Local Chilliwack residents cherish Hope Slough. Walkers, joggers, and cyclists can be found enjoying the roads, paths and parks along the slough any day of the week. Leisurely paddles are taken by canoe or kayak. Somehow "progress" seems to have relegated swimming in Hope Slough to the memories of only older residents, but fishing is popular for old and young alike. It doesn't take much luck to spot otters, mink or beaver along the slough, while herons and kingfishers are a daily sight. A viewing platform overlooks spawning salmon in season.
Sunday morning July 24 people out to enjoy the slough were shocked to get a little too close a view of the variety of fish species found in this much loved place.
That morning hundreds of dead fish were found in the waters of the slough. While the sight was a shock, the variety of species found speaks well for the habitat the network of sloughs in Chilliwack provide: northern pikeminnow, prickly sculpin, pumpkinseed, large-scale sucker, peamouth chub, redside shiner, crayfish, juvenile coho, and pacific lamprey were found in the area of the slough from the confluence with Gravelly Slough to Corbould Park.
City staff were out quickly when reports of the fish die-off surfaced on social media. A sustained pulse of low oxygen saturation was suspected as the direct cause of the fish deaths. An oxygen saturation of 27% was recorded by city staff at around 11:15 that morning. WaterWealth tested at the Menzies St. bridge at 2:40 pm and the oxygen saturation was back up to 85%, a healthy afternoon level.
Residents shared their concerns, information, and love of the slough on the facebook group #SOS Save Our Slough, while over the following ten days WaterWealth focused its streamkeeping efforts primarily on Hope Slough, as well as Gravelly Slough and Camp Slough which flow into Hope Slough above where the dead fish were found.
Previous streamkeeping experience further up Hope Slough suggested that the problem hadn't originated there. A canoe trip down Hope Slough from Gibson Road supported that hypothesis. The waters of Hope Slough were relatively healthy above the problem area. Less algae was found and water temperatures were a welcome (for fish) few degrees cooler, while oxygen levels were higher.
Turning the canoe up Gravelly Slough, the difference was jarring. While the presence of a heron fishing in the lower end of Gravelly Slough was encouraging, paddling upstream was a mucky undertaking compared to the pleasant paddle down Hope Slough. Subsequent water testing further up Gravelly Slough and Camp Slough bore out the suspicion that the hypoxic water that caused the fish die-off originated in that direction. An 8% oxygen saturation was found in Gravelly Slough at essentially the same time as a 60% saturation on Hope Slough just metres above where the two sloughs meet. For the first time ever WaterWealth streamkeepers encountered a 0% oxygen test, on Camp Slough at Gill Rd.
Early morning is when the lowest dissolved oxygen (DO) levels will be found. During the day photosynthetic processes of plants and algae produce oxygen. At night those same plants and algae consume oxygen.
Very high oxygen saturation levels in the afternoon can actually indicate a problem because the same populations that produce high levels of oxygen during the day will consume large amounts of oxygen during the night.
Camp Slough starts out well oxygenated where water from the Fraser seeps into it. Late morning oxygen saturation was 108% on August 2nd. A short distance away, at McGrath Rd, dissolved oxygen (DO) was 75% at about noon August 2nd and 58% on the morning of the 3rd. Things change rapidly from there. Only about 900 metres (800 as the crow flies) downstream at Willbourn Rd morning DO on the 3rd was a deadly low 2%. It does not seem to recover. The best morning DO number found downstream on Camp Slough was 16%. Poor conditions and heavy algae continue down Hope Slough after water from Camp & Gravelly Slough join it, though the quality of Hope Slough's own water means that conditions downstream of that confluence are not typically hostile to fish.
Jennifer Feinberg reported in the Chilliwack Progress that 'Staff were in contact with Ministry of Environment to report the event, and they reportedly agreed, that it was most likely "a low dissolved oxygen plume"'. Which of course begs two questions:
What is causing the low dissolved oxygen levels?
What do we do about it?
An opportunity to explore these questions will be at the city's upcoming Camp Slough Public Consultation, Thursday August 18 from 6 to 8 pm at the Camp River Community Hall, 50246 Camp River Rd. Hope to see you there!
The map below shows the streamkeeping test results WaterWealth volunteers recorded on these sloughs between July 24 and August 3. Click on the markers to see details for that location.