Channel Morphology and Hydraulics as Controls on Spatial Patterns of Invertebrate Drift in a Mountain Stream.

Abstract:

In this research we linked spatial variability of invertebrate drift characteristics (e.g. flux, concentration, mean body size) in a mountain stream to channel morphology and hydraulic properties such as at-a-point and depth-averaged velocity and shear velocity. The study was conducted in East Creek, a small stream in British Columbia in which reach-scale morphology transitions from cobble-dominated plane-bed to gravel-bed pool-riffle. To achieve our goal, we collected vertical profiles of invertebrate drift and time-averaged velocity in various morphological units within the study reaches. The data were analyzed using linear mixed model. Our reach-scale results suggested that, generally, the study reaches had statistically similar drift characteristics despite their contrasting morphologies. At the within-reach scale, different drift characteristics displayed different trends in relation to morphological and hydraulic properties of the channel. Longitudinally, highest drift flux occurred in riffle-pool transitions. We attributed this finding primarily to higher flow velocity because there were no statistically significant differences in drift concentration between morphological units. In the vertical dimension, highest drift flux occurred near the surface owing to a combination of higher drift concentration and higher flow velocity. A different pattern was observed for mean body size of drifting invertebrates. On average, body size was smallest in riffle-pool transitions and largest near the bed. The combination of velocity, drift concentration, and drift body size structure resulted in similar biomass flux estimates in all morphological units. In the vertical dimension, biomass flux appeared to be highest near the water surface. Generally, hydraulic variables seemed to be relatively poor predictors of drift concentration and mean body size of drifting invertebrates. Our findings reveal a complex relationship between channel morphology and hydraulics and various characteristics of invertebrate drift in the study stream. We hypothesize that this may be representative of similar mountain channels.