Fluid patterns, sediment pathways and woody obstructions in the Pine River, Angus, Ontario

The objective of this research was to determine the effects that woody obstructions have on flow patterns and sediment pathways in a sand-bed river in Central Ontario. The Pine River near Angus, a tributary of the Nottawasaga River, was chosen because it contained a number of woody obstructions to flow from which to choose study sites. Two reaches, one containing a trunk oriented in the direction of flow, were chosen as the study reaches. Sampling of fluid speeds through the study reaches showed that the cross-stream obstruction acted to slow fluid as it approached and exited the obstruction zone, while at the same time it increased the variability within the flow. At the downstream obstruction, flows became concentrated and accelerated as they moved through the obstruction zone, then decreased in speed and increased in variability once the obstruction was passed. Results of fluid speed patterns and direction of flow allowed for the delineation of “zones” of fluid behaviour. Bed material particle size distributions showed differences within the obstruction zones as opposed to areas unaffected by the obstructions. Particle sizes tended to increase underneath both obstructions, indicating acceleration of fluid underpassing the trunks. Near-bed transport rates showed high variability within the reaches, with the greatest rates of transport occurring where downstream-oriented trunks were nearby. Results also showed that rates of transport entering and exiting one pool were almost identical, indicating that under those conditions the pool could be considered conditionally stable. This is an important result for aquatic biologists in that it results in stable aquatic habitat. In order to test field results under controlled conditions, flume studies on cross-stream obstructions with varying flow depths were conducted. Results showed that increasing the flow depth decreased the spatial extent of the influence zones, indicating that the spatial extent of obstructed flow is depth-dependent. This has implications for management of woody debris as in-channel habitat structures. A conceptual model of flow through obstructed zones was created for study reach. Spatial extent of these zones were determined. Finally, information on the role that obstructions could play in providing habitat for aquatic organisms, as well as how woody debris may be used in bioengineering, is presented