Okanagan Eco-Noggin  

Practise coastal distancing

Now that Okanagan residents are mastering social distancing, it’s time for Okanagan boaters to practise coastal distancing.

With high water levels in Okanagan lakes in the past few years, boating has been discouraged near shorelines to protect against erosion and property damage.

A recent study commissioned by the Regional District of North Okanagan and District of Lake Country suggests coastal distancing should be applied permanently.

Motorboats can disturb shorelines and sediment through three main mechanisms: the wake, waves and prop wash.

The study focused on:

  • Prop wash, which is the turbulent stream of water beneath the water surface from the boat’s propeller
  • The wake, which is the visible wave from the boat’s hull as it moves through the water.

The study was conducted in Kalamalka and Okanagan lakes by Larratt Aquatic Consulting. They used aerial drones and submerged instruments to record the movement of sediment as boats passed by at depths ranging from 2-8 metres.

When boats passed over sediments that were less than three metres deep, the sediments were completely stirred up. At depths of 4-5 metres, the re-suspension was less extreme, but still caused disturbance, whereas little interaction occurred at depths beyond five metres.

At eight metres, the effect was measurable, but not likely harmful to aquatic life. It is, however, important to manage near drinking water intakes.

Disturbing the sediment can affect water quality at drinking water intakes in multiple ways.

The sediment can carry bacteria and metals, and it can clog filters, making the water more difficult and expensive to treat. Disturbing sediments can also release nutrients into the water, potentially causing a cascade of changes to the ecosystem and to water quality.

This is a real risk in a lake like Wood Lake, which UBC research has shown to have elevated nutrients in its sediments.

The recent study was prompted by data from District of Lake Country’s water intake, which has seen elevated turbidity on long weekends.

Stirring up the sediment can cause a number of other impacts in a lake. The sediments host a community of insects and animals that form a link in the food chain between primary producers such as algae and higher order consumers such as fish.

Stirring up their habitat with prop wash is the lake’s equivalent of a tornado in a trailer park.

You can see these effects in an aerial drone video, and you can imagine the plume from the perspective of the insect or animal burrowed in the sediment that is now being hurled through the water.

Vegetation growing in the sediments can dampen the disturbance caused by prop wash, but the plants themselves can be washed away by high turbulence, or they can become tangled in props. Worse, if the plants are invasive species like Eurasian water milfoil, prop wash can spread these plants to other areas.

An earlier study by researchers at Penn State titled Stirring Up Trouble? provides additional details on how sediment is re-suspended by prop wash. The amount of re-suspension depends on boat size, shape, speed, prop dimension, motor trim and other factors. 

Likewise, the potential damage to the sediments depends on their size, shape and density. Boat speed had an interesting relationship with sediment re-suspension — the greatest amount of sediment was moved when the boat was running about 16 kilometres an hour, immediately before the boat went into plane.

This makes sense, because boats are working hard and pointed more vertically during that time.

Water depth is the most important factor, because in deep portions of the lake, the prop wash has no effect on the lake bed. Larratt recommended travelling slowly in waters less than eight metres deep.

As she pointed out, wake boat manufacturers recommend operating in deeper waters for optimal wake generation, so deep water is a better place to play.

In addition, staying away from the shoreline can protect against erosion. This is particularly important for the Okanagan Rail Trail that runs along Wood and Kalamalka lakes. 

The Regional District of North Okanagan owns the northernmost section of this system.

“We know that natural factors like high water can cause shoreline erosion, but adding waves from boats can accelerate this erosion,” said Ashley Gregerson, RDNO’s communications officer.  

“If we can take the human factor out of that equation, we can avoid additional erosion and potentially save millions of dollars in mitigation simply by staying away from shore.

“Accordingly, RDNO recommends maintaining a distance of at least 40 metres from the shore along the Okanagan Rail Trail,” Gregerson said.

These seem like reasonable precautions that would not limit boating opportunities; the vast majority of open water is deeper than eight in large lakes such as Kalamalka, Wood, Skaha and Okanagan.

Other than launching, I keep my boat well away from shore. It’s safer in the middle of the lake where there are no rocks, weeds or other hidden obstructions – or buoys, swimmers and paddlers who recreate in the shallow areas.

Think of this as coastal distancing — it will keep our lakes healthy for everyone to enjoy.


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About the Author

Jerry Vandenberg is an environmental scientist and owner of Vandenberg Water Science. He lives in the Okanagan region where he is also a paid-on-call fire fighter.

He can be reached at (250) 491-7260; [email protected]; https://www.linkedin.com/in/jerry-vandenberg/

Website: www.vws.ltd


The views expressed are strictly those of the author and not necessarily those of Castanet. Castanet does not warrant the contents.

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