Freshwater Lakes and Rivers |
Freshwater Lakes and Rivers |
Rivers and streams are the routes through which fresh water travels to the oceans. Rivers can carry large amounts of water. Earth's largest rivers include the Nile in Africa, the Amazon in South America, and the Mississippi in North America. Thousands of smaller streams, creeks, and brooks carry water into these larger rivers. The fresh water comes from a number of sources, including melting snow, rain, and glacial meltwater.
Fresh water has a low salt concentration—usually less than 1%. Plants and animals in freshwater regions are adjusted to the low salt content and would not be able to survive in areas of high salt concentration, like the ocean.
Courtesy of Jacques Descloitres, NASA/MODIS Land Science Team Courtesy of NASA The Great Lakes are the largest natural freshwater lakes in North America. They were formed by continental glaciers. This photo taken by space-shuttle astronauts includes the Mississippi River as it meanders near St. Louis, Missouri.
Lakes can range in size from small ponds to huge bodies of water such the Great Lakes in the United States. Lakes develop in depressions or basins on Earth's surface. They are filled by water flowing in from streams, from groundwater beneath Earth's surface, and by precipitation and melting snow and ice. Lake water stays fresh as long as water flows in and out of the basin, usually through streams.
If there is no outlet, as in the Great Salt Lake in Utah, the water leaves through evaporation. Minerals that are dissolved in the water remain in the lake, and their concentration increases. The lake becomes saline or alkaline.
Freshwater rivers and lakes account for approximately 0.009% of Earth's total water and cover a little over 0.3% of the land surface. Lakes and rivers are closely tied. Some lakes are the source for rivers, and some rivers end in lakes. Since both rivers and lakes have fresh water and flow in and out of each other, they share similar characteristics and many plant and animal species.
Rivers and Streams
A river is a natural stream of water that flows in one direction in a channel with defined banks. A river or stream forms whenever water moves downhill from one place to another. Some rivers begin high up in the mountains, fed by melting snow or ice. Others collect water from rain and melting snow and ice and from other streams. Others may have their source in springs flowing out of the ground.
All rivers flow down a slope. The steeper the slope, the faster the water flows. As the water reaches flatter areas, it slows down. Water in the Mississippi River moves so slowly that it may appear still. But it is still flowing downhill to the Gulf of Mexico.
Courtesy of Sue Jagoda Courtesy of Upper Midwest Environmental Sciences Center,
U.S. Geological Survey The Colorado River flows swiftly over hard rock in the Grand Canyon. The Mississippi River flows over rolling terrain composed of soft rock and soil near New Madrid, Missouri. Notice the oxbow lakes formed from the meandering of the river in the past.
Rivers flow in channels over different types of ground, or substrate. Water can move into the river channel from the ground on either side of it. The area underground where the water flows is the hyporheic zone (hy-po-REE-ik). Depending on how gravelly or rocky the substrate is, a river's hyporheic zone can extend far away from the open river channel. The area next to a river channel that is covered with water during a flood is called the floodplain. Some rivers meander through wide valleys with large floodplains. Others may flow swiftly through narrow canyons, with little or no floodplain.
Many abiotic factors affect rivers and streams. The temperature of the water depends on where the river is located. Rivers that have their source high in the mountains are fed by melting snow and glaciers. This water is very cold. When it flows through warmer climates, the water is usually warm. But even in a warm place, it is cold if its source is underground.
If the river is shallow and flows over a rough substrate like solid rock, the flow will be rough and turbulent. A turbulent river constantly adds oxygen to its water. Deep water flowing slowly over a smooth bottom, such as mud or clay, forms pools. Where deep water flows rapidly over a flat bottom, smooth runs of water are common. Ponds and smooth runs have less oxygen in the water than turbulent areas.
Lakes and Ponds
Ponds are small bodies of fresh water shallow enough to support rooted plants. Water temperature is fairly uniform from top to bottom. It changes with air temperature. There is little wave action, because the water is shallow. The bottom of a pond is usually covered with mud. The amount of dissolved oxygen in a pond may vary greatly during a single day. In very cold locations, an entire pond may freeze solid.
