Surface runoff is the flow of water across the land’s surface when the ground cannot absorb it quickly enough. This water travels over fields, roads, and other land areas before entering streams, rivers, and oceans. While runoff is a natural part of the water cycle, human development has created impervious surfaces like pavement and rooftops that prevent water from soaking into the soil. This forces a greater volume of water to move rapidly over the surface, transforming a natural process into a major environmental concern that affects water bodies and the landscape.
Physical Damage Through Sedimentation and Erosion
The physical force of uncontrolled runoff is a primary driver of soil erosion, detaching and transporting topsoil from agricultural fields and unprotected land. This process is detrimental to farming, as runoff removes the most fertile layer of soil rich in organic matter and nutrients. The loss of topsoil diminishes the land’s ability to retain water, making crops vulnerable to drought and requiring greater inputs of synthetic fertilizers.
The material carried away by the runoff eventually settles out in waterways through sedimentation. This deposited sediment physically alters aquatic environments, filling drainage ditches, rivers, and reservoirs. The accumulation of fine particles can smother the gravel beds where fish lay eggs and eliminate the habitat of benthic organisms like macroinvertebrates.
Fine soil particles suspended in the water cause increased turbidity, or cloudiness. Turbid water absorbs more sunlight, which can lead to warmer water temperatures that stress cold-water fish species.
Impairment of Water Quality by Pollutants
As runoff moves across different landscapes, it acts as a nonpoint source of pollution, collecting and carrying a diverse array of chemical and biological contaminants. A major concern is the mobilization of nutrients, primarily originating from agricultural fertilizers and animal manure. These compounds are rapidly washed from fields into surface waters during storm events.
Runoff from urban and industrial areas introduces various toxic substances that accumulate on impervious surfaces. Vehicle wear is a significant source of heavy metals, with copper originating from brake pad abrasion and zinc coming from tire wear and galvanized materials. These heavy metals bind to fine sediment particles and are transported into aquatic systems, posing a toxicity risk to aquatic life.
Another consequence is the transport of pathogens. Sources of these biological contaminants include failing residential septic systems, improperly managed livestock operations, and pet waste in urban areas. Runoff mobilizes pathogens, such as E. coli and Cryptosporidium, into local waterways, directly affecting water quality and posing a threat to public health.
Ecological Disruption of Aquatic Habitats
The influx of excess nutrients carried by runoff triggers eutrophication in aquatic ecosystems. Nitrogen and phosphorus stimulate the rapid, excessive growth of algae and cyanobacteria, often resulting in dense, harmful algal blooms. These blooms block sunlight from penetrating the water column, inhibiting the photosynthesis of submerged aquatic vegetation (SAV) like seagrasses, which provide habitat and food sources for many species.
When algal blooms die, they sink to the bottom where decomposer bacteria consume them. This decomposition rapidly depletes the dissolved oxygen in the bottom water layer. The resulting low-oxygen conditions create hypoxic zones, or “dead zones,” where fish and other mobile organisms cannot survive and benthic life is suffocated.
Increased turbidity from suspended sediment and algal growth further disrupts the ecosystem balance. Reduced light penetration means the water column can no longer support aquatic plant growth, leading to a loss of structural habitat for fish and invertebrates. The subsequent reduction in biodiversity impacts the entire food web, favoring only species capable of surviving the altered, degraded conditions.
Consequences for Human Infrastructure and Health
The volume of uncontrolled runoff from impervious surfaces frequently overwhelms aging stormwater management systems, leading to urban flooding. This flooding causes costly damage, including the collapse of roadways, the formation of sinkholes, and structural damage to private property and building foundations. The annual damage from urban flooding in the United States alone is estimated to average billions of dollars.
Physical infrastructure is also damaged by the erosive force and sediment load of the water. Culverts and drainage pipes become clogged with sediment and debris, causing water to back up or be redirected, which further erodes nearby roads and bridges. Repairing or replacing this damaged infrastructure requires significant public spending and diverts municipal funds from other services.
Contaminated runoff poses a direct risk to human health, especially through recreational water use. Pathogens like E. coli enter recreational waters, and when bacteria counts exceed safe limits, public health officials must issue advisories or close beaches entirely. Swimmers exposed to this contaminated water risk gastrointestinal illnesses, including gastroenteritis, as well as ear, eye, and respiratory infections.
The degradation of source water quality by sediment and pollutants significantly increases the cost of providing clean drinking water. Water treatment plants must use more complex and energy-intensive processes to remove high levels of turbidity, pathogens, and chemical contaminants. Studies show a direct correlation between source water quality and operating costs, with lower turbidity levels leading to a measurable reduction in the chemical costs required for water purification.