Figure 4. Profile schematics of a low gradient rock weir (a) and high gradient rock weir (b). The difference in gradient depicts how embeddedness changes depending on how gentle or steep the channel bed is. A greater level of embeddedness provides upstream and downstream connectivity without obstructions to the flow path (b).
Depending on water level condition and gradient, rock weir keystones are either exposed or experience ‘drowned conditions’. Under low water level conditions, low gradient rock weir keystones are exposed, where the keystone surfaces are above water level, and water moves downstream through orifice flow pathways. Under intermediate water level conditions, low gradient rock weir keystones begin experiencing ‘drowned conditions’, where approximately 50% of the keystone surfaces are submerged, and water moves downstream through orifice and gap flow pathways. Under high water level conditions, low gradient rock weir keystones experience fully ‘drowned conditions’, where keystone surfaces are submerged, and water moves downstream through orifice, gap, and over-weir flow pathways. Particularly in locations where low water level conditions are dominant (i.e., Weslie Creek), low gradient rock weirs may impede fish passage by limiting gap and over-weir flow pathways for upstream movement. High gradient rock weirs, and the connectivity between upstream and downstream flow that is already established (Figure 4a), experience more ‘drowned conditions’ than exposed conditions, and therefore, provide greater opportunities for local fish species to maneuver through gap and over-weir flow pathways.
When velocities for upstream or downstream movement are not favourable, habitat conditions within the pool features need to serve as refuge with respect to flow velocities and thermal conditions. The range of preferred water temperatures for all local fish species was identified and compared to the average daily water temperature collected in Weslie Creek. The average daily water temperature was consistently within the preferred water temperature range for local fish species from June 6, 2018 until September 24, 2018. The water level logger was placed in pool 7 (Figure 2), which is deeper than the other pool features within the reach, thus it is important to note that the water temperature is likely cooler than in other, more shallow pools (i.e., Pool 5 – Figure 2).
To assess pool features in terms of both fish passability and suitability to serve as refuge with respect to flow velocities, the length of each pool feature in Weslie Creek was identified and compared to downstream fish passage effectiveness and the number of refuge opportunities available at the sampling cross-sections, respectively (Table 2). These results indicate that longer pool features facilitate increased fish passability downstream, and 10/11 pool features in Weslie Creek provide opportunities for fish habitat and/or refuge under all water level conditions.
Table 2. Pool length influences fish passability through the downstream VRW and available locations for fish refuge.