Courtesy of Karen Holland, U.S. Environmental Protection Agency Courtesy of Julie Donnelly-Nolan, U.S. Geological Survey The shore is rocky at Chequamegon Bay in Lake Superior. Medicine Lake formed in the caldera of Medicine Lake Volcano. This view is toward the west-southwest. Mt. Shasta is on the horizon.
Lakes are bigger than ponds and are too deep to support rooted plants except near the shore. Some lakes are deep enough and wide enough for waves to form. Water temperature during summer months is not uniform from to to bottom. Three distinct layers develop.
Light does not penetrate to the bottom of the lake, so photosynthesis is limited to the epilimnion. Almost all organisms spend the summer months in the epilimnion with its warmer waters and more abundant food supply.
During the fall, overturning occurs. The epilimnion cools with the cooling air and becomes more dense than the water below. It sinks and mixes with the hypolimnion. Wind helps mix the water. The lake circulates, or overturns, completely. The lake overturns again in the spring, when the surface ice melts and the resulting cold water sinks.
Most lakes are large enough to not freeze solid in winter months. A layer of ice can develop on the top of lakes during winter. The ice blocks out sunlight and can prevent photosynthesis. Without photosynthesis, oxygen levels drop, and some plants and animals may die. This is called winterkill.
Because rivers and lakes are often interconnected and water flows between them, many aquatic creatures move between them in search of food. Some creatures prefer to be in either a river or a lake, but many spend some of their lives in both environments.
During the winter months some animals hibernate in the quiet bottom mud of rivers and streams. Some fish continue to feed, but less actively.
Rivers and Streams
The characteristics of a river or stream change during the journey from its source to its mouth. The temperature is usually cooler at the source than it is at the mouth. The water is also clearer and has higher oxygen levels. Freshwater fish, such as trout, and heterotrophs can be found nearer the source.
Courtesy of Don Breneman,U.S. Environemntal Protection Agency Courtesy of U.S. Fish and Wildlife Service The great blue heron is a common sight near lakes and rivers and in wetlands. Salmon hatch in the clear, cold streams in Alaska, the Pacific Northwest, and New England. They spend their adult lives in the ocean and then return to the streams where they were born to lay their own eggs. Salmon are important commercial and sport fish. In Alaska and Canada, salmon are a very important part of the diet of native people and bears.
As the water flows downstream, the rive's width increases and its flow slows down. Species diversity increases with width. Numerous aquatic green plants and algae, which could not live in the faster-moving water upstream, can be found. The nutrient content of the water is largely determined by the land and vegetation surrounding the river. Erosion of the streambed can add to the inorganic nutrients in the water.
As water reaches a river's mouth, it becomes murky from the sediments it has picked up. The sediments decrease the amount of light that penetrates the water. With less light, there is less diversity of flora. The river also has less oxygen. Fish that live near the mouth of a river, such as catfish and carp, tolerate low levels of oxygen.
Many life-forms live in a river's hyporheic zone. Mayflies and other insects and crustaceans can move into the hyporheic zone to hide between rocks. They are protected from predators, yet are in flowing water that brings them nutrients.
Lakes and Ponds
Ponds and lakes may have limited diversity of species when they are isolated from one another and from rivers and oceans. Lakes and ponds are divided into three different zones, which are usually determined by depth and distance from the shoreline.
Courtesy of National Park Service, Indiana Dunes National Lakeshore, U.S. Environmental Protection Agency Courtesy of National Park Service, Indiana Dunes National Lakeshore, U.S. Environmental Protection Agency A variety of gulls can be found along lakeshores. This one lives on the shore of Lake Michigan. The common snapping turtle is often found buried in mud in shallow water of ponds and other wetlands. It is omnivorous. In the water it is usually passive. On land, it may lunge and snap.
