Laura is a senior in at Castlewood High School

Laura is a senior in at Castlewood High School. After her graduation, May 17, she is planning to attend South Dakota Sate University, majoring in journalism. She lives on Lake Poinsett, which was one of the reasons the particular subject picqued her interest.

“Living on the lake and hearing the concerns of neighbors made me wonder what made the lake work. When the opportunity to write a research paper arose, it was the first subject that I thought of. I wanted to create a single essay that clearly explained why the water in the lake turned green in the summer, why it floods, and why we have low lake levels. Over the last several years we have heard much about adding water to the lake. I understood that we could not do it, but did not fully know the complete reasoning behind it. One of the goals of my paper was to define why that was not possible and identify what we were actually adding to the lake. I cannot say that I did more than touch that subject, but do hope that it helps others to understand also.”
Laura Katlynn Cox

South Dakota has many natural lakes, including Lake Poinsett and those in its watershed. Most of the lakes are glacial in origin and were formed close to 13,000 years ago as the glaciers began to move out of the Choteau des Prairies region within South Dakota. The lakes left behind were deep and filled with clear water. (Hauschild and Hauschild 1-2).

The climate of the watershed is sub-humid, which is cooler than the surrounding areas of South Dakota due to its raised elevation. The precipitation received in the watershed averages 22-24 inches per year. Up to seventy-five percent of this is received between April and September. Snowfall, which accounts for much of the spring runoff, averages 25-30 inches per year (Smith 3, 4, 6).

The landscape of Lake Poinsett’s watershed varies greatly from gently sloping fields to steeply rolling hills. There are 287, 628 acres within the watershed with close to 95 percent of it privately owned and used for agriculture. Some of the land is left for animal habitat, which may be tall grass prairie or seasonal and temporary wetlands. The watershed also encompasses nine lakes that feed Lake Poinsett (Smith 2, 4-6).

The nine lakes that feed Lake Poinsett do so through three chains. The first chain, which feeds Poinsett from the northwestern area of the watershed, consists of Lake Marsh, Lake Norden, Lake Mary, Lake St. John, and Lake Albert. (All lakes are listed from furthest to closest in relation to Lake Poinsett.) Albert is located west and slightly south of Lake Poinsett with its outlet flowing into Lake Poinsett (Smith 5, 6).

The second chain also feeds Poinsett through Albert, but includes only two additional lakes, Lake Badger and Lake Thisted. These lakes are located to the south of Lake Albert and receive the runoff from the southwestern portion of the watershed (Smith 5).

The third and final chain includes Lake Florence and Dry Lake in Hamlin County. These lakes are located to the north of Lake Poinsett with Dry Lake directly connected to Poinsett at Stone Bridge on Hwy 28. This chain collects the runoff from the northern portion of the watershed and also receives the floodwater from the Big Sioux River located to the east of Lake Poinsett (Smith 5).

Lake Poinsett itself is a 7,868 acre glacial lake, which makes it one of the largest natural lakes in South Dakota. The outlet, which flows into the Big Sioux River, is a three mile long channel. The majority of the inflow comes from Dry Lake to the north and Lake Albert to the southwest, as stated above. Lake Poinsett has approximately 625 residences and 10 businesses. There are four public water access areas around the lake that are maintained by the South Dakota Game, Fish and Parks (Smith 5, 6).

What factors affect the trends in the water level and water quality of Lake Poinsett?

Hundreds of years ago, before the settlers had come to the territory, Lake Poinsett had less dependence on runoff to fill it. As rain fell onto the prairie, it soaked into the soil very quickly, due to the health of the soil. As it moved down through the soil, it filled the aquifer. The aquifer in turn filled the lakes from underneath. It is believed that the water level of the lakes varied little throughout the seasons because of the constant water source (Smith 1 Feb.).

Runoff is the result of water, from rain or snowmelt, flowing over the land into rivers, streams, and lakes. Runoff occurs when soil cannot absorb water.

