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[Rodney]

https://www.chilliwack.com/main/page.cfm?id=3074

[Lunk Louie]

Finally....much welcomed by me

The (tree-hugger-induced???) suspension of the semi-annual (flood control) dredging of the lower section finally comes to an end. The practice of no flood mitigation the past many years, has immensely contributed to in at least the majority of the flow, from roughly the crossing down to the Fraser pretty much filling in with gravel....imo, exacerbated by increased flow and higher than usual flood conditions???....leaving flat, structureless glides virtually the entire length built up over years of no flood control...the shallower than normal glides/runs was probably going to enhance the snagability of the salmon this year more so than any other, with no where left for the fish to hide or hunker down, IMO... At the very least, as far as the salmon are concerned, they probably won't be as easily susceptible to what I was forseeing as being a much bigger than usual epic, bonanza snagfest for the long line bead swingers and snaggers this coming Fall....not scientifically based, just my biased opinion/rant based on what I've seen on this flow over many decades

[Roderick]

Wow! It says over 440,000 cubic meters of gravel shifted into the canal during the flood.  That's more then 10X the annual amount.  I would think that most of it came from the area just below the crossing. 

To put it into perspective, 440,000 cubic meters is an area of about 2/3 of a kilometer long, 2/3 of a km wide, and 1 meter deep. 

[santefe]

I may be pilloried and castigated but after walking over acres of gravel on the lower reaches I think it is high time some of that gravel should be removed.

[geomorphin]

Historically, downstream of the crossing used to be an alluvial fan. Sediment would come down from the steeper part of the river above the crossing and deposit in the lower slope valley on a fan shaped landform. Channels on the fan would fill with sediment to a point where the could no longer flow, then they would change its direction (avulse) and form a new channel in a different location. This obviously wasn’t great for folks that live there so we built the canal and dykes to lock the Vedder into place.

Because of the high sediment supply from above the crossing, and the fact the channel now can’t really shift, we are always going to have to remove sediment or else the channel will eventually fill and flood. It’s pretty costly and invasive to habitat but this is the way. There are some options to make it more effective. For example, more targeted removals where the flood risk is higher such as a narrow part of the river (like the rail bridge) rather than just removing everywhere for the sake of removing.

Another thing we, especially as fish lovers, should advocate for is setting back the dykes where possible. If we moved dykes further back, the channel could store more gravel so we would have less frequent need for gravel removal and less flood risk. It would also add more side and back channels which prove to be important salmon habitat.  We don’t have to look much further than the November flood to know that those back channels, which have been largely cur  off by the dykes in the Vedder, were crucial places for fish to escape those gnarly flows.

[Roderick]

You should read this history of the Chilliwack/Vedder system. 

http://www.yarrowbc.ca/pioneers/veddercanal.html

The Vedder below the crossing used to be a creek that came off Vedder Mountain and drained into Sumas lake.  The straight part of the canal down to the confluence with the Sumas River, of course, is 100% artificial and will need to be dredged regularly to prevent flooding. 

[roeman]

You should read this history of the Chilliwack/Vedder system. 

http://www.yarrowbc.ca/pioneers/veddercanal.html

The Vedder below the crossing used to be a creek that came off Vedder Mountain and drained into Sumas lake.  The straight part of the canal down to the confluence with the Sumas River, of course, is 100% artificial and will need to be dredged regularly to prevent flooding. 

By far the best information ever read on this site