Surface water, including rivers, lakes, streams, and wetlands, is the most visible and accessible source of water for human use. These systems are profoundly interconnected, with the health of one water body influencing others within the same watershed. Human civilization relies heavily on these sources for drinking, agriculture, and industry. However, human activities consistently introduce alterations that modify the natural functions of these aquatic environments, making understanding these changes crucial for preservation.
of Contaminants and Pollutants
The direct discharge of waste into surface water bodies represents a point source of contamination that immediately degrades water quality. Industrial facilities often release chemical pollutants, including heavy metals like lead and mercury, which can bioaccumulate in aquatic organisms and pose health risks to humans. Pharmaceuticals and personal care products are also emerging threats, contributing to antimicrobial resistance.
Municipal wastewater treatment plants (WWTPs) are a primary source of nutrient loading, discharging effluent rich in nitrogen and phosphorus into rivers and lakes. This introduction of excess nutrients accelerates cultural eutrophication, leading to explosive growth of algae and cyanobacteria. When these blooms decompose, they consume vast amounts of dissolved oxygen, creating hypoxic “dead zones” that cannot support aquatic life.
Untreated sewage is a major vector for pathogenic microorganisms, including bacteria like E. coli and viruses such as Norovirus, which can persist in surface water. Combined sewer overflows (CSOs) exacerbate this when rainfall overwhelms municipal sewer systems, causing raw sewage to discharge directly into waterways and threatening public health.
Alterations to Water Quantity and Flow
Human activity extensively manipulates the physical volume and movement of surface water, beyond just affecting quality. Large-scale water withdrawal for municipal supply, industrial cooling, and irrigation significantly reduces the volume of water available in rivers and lakes. Pumping from shallow aquifers connected to surface water can intercept groundwater, reducing the base flow that sustains streams during dry seasons.
The construction of dams and reservoirs fundamentally alters a river’s natural flow regime and ecological function. Dams create large impoundments upstream, transforming free-flowing rivers into slack-water environments that trap sediment and change the water temperature profile. This interruption starves the downstream channel of the gravel, sand, and silt needed to maintain its natural shape and substrate composition.
The water released below a dam, often called “hungry water,” retains its erosive energy but has lost its sediment load. This clear water scours the downstream riverbed, causing channel incision and bank erosion that degrades aquatic habitats. Furthermore, water release timing is managed for human needs, such as power generation, disrupting the natural seasonal flow pulses necessary for native species reproduction and floodplain connectivity.
Impact of Land Use and Watershed Changes
Land use practices introduce pollutants diffusely, known as non-point source pollution, which is difficult to trace. Agricultural runoff is a major contributor, carrying excess fertilizers, herbicides, and pesticides into streams and rivers during rain events. The delivery of nitrogen and phosphorus contributes significantly to regional eutrophication.
Urbanization fundamentally changes how water moves by replacing permeable soil with impervious surfaces like roads and parking lots. These surfaces prevent rainwater infiltration, dramatically increasing the volume and speed of surface runoff. This rapid runoff often overwhelms drainage systems, leading to more frequent flash flooding in urban streams.
Urban runoff also carries a concentrated load of pollutants, including petroleum products, heavy metals, and road salts. Impervious surfaces also prevent groundwater recharge, reducing the water available to feed surface streams during dry periods.
Deforestation and logging activities further contribute to non-point source pollution by removing protective canopy and root systems. This loss of vegetation accelerates soil erosion, leading to increased sediment loads in surface waters. It also results in the loss of riparian buffers that naturally filter pollutants and shade the water.
Thermal and Biological Shifts
Human activities cause direct changes to the temperature of surface waters, creating thermal pollution that stresses aquatic ecosystems. Power generation and industrial manufacturing often utilize surface water for cooling and then discharge the warmed water back into the source body. Since warm water holds less dissolved oxygen, this discharge creates localized zones of low oxygen that are inhospitable to many fish species.
A sustained increase in water temperature disrupts the metabolism, reproduction, and migration patterns of temperature-sensitive aquatic organisms. This thermal stress may favor the growth of thermophilic species, altering the biological community. The loss of natural stream shading due to the removal of streamside vegetation also contributes to thermal stress by allowing direct solar heating.
Human commerce and recreational activities serve as pathways for introducing non-native species, which drastically alter the biological composition of surface waters. Vectors include the discharge of ballast water from ships and the movement of recreational equipment, transporting organisms like zebra mussels or invasive aquatic plants. Intentional introductions, related to aquaculture or sport fishing, introduce new predators or competitors that displace native species.
These biological shifts are compounded by human-caused climate change. Climate change introduces long-term temperature stress, altered precipitation patterns, and increased evaporation, further disrupting the delicate balance of surface water ecosystems.