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NatureSpeak Articles

"Dirty Rivers"

We’ve had a lot of rain to usher in 2002, which is not uncommon for this time of year, although it would have been more welcomed as snow. All the rain has increased river flows. A friend recently asked me about the "dirty water" he was kayaking. "Dirty" referring to the brown discolouration of the water. All you anglers, dog walkers, water rats, and observant individuals out there will remember that most of our watercourses turn brown during spring snowmelt and intense fall/winter rain on snow events.

"Why do they turn brown?" A combination of factors including high river flows, erosion of streambanks, landslides, and runoff increase the quantity of sediment carried by rivers. Given that high river flows are the primary factor determining when we observe "dirty water," we tend to make this observation during spring and fall runoff events. As water flows increase, rates of erosion, potential for slides, and movement of sediment within the river increases.

"Where does all the sediment come from?" Needless to say, it is difficult to point to one particular source. Some of our rivers begin their journey at the foot of glaciers. We’ve read in previous naturalist columns that local glaciers are receding. As they continue to melt, more bare earth is exposed to the elements. The bare earth or unconsolidated sediment is then readily available to be swept away during heavy rain or runoff events. Blackcomb Creek exemplifies this situation.

Some of our larger watercourses, such as Fitzsimmons Creek, flow through and erode into remnant glacial sediment. Next time you’re on the mountains, look at the terraces (the almost horizontal areas in the valley between Whistler and Blackcomb Mountains). The erosion of Fitzsimmons Creek into those sediments for the past 10 000 years, has provided a constant source of sediment to the river. Rivers continually erode sediment and transport it downstream. Erosion of sediment may prompt localised landslides or slump events, which have been known to occur in the Whistler area.

Some may recall the events of August 1991, when a prolonged and intense rain event triggered a mass movement of sediment half way up the Singing Pass Road, resulting in a debris flow in Fitzsimmons Creek. Erosion of streambanks also contributes to the "dirty water" syndrome. A paddle down the River of Golden Dreams will illustrate localized erosion of stream banks. As banks erode and fall into the river, they provide another source of sediment to watercourses.

"Does the dirty water affect water quality?" Yes. You may recall that during our recent rain events, the City of Vancouver increased its rate of filtration and treatment of surface water due to high concentrations of sediment. Fine sediment may also pose a threat to spawning salmonids and their habitat. Bear in mind however, "dirty water" has been a natural occurrence in our rivers draining steep valleys for thousands of years. Resident fish time their spawning so they do not coincide with "dirty water" events.

"Has logging and/or human influence played a role?" To some degree yes. Many researchers have studied the role of logging in watersheds and the link to increased rates of erosion. Most observations note that periodic occurrences of "dirt water" coincide with road construction activity and burn and slash operations. These practices have radically changed over the past decade to minimize their role of increasing sediment to rivers. Prolonged occurrences of "dirty water" have largely been shown to relate more closely to the nature of the watershed, particularly, the geology. For instance, basalt tends to erode more quickly than granite, providing a higher rate of sedimentation to the river.

"Why are some rivers dirty and others not?" We can now consider that there probably isn’t much of a sediment source upstream of the "clean river." More likely however, the river may have a reservoir immediately upstream. One peek at the Cheakamus River illustrates this scenario, as it drains from Cheakamus Lake through bedrock channels. The lake provides a settling area for any "dirty water" and the bedrock channels do not easily erode, offering limited sediment sources – but great kayaking potential!

The rivers in our backyard are highly dynamic and unpredictable, but they offer us our drinking water, nourish a diverse environment and aquatic habitat, and provide incredible recreational opportunities. Some "dirty" things aren’t so bad after all.

Upcoming Events:

Saturday, February 2nd— Monthly Bird Walk . Meet at the base of Lorimer Road at 8 a.m. Contact Michael Thompson for details.

Written by: Channa Pelpola


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