Approximately half of the freshwater lakes in the U.S. are man-made at 48%, and the other 52% are natural freshwater lakes. The most popular natural aspects of U.S. states are their lakes, which provide amazing views and vistas, habitats for abundant wildlife, electricity generation, transportation, water usage, and especially recreational opportunities. The largest group of lakes shared by Canada and the U.S. are the great lakes in North America.
Can Lakes Exist Without Rivers?
We call many reservoirs lakes, but they are not technically lakes. Lakes are naturally formed bodies of water surrounded by land, and reservoirs are typically created by damming a river. Open lakes or exorheic lakes drain into another body of water like a river that drains into the ocean, eventually. Closed lakes or endorheic lakes are reduced by evaporation and/or water seepage into the ground.
Generally no, few lakes in the U.S., both man-made reservoirs and natural lakes, can exist without rivers. Most depend on rivers or other sources for an inflow or outflow. Man-made lakes not technically lakes, but reservoirs formed by dams. Some have only one inflow or one outflow. Many lakes have multiple inflows, but fewer outflows.
For example, the United States Geological Survey (USGS) defines a reservoir like this: “A reservoir is the same thing as a lake in many people’s minds. But, in fact, a reservoir is a man-made lake that is created when a dam is built on a river. River water backs up behind the dam, creating a reservoir.”
The USGS defines a lake like this: “A lake is where surface-water runoff and some groundwater seepage have accumulated in a low spot, relative to the surrounding countryside. It’s not that the water that forms lakes gets trapped, but that the water entering a lake comes in faster than it can escape, either via outflow in a river, seepage into the ground, or by evaporation.” Most U.S. lakes are open lakes or exorheic.
Only a few lakes in the U.S., like the Great Salt Lake in Utah, are a closed or endorheic lake. There are several closed lakes in California, Nevada, North Dakota, Oregon, and Utah. States with one or two closed lakes are in New Mexico, Washington State, and Wisconsin, plus Devil’s Lake in both Wisconsin and North Dakota. Cryptoheic basins are lakes and rivers that typically drain underground into aquifers and sinks, but their waters can eventually reach the ocean.
How Do Lakes Stay Full?
Not all lakes stay full. Right now in the Colorado River Basin, Lake Mead in Nevada and Lake Powell in Arizona are experiencing dangerously low water levels due to the ongoing double-decade drought in the western U.S. There is not enough water flowing into the Colorado River to keep these lakes at a normal or slightly less-than-normal water level. Also, some lake beds are leakier than others, leading to seepage into the ground.
For a lake or reservoir to retain its water level over time, it has to refill. The primary way reservoirs replenish is from the rivers that were dammed to create them. Also, other rivers, tributaries, and streams may flow into them, plus rain and runoff from mountains and hills. Rivers and streams replenish most natural and man-made lakes.
Water can flow in or out from underneath a natural lake bed. Most natural lakes typically lie in the lowest elevation in their regions. They can receive water from underneath their lake beds, or they can leak water underneath their lake beds. Natural lakes have this advantage over reservoirs. It depends on what the lake bed’s composition is whether it seeps water in or out of the lake. Rocky-bottomed lake beds allow less seepage.
Aquatic Life Tells Us that Reservoirs Are Not Lakes
Other signs let us know that a difference between lakes and reservoirs exists. These signs come in the form of wildlife. Birds, fish, and plants that live in natural lake and river habitats have adapted to the seasonal changes that regulate changes in water levels. When a river is dammed up, the side in front of the dam will severely decrease in water levels on that part of the river. The reservoir side will drown the land in back of the dam.
These species do not quickly adapt to these and other changes. Reservoir water levels change according to human needs, like hydroelectric generation, water usage, and flood conditions, or even other lake water levels on the same river and dam systems. After damming, some reservoirs can take from five to ten years to impound. This creates serious, continuous, and erratic changes to the habitats of wildlife living there.
Fish and marine life species are important indicators of environmental changes because they cannot move to another habitat like turtles, etc., or birds. Different fish species require different conditions for a safe place to lay their eggs, like rocks or sand or water flow that only occurs at certain locations in a river. When a river is dammed, fish lose their habitat, where they search for food and where they breed depending on those conditions.
Indicator species (IS) direct scientists to understand environmental change impacts, like pollution, or if an impaired environment is managed well, or being restored. Aquatic life also gives warning signals about future shifts or changes to an ecosystem. Scientists choose only a few IS to study because it is too expensive to monitor many species in an ecosystem.
The quality of an IS must reflect a research team’s ability to record reliable and valid data. IS are bioindicators that accurately show and predict changes to changes in ecosystems. Researchers consider IS as surrogates in their environments. It is important for scientists to choose the most sensitive surrogates in the ecosystems they study. This is a relatively recent method of studying ecosystems and their environments.
For example, Bois d’Arc Lake near Bonham, Texas, is the newest reservoir in Texas in almost 30 years. The North Texas Municipal Water District (NTMWD) is employing a watershed protection management plan covering 17,000 acres surrounding Bois d’Arc Lake. This plan will mitigate environmental impacts on the reservoir for years to come based on past mitigation plans and the impacts on nature when building reservoirs. At this time, it is a 20-year plan.
A few of the most common species serving as IS in rivers, lakes, and oceans are lobsters, mayflies, mussels, and platypodes (platypus). Lobsters indicate rising water temperatures. Mayflies are extremely sensitive to pollution. Mussels cannot move, and indicate temperature, oxygen levels, and acidity in specific areas in water bodies. Platypodes serve as an indicator of rehabilitated water quality and successful conservation efforts.