With a surface area of 18,600 acres and a maximum depth of 85 feet, Mark Twain Lake is the largest reservoir in northern Missouri (Figure 1). The dam was initially proposed as a way to relieve flooding problems on the Salt River. U.S. Army Corps of Engineers completed construction in 1983, and now the lake provides drinking water, flood control, electricity and recreation opportunities for Missourians.

Figure 1. The location of Mark Twain Lake and the LMVP sample sites

Mark Twain Lake’s watershed covers 2300 square miles, with just over half of that covered by row crop agriculture. Grassland and prairie cover a quarter of the land in the watershed. These land uses are typical of northern Missouri, where the soils are deep and fertile.

The lake has water quality similar to the other lakes in the region. Chlorophyll, phosphorus and nitrogen concentrations are in the eutrophic range at the 3 sites monitored for the LMVP by the Mark Twain Lake Sailing Association. Suspended sediment is a dominant aspect of Mark Twain Lake’s water quality. Inputs of sediment, according to a 1986 study, equals 3 pounds per year for each square foot of lake area.

A study published in 1995 showed that in 79% of the observations made over a 29 month period light was the factor limiting algae growth. With so much sediment in the water, algae (which are plants) can’t get enough sunlight to continue growing. It’s similar to putting your African Violet in a closet. No matter how much fertilizer it has, it simply can’t grow in the dark. When there is enough light for algae to grow well, growth will continue until the point where phosphorus is limiting, the 1995 study showed.

(If you want to know what we mean when we say “limiting” click here to read about Liebig’s Law)

Figure 2. This graph shows Chlorophyll vs. Phosphorus in Missouri lakes. The light grey dots represent the 2004 LMVP data for all lakes. The dark diamonds represent LMVP data for Mark Twain Lake (2002-2004). Note how chlorophyll increases as phosphorus increases with all LMVP data (grey dots). In Mark Twain Lake, however, there is a mound shaped relationship. Increasing phosphorus means more sediment, which eventually shades out algae.

Phosphorus concentrations in Mark Twain Lake are tightly correlated with the concentrations of sediment in the water. This means that the more sediment you have in the lake, the more phosphorus you have. This can be a problem for the algae, which are typically limited by phosphorus in lakes. Algae need phosphorus to grow, but in Mark Twain Lake algae only get phosphorus in conjunction with sediments. These are the same sediments that block out the sunlight and limit the growth of algae! So what you get is a situation (Figure 2) where you get more algae growth as phosphorus increases…to a point. Eventually there is plenty of phosphorus, but not enough light for algae (plants, remember!) to grow. On Mark Twain Lake, light and nutrients are inversely proportionate (Figure 3), so that as you get more of one, you have less of the other.

Figure 3. Light and nutrients are inversely proportionate in Mark Twain Lake because of the suspended sediments. This means that the more nutrients you have, the less light you have (and vice versa).

The sediments that are suspended in Mark Twain lake are often very small. They can be so small, in fact, that they often pass right through the filters used by the LMVP to measure sediments. Therefore, we are likely underestimating the concentrations of sediments in the lake.

Interestingly, the 1995 study found that the amount of tiny clay particles in the Mark Twain Lake can actually decrease during an algae bloom. As algae grow, their cells tend to release organic compounds into the water. These compounds attach to the tiny clay particles and make them heavier. Eventually, the clay particles become so heavy they settle out. This is similar to the flocculation methods used in water treatment plants. Mark Twain Lake algae have the ability to make the water a little more clear so they can get more sunlight.

The 1995 study also showed that the impact of sediment inflows on Mark Twain Lake water quality is different in summer than in winter. In the winter the water column is mixed (not stratified) so inflowing sediments affect the entire water body. In the summer the lake is stratified and inflowing sediments tend to sink below the surface layer. So the surface layer is less affected by sediments in the summer than in the winter (Figure 4). It should be noted, though, that excess sediments still negatively affect Mark Twain Lake whenever they occur, they will simply have varying degrees of impact on the surface layer depending upon the time of year.

Figure 4. Due to the plunging of inflowing water in Mark Twain, sediments affect the surface layer more in the winter than in the summer. However, the lake bottom will fill in regardless of the season.

All reservoirs will eventually fill in with sediments if they aren’t dredged out. The rate at which this occurs is dramatically increased when land in the watershed is disturbed by development and certain agricultural practices. Fortunately there are several efforts to reduce the amount of sediments flowing into Mark Twain Lake due to land disturbances. For more information about these efforts, visit the links below.

Tony Thorpe

Assorted Mark Twain Lake Links

1995 Article: Temporal and spatial dynamics of suspended sediment, nutrients, and algal biomass in Mark Twain Lake, Missouri (pdf ~2.7 MB)

EPA page on Missouri Nonpoint Source Pollution, including the Mark Twain Water Quality Initiative
MDC's Salt River Watershed Inventory and Summary
LMVP's Drinking Water Quality article

Assessment of Nonpoint Source Dominated Watersheds in Missouri (Mark Twain and Smithville - pdf)
Mark Twain Lake TMDL

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