Comparing Lakes, part 1 – the natural factors

There are many factors that go into evaluating a lake. Once you understand some of these factors, you should have a better understanding of your lake, where it came from and where it’s headed.

Lake1There are natural factors that contribute to a lake’s condition, and in most cases around here, there are also human factors that have affected the lake. This week I’ll explain the natural factors since those came first. Next week I’ll explain the human factors and then put them together.

Most of the lakes in Minnesota were formed as glaciers receded during the last ice age. Approximately 15,000 years ago to about 9,000 years ago, glaciers alternately retreated and advanced over the landscape, carving out holes and leaving behind ice chunks. As these ice chunks melted in the holes left behind, lakes were formed. Northern Minnesota was scraped fairly clean down to the bedrock, with boulders, sand and clay left behind, while southern Minnesota was left with a rich, fine prairie (now agricultural) soil.

The first thing that goes into understanding a lake is what sort of geological area it is in. Bad Medicine and Juggler lakes are very deep, rocky lakes in forested areas. These lakes have very clear water and characteristically low phosphorus and algae concentrations due to the abundance of sandy, relatively infertile soil. The lakes around the Lake Park area are shallower prairie lakes surrounded by fertile soil. Lakes in this area tend to have more nutrients available for plants and algae to grow, and therefore get “greener” in the summer.

The geology and glacial formation of a lake usually determines its shape, size and depth. These factors contribute to nearly all physical, chemical and biological properties of a lake. Lake users such as fishermen are probably aware of these characteristics already because they also determine where the fish are. A lake that is one large round hole is different than a lake that has a lot of bays, points and bottom structure. A long narrow lake is more affected by wind (which mixes the lake) than a round lake.

Deep lakes have different dynamics than shallow lakes, and most of all, deep lakes have more water. The more water a lake has (volume), the better it is able to dilute what runs into it. Now this of course doesn’t mean we don’t have to worry about what we allow to run into large deep lakes! Even Lake Superior, which is the largest freshwater lake in the world by area and the third largest lake in the world by volume, is affected by what runs into it.

Shallow lakes are lakes where the sunlight can reach the entire bottom. Generally, this corresponds to about 15 feet deep or less. Since the sunlight can reach the bottom, aquatic plants are able to grow there. In deep lakes, the bottom does not receive sunlight, so no plants grow there and it stays dark and cold.

Another major factor affecting lake condition is the size of its watershed and where the lake sits within the watershed. A watershed is an area of land where all the water drains into the same river system. These watershed areas are defined by topography, or ridges of elevation. Therefore, watersheds are mainly driven by gravity – water runs down hill.

If a lake has a very small watershed or is at the top of a watershed (in topography terms), the lake usually has better water clarity than a lake at the bottom of a large watershed. As water flows downhill through a watershed it picks up sediment from erosion and nutrients from runoff. This sediment and nutrients can feed algae and cause the lake to become “greener”.

Next week, I’ll combine these natural characteristics of lakes with human factors and sum up how to track trends and changes.

For more information about lake biology, a great resource is the Guide to Lake Protection and Management, published by the Freshwater Society in cooperation with the Minnesota Pollution Control Agency. It can be found at: