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.