| Abstract/Description or Keywords |
As part of the ongoing sturgeon recovery effort that is underway for the Nechako River, Northwest Hydraulic Consultants Ltd. (NHC) conducted a substrate assessment at four locations near Vanderhoof BC. Coarse substrate was placed at two of the sites in May 2011 and the quality of the substrate was the primary purpose of the assessment. Of particular interest was whether interstitial spaces between the stones that may provide refuge for sturgeon eggs and larvae still exist. A secondary objective was to evaluate the condition of the substrate at a downstream site where a large number of eggs was found in the spring of 2012, and the condition of the substrate at an upstream site that was historically used for spawning. For the purposes of this report the placed material sites have been labelled the Middle and Lower Patch, while the natural substrate sites have been labelled the Upper and Lower Site. The assessment was conducted with Ministry of Forests, Lands and Natural Resource Operations personnel and boat and driver support supplied by EDI Environmental Dynamics Inc. The assessment consisted of taking freeze core samples of the substrate and collecting underwater images of the substrate. Visual inspection of the freeze cores and underwater images was used to identify if the interstitial spaces between the placed stones had filled with fine sediment. The freeze cores and underwater images demonstrate that good quality substrate exists at the Upper Site and the Middle and Lower Patches. The Lower Site has a distinct lack of cobble substrate and the substrate is generally of marginal quality; some interstitial spaces do exist between gravel grains, but these spaces are smaller those found at the three upstream sites. The upstream most site is the only location with naturally occurring cobble substrate and the substrate is quite imbricated. There does not appear to be sufficient energy to move these cobbles to the downstream areas. At the Middle Patch some infilling has occurred on the patches, and some filamentous algae is growing in other areas but nevertheless, the downstream portion of the patch remains free of fines and functional. The 2012 field work revealed that the Lower Patch is in better condition than expected. A large proportion of the placed substrate along the left bank side of the channel is free of fine sediment and the placed substrate remains functional. The right bank side of the placed material does have areas that were extensively infilled with fines. The placed material at both sites does not show any signs of being moved due to the 2011 and 2012 freshets. The Lower Site is predominantly gravel and has more sand than any of the other sites, as such it has the poorest substrate quality. Based on the site investigations the quality of the substrate in the region could possibly be improved by hydraulically cleaning the Upper Site and placing cobble substrate at the Lower Site. At this time the two placed patches appear to be functioning reasonably well and remedial work does not appear warranted. A decision to place substrate at the Lower Site should only be done after considering all of the biological, geomorphic and potential flooding implications. Of particular interest is why the sturgeons appear to be spawning further downstream now compared to before, and if placing substrate at the Lower Site could result in a change in their spawning behaviour. |
