower gradient units (i.e. riffles and rapids) were common in semi-alluvial streams with more mild slopes. Accordingly, channel classes with opposing bed material and gradient designations exhibited notable differences with respect to relative proportions of geomorphic units while width and entrenchment designations exerted little influence on channel organization. Ultimately, only two of the four CSP classification criteria effectively systematized channels on the basis of channel geomorphic units within them. Abundance of benthic macroinvertebrates was greatest in riffles (≈100 individuals per two minute kick sample), followed by rapids (≈80 individuals/sample), pools (≈70 individuals/sample), boulder cascades (≈60 individuals/sample), chutes (≈50 individuals/sample), and lowest in bedrock cascades (≈25 individuals/sample). In addition, abundance of invertebrates in channels with ephemeral flow regimes was considerably lower compared to channels with seasonal or perennial flow regimes. Ordination of macroinvertebrate taxa showed that community structure of bedrock cascades and chutes were similar but different from other habitats. Similarly, the benthic macroinvertebrate community structure of channels with ephemeral flow regimes was very distinct.