The shallow-water, or littoral, zone is the topmost zone near the shore of a lake or pond. It absorbs the Sun's heat and tends to be warmer than deeper zones. Usually a diverse community of organisms lives in the littoral zone. These include several species of algae (like diatoms), rooted and floating aquatic plants, grazing snails, clams, insects, crustaceans, fishes, and amphibians. For some insects, such as dragonflies and midges, only the egg and larval stages are found in this zone. The vegetation and animals living in the littoral zone are food for other creatures such as turtles, snakes, and ducks.
The open-water, or limnetic, zone is near-surface open water surrounded by the littoral zone. The limnetic zone is well-lighted like the littoral zone. Both phytoplankton (plants) and zooplankton (animals) thrive in the limnetic zone. Plankton are small, floating organisms that play a crucial role in the food chain. A variety of freshwater fish also occupy the limnetic zone.
Courtesy of Duane Raver, U.S. Fish and Wildlife Service Several kinds of trout live in rivers, lakes, or both. This is a rainbow trout. It can reach a weight of 24 kilograms (52 pounds) and a length of 1.2 meters (3.75 feet). It thrives on a diet of small fish and invertebrates. 
Courtesy of D. Mason, University of Oregon The mayfly is a common freshwater, aquatic insect. It is the only insect to molt after its wings become functional.
Plankton have short life spans. When they die, they sink into the deep water of a lake or pond. This deep-water zone is the profundal zone. This zone is much colder and denser than the other two. Little light penetrates all the way into the profundal zone. Heterotrophs are the organisms that live here. They eat dead organisms and use oxygen for cellular respiration.
The issues that affect freshwater lakes and rivers are similar to those that affect other bodies of fresh water. Habitats are altered by human development. Development along river and lakeshores increases erosion and causes more sediments to wash into rivers and lakes. The extra sediment lowers the amount of light that passes through the water.
Courtesy of Ohio Department of Natural Resources, Fairport Fisheries Station, U.S. Environmental Protection Agency Courtesy of Karen Holland, U.S. Environmental Protection Agency A sport-boat marina on Lake Erie. Families enjoy a day at the lake at Warren Dunes, Indiana.
Many lakes and rivers are popular recreation sites. As more people use these areas, valuable habitat for native plants and animals is lost.
Courtesy of Romy Myszka, U.S. Department of Agriculture, Natural Resources Conservation Service, Environmental Protection Agency Courtesy of National Park Service, Indiana Dunes National Lakeshore, U.S. Environmental Protection Agency Junked cars along the Illinois River are an eyesore and a potential source of pollution. This pipe dumps pollution right into a river. This type of pollution is identified as a point source.
Pollution is introduced by industry and other human activity. Toxic chemicals can poison organisms and cause illness. Mining can damage groundwater sources next to rivers and streams. Vegetation that supports lake and river habitats may disappear.
Lake and river levels can be changed. Humans may dredge out areas to make water deep enough for ships. Dams are built on rivers to form reservoirs. Reservoirs may provide drinking water for neighboring cities or hydroelectric power. Dams change the water levels above and below the dam site as well as the volume of water flow. A dam may block the path of fish that live in different parts of the river at different times during the year.
Courtesy of National Biological Service, U.S. Environmental Protection Agency Courtesy of Sue Jagoda Polluted lake and river water can harm fish. This bullhead catfish has developed a tumor in its mouth. Dams, like Hoover Dam on the Colorado River, change water levels both above and below the dam site.
Changes in the food web can occur when exotic (nonnative) species are introduced to a river or lake. These new species compete for food and space with the native species. Exotic species often have no enemies in their new environment. They may thrive and take over.
Courtesy of Center for Great Lakes and Aquatic Sciences,
U.S. Environmental Protection Agency Courtesy of Karen Holland, U.S. Environmental Protection Agency These zebra mussels are an exotic species introduced into the Great Lakes. They are a threat to the health of native species. Purple loosestrife growing along the shore of Lake Huron can clog the flow of water in and out of the lake.