Pennsylvania is a well-drained state. We have a tremendous quantity of streams and waterways, both above and below ground, that contain and move an enormous quantity of water. With so much moving water, we have very few natural lake systems. We simply don’t have the right conditions throughout most of the state to afford water the luxury to just sit there. Therefore, most of Pennsylvania’s lakes are manmade and engineered to act as a reservoir or for flood control, such as Raystown Lake.
Manmade lakes often require dredging or other active control methods to ensure the lake remains a lake and does not transition into other forms of wetlands. Periodically they can even be completely drained for extensive rejuvenation projects. Locally, Colyer Lake and Lake Perez underwent this level of work, revealing a strange landscape of mud, partly decomposed trees, and the remnants of the original stream channel.
Of course, people aren’t the only hydrologic engineers capable of altering the flow of streams for their benefit. The state’s largest rodent, the beaver, is an expert builder. Beavers were extirpated, or extinct within a region, in Pennsylvania at the end of the 19th century but were successfully re-introduced over the last hundred years to reach stable populations. These fuzzy friends can sometimes be seen along slow-moving streams and smaller lakes as they create habitat for a number of bird, mammal, and reptile species that rely on the unique wetland they create for survival.
Some bodies of water in PA were formed naturally during the retreat of the glaciers. While there are hundreds of these geologic puddles in the far northeast of the state, they are very rare in our region. Most of these lakes are managed by humans for recreational or scenic value and often involve some form of outflow control to stabilize their level.
One benefit to manmade lakes is that their ecosystems can exhibit fascinating stability, often providing habitat and prime space for what normally would be transient species. Human intervention also helps prevent destructive cycles from taking over lakes, even if some of the initial disturbances are human caused. Chief among these are the nutrient distributions and algal blooms, which can result in so much plant growth in a lake that the animal life cannot survive due to lack of oxygen.
A stable and healthy lake, on the other hand, exhibits a process where its waters actually “turn over” in the spring and fall. This circulation ensures that oxygen is delivered into the deeper reaches of the lake and allows for fish to survive the surface completely freezing over. In the summer, the warm surface water becomes stratified, or separated into layers, as it is unable to cool due to the constant transfer of heat from the summer sun. As temperatures begin to fall that same water which was constantly heated now cools at a faster rate than the deeper waters. The layers then begin to intermix as the cold water sinks until the entire water column is about the same temperature. When ice begins to form over the surface an interesting inversion occurs where a layer of near-freezing water is trapped next to the ice but the water towards the bottom of the lake retains its temperature. When the ice melts the same mixing begins again, keeping all the fish happy and allowing them to breathe.
Source: National Geographic
If we did not actively control the form of our lakes, slow but inevitable succession, or hydrosere, would take hold. Lakes would transform from open water to different types of wetlands until ultimately becoming mature forest. Natural lakes typically evolve through these stages over the course of thousands of years depending on the exact amount of siltation present in the water. Slowly, inorganic mater builds up on the floor of lakes until it is close enough to the surface that plant life can begin to anchor itself to the bottom and still reach the upper layers. These anchoring plants trap even more sediment and contribute to the gradual shallowing. As the plants and dead plant matter close in, larger and thirstier species begin to drain away much of the water drying out the wetland and transitioning into a forest.
Source: Texas Gateway