There are many reasons that soil cannot absorb water, with the main reason being there are not enough open cavities in the soil for the water to fill. Organic matter must be a component of the soil to allow for that space (Smith 7 Mar.).

The effects of adding organic matter to soil are simple. Picture a coffee can filled with plain black soil containing little organic matter, similar to what could be found in a corn field. It is known that the can would be fairly heavy. Now imagine adding water to that can filled with dirt. The water will not soak into the soil. It will run off of the surface because the plain dirt has no cavities for the water to fill. This is the condition of much of the soil in the watershed where continuous tillage farming has reduced the level of organic matter within the soil (Smith 7 Mar.).

Let’s return to our can. Fill it halfway with the same kind of dirt and then mix in a half can of dried leaves and grasses and mix it all together. This time as water is added, it soaks into the mixture. When the water finally starts to show at the top, you would have put nearly half of a coffee can of water into the soil. This soil condition, which was much more common before the land was settled by Europeans, does not allow for runoff, as does the first (Smith 7 Mar.).

Farming is a major factor in the change of soil condition. The majority of farmers plant annual crops every spring with tillage and harvest the grain every fall. However, mechanical tillage, which speeds breakdown of organic matter, is not conducive to ideal soil conditions in the watershed. By repeating this cycle farmers are reducing the amount of organic material in the soil and therefore reducing the soil’s ability to take in and store water causing an increase in runoff. Perennial crops, though, add organic material to soil and reduce the amount of runoff by increasing a soil’s ability to take in water (Smith 7 Mar.).

A drop of water moves in one of four ways once it touches the surface of the earth. It can move up, as evaporation or transpiration, it can move sideways, as runoff or laterally below the surface of the soil, it can go down into the aquifer, or the drop can be held in place before moving in one of the other ways (Jones 4).

As the drop of water moves through, over, or is held in the soil, it picks up the nutrients that are in the soil and carries them with it as it travels. This means that what is in the soil travels throughout the watershed, until it either settles out in one of the lakes, streams, or wetlands or is carried into Lake Poinsett. Once again farming practices come into play (Smith 7 Mar.).

Many farmers have their cycle of annual crops they use. In this area corn and soybeans are a very common cycle. Plants in general are natural mines. They mine the nutrients they need from the soil and use them. The nutrients are then used in building the structure of the plant and in photosynthesis. Each plant species has a set of ideal nutrients that it needs and a certain quantity that it must have to live. In a natural setting, grasses, which are perennial, bring the nutrients that it needs up from deep in the soil and uses them. Then, when winter comes, the roots die and redeposit the nutrients where they can be used again next year. This helps to increase soil permeability and tie up the nutrients when water is moving through the soil. The annual crops many farmers use do not redeposit the nutrients, because they are harvested, and do not add to the amount of organic matter left within the ground because annual crops do not grow the extensive roots that perennial plants do (Smith 7 Mar.).

In order to have a good crop next year, farmers must replace the nutrients that they have harvested, particularly nitrogen and phosphorous. The most common ways to do this are spreading manure or applying a nitrogen fertilizer on fields (Smith 7 Mar.).

There are two nutrients found in soil that affect the quality of water in the lake. These are nitrogen and phosphorous. Harmful algae growth in the lake is dependent on nitrogen and phosphorous nutrients being present (Smith 7 Mar.).

Nitrogen is a soluble nutrient found in soil. As the water moves through the soil, it dissolves nitrogen and carries it along. Basically, anywhere water can go, the nitrogen can go, too. It travels over land, into aquifers, into plants, and into lakes. Nitrogen is very common and is present in virtually all lake systems (Smith 7 Mar.).

Nitrogen is found most commonly in the atmosphere, but can be released from the atmosphere by adding energy, such as lightning. After the nitrogen is released, it combines with rain and falls to the earth. Nitrogen also finds its way into the system as fertilizer on fields. Nitrogen is in all water, and the plants that animals graze on. Because animals retain little of the nitrogen they eat in their systems, it is also found in dung (Smith 7 Mar.).

Nitrogen is impossible to eliminate from the lakes; therefore if phosphorous enters the system, it allows the growth of algae and pondweed. Phosphorous is a positively charged molecule that likes to attach to negatively charged molecules. Soil is negatively charged, so the phosphorous attaches to the soil (Smith 7 Mar.).

How does it get into the lake, then, because soil does not move as easily as water? In early days the phosphorous did move very little and the lakes generally had very clear water. The prairie covered the soil and the grasses held it in place, so its only movement was carried out by animals or as rivers eroded their banks. Today, farming has left much soil exposed to rain and wind with very little plant cover to hold the soil in place. As the soil is washed or blown off of the fields, it finds its way into the bodies of water within the watershed, allowing for the growth of algae and pondweed in the lakes (Smith 7 Mar.).

Phosphorous is nearly impossible to remove from a body of water once it gets there. It is a long term nuisance that, once it gets into a system, gets tied up in the sediments at the bottom of the lake. As the water moves in the lake during the warm summer months, the phosphorous is released again, only to be available for continued algae growth. The cycle makes it difficult to remove from the system, without dredging or continually harvesting the algae and pondweed from the lake (“Lake Poinsett” 1).

There are also health concerns connected with nutrients in water. For example, the state has a maximum of 10 milligrams of nitrogen per liter water in any ground water source. If the nitrogen levels exceed this, the water becomes unsafe for babies, very young children and senior citizens. Drinking the water may cause the development of an oxygen depriving illness in those certain people (“Ground Water”).

Another health concern is the amount of fecal coliform that has entered into the system. Fecal coliform comes from anything that has been excreted from an animal. It has little or no effect on the algae growing in the lake. However, the state threshold for immersion activities (e.g., swimming or skiing) is set at 150 parts per million. If the level exceeds this, the public beaches must be closed until the level has returned to an acceptable number (Smith 7 Mar.). Only once has a beach been shut down on Poinsett due to the level of fecal bacteria in the water. It occurred at the state park and was quickly taken care of (Myers).

Not everything that enters the water system in the watershed enters Lake Poinsett. There are two main ways that nutrients and particles of soil can be filtered from the water. The filtering occurs as either the water moves through a grassed area or the particles settle out in a lake (Smith 1 Feb.).

As one looks at a map of the watershed, they may notice that all of the lakes are not directly connected by streams. Those lakes are connected by a series of wetlands, through which the overflow of one lake runs into another. For example, Lake Marsh’s outlet is directed into a predominately grassland area of the watershed. The overflow travels through the grassy area and enters into Lake Norden (“Watershed Quality”).

The water flowing through the grasslands has the potential to reduce the amount of phosphorous it contains and drop out some of the fecal coliform it had picked up (“Watershed Quality”).

Another thing one may notice is that some of the lakes in the watershed are directly connected by streams of water. For example, Dry Lake flows directly into Lake Poinsett at Stone Bridge on Hwy 28 (Smith 5).

However, the sediment that enters Dry Lake does not necessarily enter Lake Poinsett. If the movement of the runoff water slows once it enters into a lake, it can diffuse evenly into the water it joins. The sediment that was once suspended in the runoff no longer has the support of turbulence and speed and will settle out. Another way to think of it is that the heavier particles soil drop out of water However, if the runoff volume is high and there is not sufficient time to allow for settling, the suspended solids will move into Lake Poinsett (Smith 1 Feb.).

A good example of this is adding sugar to a cup of coffee. When you first add the sugar it drops directly onto the bottom of the glass. The heavy particles do not stay suspended in the liquid. Naturally, the coffee will be stirred to dissolve the sugar faster. As the coffee is stirred, notice that the sugar is not at the bottom of the glass. That is because the speed and turbulence of the coffee has suspended the sugar. As soon as the stirring stops, the sugar will begin to fall to the bottom of the glass once more. That is the same concept as suspending and settling of soil in runoff.

