Gauging the impact of pollutants

If you travel anywhere in Wolfeboro where one of the major tributaries of Lake Wentworth crosses a roadway, chances are good that, over the course of the last six months, you’ve spotted something unusual in the streambed. Poking out of the water is what probably looks like a large white yardstick attached to a fencepost.

Stream gauge

Stream gauge

What you’re seeing is a critical tool in the recently initiated watershed management plan for Lake Wentworth and Crescent Lake – a stream gauge. As the name implies, these gauges, marked in increments of feet and tenths of a foot (rather than inches) measure the height of the water flowing in the stream.

Why is that important? As it turns out, one of the critical aspects of the watershed management plan is getting a handle on how much phosphorous is entering the lakes each year. Phosphorous, you’ll remember, is a plant nutrient that’s great for suburban lawns but highly disruptive to clean lakes like Wentworth and Crescent, because it feeds weeds like milfoil as well as microscopic algae that rob a lake of its clarity.

As it turns out, much of the phosphorous that enters the lakes does so via the streams that feed the lakes. Often, the phosphorous is hitching a ride on sand and other debris washed into the water by runoff from a storm. We’ve all seen the evidence: gravel roads, roadside culverts, and sandy beaches marred by deep trenches where stormwater has eroded the soil and carried it off to a nearby marsh, brook, or even lake.

So how do the stream gauges work? The answer is that, in order to protect a lake, you need to know not only how much phosphorous it currently has but how much is coming in. If, as is the case, with Wentworth and Crescent, a lake is still relatively healthy, you want to know if phosphorous is coming in at a rate that exceeds the lake’s long-term ability to absorb it.

Making that calculation for each stream requires two types of measurements: one determines how much phosphorous the stream is carrying for a given volume of water; the second determines the volume of water actually flowing down the stream. That’s where the gauges come in.

Using special flow meters, scientists can determine how much water the stream is carrying at a given water depth. By taking flow measurements when the stream depth is at different levels, they can use a mathematical calculation to determine the volume at any given height.

From that point on, it’s easy to read a gauge and know how much water is currently flowing – and, by extension, how much phosphorous the stream is carrying down to the lake. By adding those calculations from all tributaries, lake managers can determine the phosphorous “load” that the streams are putting on the lake.

Using funding from the Lake Wentworth Foundation, volunteers have placed gauges in the streambeds of 12 tributaries: Harvey (Hooper) Brook, Hersey (Tyler) Brook, Willey Brook, Fernald Brook, Claypit West, Claypit East, Frost Brook, Heath Brook, Red Brook, Breezy Brook, Townsend East and Townsend West. Warren Brook, while a major tributary to Lake Wentworth, has no gauge because its channel is flat and at the same elevation as the main body of the lake, making flow measurements all but impossible.

As work on the watershed management plan gets under way in earnest in the summer of 2011, the stream gauges will provide invaluable information that will lead to a better understanding of where stormwater runoff poses the greatest threats to the quality of our lakes.


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