Our program is called the Lakes of Missouri Volunteer Program, though it would probably be more aptly titled “The Reservoirs of Missouri Volunteer Program”. Except for a single oxbow lake, Creve Coeur in St. Louis, all of the water bodies ever sampled for the LMVP have been man-made reservoirs. For this reason, it seems appropriate that we discuss what it means to be a reservoir.

Reservoirs are constructed to address one or more needs such as drinking water supply, irrigation, recreation, flood control, power generation, or navigation. There are 2 broad categories of reservoirs, the valley reservoir and the off-river storage reservoir. The valley reservoir is most common in Missouri. To create a valley reservoir, a dam is constructed perpendicular to a river and the river subsequently pools up behind the dam. The off-river storage reservoir is created near a river and water is diverted to it via gravity or by pumping. Delaney Lake in Mississippi County is an example.

In reservoir construction there are 4 basic dam types:
Arch dams are made out of concrete in an arch shape, with the convex side facing upstream. Arch dams can be much thinner than other types of dams, because they incorporate the inherent strength of the arch shape in their design. This type of dam requires very solid rock and for that reason is reserved for narrow, steep sided valleys. An example is the Hoover Dam on the Colorado River.

Buttress dams are made out of concrete (or masonry) and are triangular in cross section. This type of dam is strengthened by regularly spaced supports, or buttresses, along the downstream side of the dam. The supports help to counter the pressure of the water against the dam wall. An example is Powersite Dam at Lake Taneycomo.

Gravity dams are also made of concrete (or masonry). They are essentially giant blocks that use their weight to hold back the water. Gravity dams need to be built on a solid foundation of rock. An example of this type is Bagnell Dam at Lake of the Ozarks.

Embankment dams are constructed of rock or earth. They may have either a central core or a cover layer of impenetrable material, usually clay or concrete. Embankment dams are suitable for areas with wide valleys and may be built on softer soils. An example is Clarence Cannon Dam on Mark Twain Lake.

According to UN data, there were 800,000 dams worldwide in 1997. Around 25% of the water that previously flowed to the oceans is impounded in reservoirs. These dams are more concentrated in the temperate and sub-tropical latitudes. Most of the 1,700 or so new large dams under construction are in developing countries.

Reservoirs vs. Lakes

Physical Differences

Though they seem similar to one another, reservoirs and lakes are different in a number of ways. One way to differentiate them is by their shape when viewed from above. Reservoirs usually have a dendritic appearance, thin at the upper stretches and thicker toward the dam. This is a result of the water backing up into the feeding streams as the reservoir filled in. Natural lakes, on the other hand, are usually much more rounded, due to the processes that form them (glacial, volcanic, tectonic).

Reservoirs are deeper at the dam and become shallower at the upper end. The deepest part of a natural lake is generally at the center.

Note the contrasting shapes of these 2 waterbodies. The man-made reservoir has a dendritic appearance, while the natural lake has a round shape. pictures are NOT to scale

Differences in Water Quality
Reservoirs usually have greater inputs of pollutants than natural lakes. This is primarily due to the greater volume of water flowing into reservoirs than lakes. However, this greater inflow also means that water must be flowing out of the reservoir at a faster rate. The amount of time a waterbody holds a particular unit of water is called its residence or retention time. Among Missouri reservoirs, retention times vary considerably. For example, Taneycomo water is replaced every 3 days on average, while Stockton Lake water sticks around for 1.2 years. The great Lake Superior, a natural lake, has a retention time of 500 years.

The water quality in natural lakes tends to improve toward the center of the lake. In reservoirs, water quality improves longitudinally, toward the dam. For reservoirs, this is largely due to the deposition of sediments and other materials as the water velocity decreases.

What happens downstream of a dam in another thing to consider. Typically the water coming out of the dam has lower concentrations of nutrients and sediments than the stream . In some cases, though, reservoir managers may choose to pull water through the dam from near the bottom of the lake. This is called selective withdrawal. Deep, hypolimnetic (see page 5 of the Spring 2002 Waterline) water is colder, has higher concentrations of nutrients and less oxygen than the surface water. Sending this water downstream can have significant effects on the physical, chemical and biological characteristics of the stream.

When several reservoirs occur on the same river, the first reservoir often has higher concentrations of nutrients and suspended sediments. An example of this is the Jackson County lakes, Jacomo, Blue Springs and Prairie Lee. See the figure below.

The reservoirs 'Lake' Jacomo and Prairie Lee are located on the East Fork of the Little Blue River in Jackson County. The numbers represent the average 1999 Secchi transparency values in inches. Prairie Lee catches much of the sediment that would otherwise flow into Lake Jacomo.

Secchi transparency is a measure of water clarity. Higher numbers indicate clearer water.

I hope this helps in your understanding of reservoirs. I apologize in advance, but I’m going to continue calling our reservoirs “lakes”. The RMVP just doesn’t have the same ring to it.

Tony Thorpe

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