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Turbidity in Johnson Creek is cause for concern

On April 26, 2012, there was a record-high turbidity spike in Johnson Creek, measured at the U.S. Geological Survey's stream gauge in eastern Gresham.

Turbidity is an important measure of water quality. It indicates how cloudy the water is, based on what's in it - for example, soil particles - as well as how much is in it.

Turbidity is commonly measured by how quickly the strength of light is reduced as it shines through a water sample. The more particles in the water, the cloudier it is, and the more quickly light will fade. Think of it as measuring whether Johnson Creek looks like tap water, weak tea or chocolate milk. You couldn't shine a flashlight very far through a glass of chocolate milk.

Turbidity is measured in Formazin Nepholometric Units, or FNUs for short. The higher the number in FNUs, the dirtier the water is.

Turbidity is a form of pollution. It's a problem that negatively affects the health of Johnson Creek.

For example, turbidity interferes with salmons' ability to find food as well as to avoid predators. The soil in turbid water often settles to the bottom of streams, filling in pools, which are prime habitat for salmon. The soil can smother and kill salmon eggs and reduce populations of aquatic insects that serve as food for salmon.

How turbidity affects fish isn't just an academic point. Fish surveys conducted last year confirmed coho salmon, steelhead, rainbow and cutthroat trout, all native species, can be found in upper Johnson Creek and many of its tributaries.

Turbidity also is often linked to other types of pollution that affect human health. For example, in addition to causing turbidity, the sediment in urban stormwater frequently carries pollutants such as heavy metals and combustion by-products that are known to cause cancer.  

Agricultural runoff can carry pesticide-laden soil particles into streams. For example, legacy pesticides like DDT and dieldrin, which haven't been used for years, are still present in Johnson Creek. They enter and pollute the creek through soil erosion.

Other modern pesticides may be attached to these soil sediments as well.

During the April 26 turbidity spike in Johnson Creek, turbidity values peaked at 881 FNUs, which is very turbid (definitely in the chocolate milk range).

For reference, a normal April turbidity value at the same location would be 10 to 20 FNU.

Between 2006 and 2010, the highest turbidity value measured at the Gresham gauge was 664 FNUs. And the highest peak during the January 2009 Johnson Creek flood (which was the third highest Johnson Creek flood ever recorded, when you'd expect high turbidity levels) was 561 FNU.

Just after midnight on May 5, 2012, there was another smaller turbidity spike, registering 351 FNU at the gauge in East Gresham.

Stream turbidity can come directly from a single point source. For example, in June 2011, a farm pond was drawn down and drained directly into upper Johnson Creek, releasing a plume of sediment that was still measurable miles downstream at the USGS Milwaukie gauge on Johnson Creek.

In the cases of the recent turbidity spikes, no single source was identified.

However, grab samples were collected and tested in an upstream direction in mainstem Johnson Creek and its tributaries and found that the suspended sediment came from the rural agricultural area of Johnson Creek watershed, upstream of the urban area.

This suggests that there is a systemic issue here, caused by widespread land-use activities (for example, crop planting and tree harvesting), which loosen soil throughout upper Johnson Creek Watershed at this time of year.

These types of activities then combine with the erosive effects of stormwater run-off from intense rainstorms and cause loosened soil to wash into Johnson Creek, raising turbidity levels.

The April 26 turbidity spike occurred near the end of 16 hours of rain. During that time, 1.37 inches of rainfall were recorded near the Johnson Creek headwaters in Boring.

The point of this is not to cast blame, but to make people aware that water turbidity can have many causes and that excessive water turbidity harms the health of Johnson Creek.

The good news is that there are resources available to help minimize soil erosion and reduce turbidity pollution in Johnson Creek.

Contact the Clackamas Soil and Water Conservation District (503-210-6000), the East Multnomah Soil and Water Conservation District (503-222-SOIL), and USDA Natural Resource Conservation Service for help with erosion and sediment concerns.

East Multnomah Soil and Water Conversation District's Stream Care program provides eligible landowners with five years of free weed control and tree planting along Johnson Creek as a way to protect the creek from soil erosion and sediment.

In Clackamas County, the Clackamas SWCD and the Johnson Creek Watershed Council provide a similar service to streamside landowners along Johnson Creek through the Creek Care program.

Turbidity is an issue of concern, but working together, we can help improve the water quality of Johnson Creek.

Matt Clark is the Executive Director of the Johnson Creek Watershed Council, an organization dedicated to the health of Johnson Creek and its surrounding watershed.



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