There are three ways in which Lake Poinsett and the Big Sioux River are connected. They are the outlet of Poinsett, the flood plain, and the Boswell gates and ditch.

The outlet consists of a three mile long channel from Lake Poinsett to the Big Sioux River. During flood conditions, the channel allowed lower quality river water to backflow into Lake Poinsett. In 1987, a flood control permit was issued by the South Dakota Water Management Board. The permit authorized construction for flood control gates at the outlet of Lake Poinsett (Smith 5, 6).

The current permit acknowledges the Lake Poinsett Water District as the operators of the gate (Smith 6). The permit does have set conditions in which the gate may be open and closed. To divert river water into the lake to raise the water levels is one scenario that the gate may never be used for. The gate’s purpose, on the other hand, is to reduce the flood damage to manmade structures along the shore of Lake Poinsett (“Lake Poinsett”).

The floodplain connects the lake and river in extreme flood conditions. These conditions occur when the river has more water than it can handle and rises above its banks. Though the river and the lake are located several miles apart, there is a natural low “channel” that directs the river water over the fields, towards the lake. The channel does have one high point or “lip” that the water level must exceed before continuing into Dry Lake. After entering Dry Lake, the water can diffuse into Lake Poinsett. Because the water must travel over land and through Dry Lake first, some of the sediment settles out before it reaches Lake Poinsett (Smith 1 Feb.).

The Boswell gates and ditch were constructed in 1929 and consist of two gated structures and a two mile long excavated ditch. One structure is on the Big Sioux River and the other, in the ditch itself (Smith 1 Feb.). The Boswell gates and ditch were originally intended to divert the floodwaters from the Big Sioux River into Lake Poinsett and Dry Lake for storage. However, South Dakota’s Game, Fish and Parks, who has responsibility for the operation of the gates, has since recognized what effects the Big Sioux’s water could have on the quality of water in Lake Poinsett and made the gates inoperable (Smith 6).

To make the gates inoperable and return the river to its original path, the gates on the river have been welded open, to allow the river through, and the gates in the ditch have been welded shut, to prevent the water from reaching Dry Lake. The gates in the ditch do prevent the river water from reaching Dry Lake in most circumstances, but when levels exceed the height of the gates, the gates only act as a partial dam, and the water that runs over the gates may run freely into Dry Lake. Also, any water that is able to flow over the “lip” of the natural channel in the flood plain runs almost directly into a portion of the Boswell ditch that is behind the gates, where it flows almost without obstacle into Dry Lake (Smith 1 Feb.).

Runoff, which is the result of water not being able to soak into soil, feeds the streams and lakes in the watershed. Each of the lakes stores the runoff until it can feed the next lake in the line. Eventually, when all of the lakes in a chain are full, the water feeds into Lake Poinsett. Flood water from the Big Sioux River is also a minor source in filling Lake Poinsett.

The water quality is mainly affected by the nutrients that the runoff carries with it.

Nitrogen, which is dissolved in the runoff, has two ways it affects the lake. It is a health concern for small children and elderly people if the levels reach too high of a level and is a necessary element in the growth of algae and weeds in Lake Poinsett.

Phosphorous, which is attached to the sediment carried in runoff, is the prime limiting or supporting element for algae and pondweed growth. Nitrogen can not be stopped from entering the lake, but if the amount of phosphorous in runoff is reduced, pondweed and algae growth may be reduced also.

Fecal coliform poses only a health threat. If the levels in the water get too high, the beach must be shut down for immersion activities until they are brought back down to a healthy level. To date the levels of fecal coliform in the Big Sioux River between Castlewood, SD, and the outlet of Lake Poinsett have rarely been low enough to allow for immersion activities.