When the river runs black

By Anne Sophie Le Brun Robles Gil Special to The Record


If you have ever been around Huntingville dam when it rains, you may have noticed that the river changes colour, going from mostly clear to an opaque brown or black. Why this change in colour? What does it have to do with rain?
Well, a local farmer downstream from the dam explained that the change in colour is due to soil runoff from nearby farms. Essentially, when rain falls on barren fields, the soil gets dissolved in the rainwater, and the water carries the soil into the river. There, all that soil is enough to change the river’s colour temporarily.
What is the problem with that?
Quite a lot actually, and it all has to do with what is in the farm soil. To get better crop yields, farmers will frequently use fertilizers, which will end up in the soil of their fields. Then, when the rain washes that soil into the river, it takes the fertilizer with it.
Fertilizers are meant to help plants grow, right? Then, why would they be bad in natural ecosystems?
Well, to help plants grow, fertilizers contain nutrients such as nitrates and phosphates that are found at relatively low levels in the soil, and at even lower levels in rivers.
When these high amounts of nitrates and phosphates get swept into rivers and lakes, they do exactly what they are meant to do: help plants grow. Unfortunately, they help them grow too much too fast and even help some bacteria, such as cyanobacteria (blue-green algae), bloom.
This leads to what is known as eutrophication. Eutrophication is a process that can transform clear, deep lakes that have low nutrient levels but lots of dissolved oxygen (these lakes are called oligotrophic) into more marsh-like lakes that have a lot of nutrients but very little dissolved oxygen (these are called eutrophic). Both kinds of lakes are valuable and have a unique ecosystem. The problem is the transition from one type to the other.
Essentially, cyanobacteria, algae, and some plants grow very quickly when there are lots of nitrates and phosphates. This creates a lot more organic waste as everything that grows eventually dies. When they die and sink to the bottom of the lake, they decompose. This has two main effects. One, it covers the lake bottom in silty sediment and makes the lake shallower. Two, as small organisms decompose this organic matter, a lot of oxygen in the water gets used up. The oxygen levels in the lake then drop, as do fish populations that needed that water-dissolved oxygen to breathe.
That is not the end of it, though. As algae and cyanobacteria bloom, they block out light from entering the lake, preventing photosynthesis from happening underwater. This kills a lot of the plants that grow underwater, like lily-pads. These plants are often very important to the ecosystem, such as for food and habitat for animals. As such, biodiversity decreases.
It is therefore important to keep fertilizers from getting into rivers. A number of local organizations are working hard to protect our freshwater systems like Bleu Massawippi, dedicated to preserving the Massawippi Lake and Memphremagog Conservation Inc. (MCI), a not for profit organization working for the protection, conservation and improvement of Lake Memphremagog and its watershed.

What can be done about soil runoff?
Quite a bit, actually. There are three main preventative measures when it comes to farms. The first is to maintain riparian buffers. Riparian buffers are strips of land along the shores of any lakes or rivers that are left forested. These help prevent erosion, as well as help keep garbage and fertilizer from flowing into the water after it rains. These buffers are supposed to be mandatory and at least five meters wide, but they are often not well regulated. The second is strategic ditches around farmland so that rainwater filled with fertilizer gets collected in these ditches instead of flowing into the river. The third is to avoid leaving fields barren. Barren fields are a lot more susceptible to erosion and loose soil is easily swept away by the rain. Instead, letting some plants grow on unused fields helps prevent soil runoff, and in doing so, keeps nutrients in the ground, making it more fertile for the next time it is used.
Many farms have already taken these steps to help preserve our lakes, but there is still work to do to get everyone on board with keeping the lakes healthy and keeping farmland fertile.

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