Runoff water is usually excess rain that falls or snow that melts when the ground is saturated and cannot absorb any more water. In an urban or residential setting can be found pooling on the grass or flowing along gutters into storm drains. In natural settings, run off feeds rivers and often fills lakes. In California, for example, melting snow from the Sierras feeds the Sacramento River and the San Joaquin River, the two largest river systems in California.
Hurricanes, flash floods, heavy storms, and simply being downstream from a major river like the Mississippi, can produce massive amounts of runoff water and trigger major floods. In fact, the Mississippi runs through Louisiana, helping make it the most flood-prone state in the US. Being a coastal state doesn’t help either; Louisiana suffered catastrophic flood damage when Hurricane Katrina struck in 2005.
Obviously, runoff water doesn’t always produce natural disasters. In fact, it is both a critical natural resource and sometimes, a real headache.
Natural Runoff
When excessive rain produces runoff in a natural, undeveloped setting, some amount of water returns to the atmosphere via evaporation and moves through the hydrologic cycle. Most, however, flows through a geographical feature called a basin and is directed through a series of streams and rivers until it eventually returns to the ocean. When both snow and glaciers melt, they can create massive amounts of runoff.
When it’s not fully saturated, soil absorbs precipitation, retaining moisture for plants to access, and trickling deep down and recharging the water table. The water table feeds wells and spring-fed lakes and ponds.
The Problem
Natural runoff, occurring in a natural setting, often isn’t identified as a problem, but it can be, and it can even be related to human activity. Even if rain falls in an undisturbed forested area, the runoff may pass through areas where the soil is disturbed by human development. Runoff can quickly cause erosion, picking up soil and silt and depositing it into rivers and streams. Not only is erosion damaging and dangerous to infrastructure like roads and bridges, but large amounts of silt and soil entering aquatic ecosystems can cause severe, long-lasting damage to the overall health of the waterway, potentially spreading for many miles downstream.
Municipal Stormwater
Runoff water can be a real challenge to manage safely in municipal areas, whether they’re sprawling metropolitan areas like Los Angeles, or quiet, residential and downtown areas in small cities throughout the country. Large or small, these developed areas are inevitably covered by hard surfaces like pavement, sidewalks, buildings, and parking lots. When precipitation hits these hard surfaces, the water cannot seep into the ground, so all of it becomes runoff. Traditionally, this kind of runoff has been managed by quickly directing it to municipal stormwater systems, which direct the water to nearby streams or lakes. When those systems can’t handle the volume of precipitation or fail for some other reason, flooding occurs.
The Problem:
Although it starts with largely clean rainwater, once it hits hard surfaces such as roads and sidewalks, municipal runoff becomes contaminated. Oils, pesticides, heavy metals, and toxic elements found in vehicle exhaust can all be picked up as flowing water washes off streets. Water that runs off sidewalks or even across residential yards and parkland picks up fertilizers, weed killers, and organic waste (feces) from pets and wildlife, which often carry parasites and diseases. Most cities are concerned, first and foremost, with avoiding flooding, so their management strategy focuses on getting rid of stormwater as fast as possible. In practical terms, this means dumping it straight into local rivers and streams. Unfortunately, a large, sudden influx of contaminated water can’t be filtered through natural means, and it can quickly overwhelm and kill the river ecosystem. As the polluted water continues to flow downstream, it can spread disease, trigger algae blooms, decimate fish populations and contaminate marine life, including commercial shellfish and shrimp.
Agricultural Runoff
Agricultural runoff originates, not only from storm events, but directly from farming practices such as irrigation and livestock management. Just like municipal runoff, farms produce both organic and inorganic contaminants, but the most troublesome contaminants are usually excess nutrients such as nitrogen, phosphorus and potassium, which come from both organic waste and fertilizers. Common sources of organic pollutants come from farmyards, cultivated fields, and wastewater. This includes water used in cleaning operations in livestock facilities like barns, swine houses, or battery cages for chickens. Processing facilities for agricultural products produce large amounts of wastewater, and some irrigation practices trigger runoff that flows directly over the fields.
The Problem
Irrigation runoff can trigger significant problems as it enters waterways. Agricultural fields are regularly disturbed with tilling, and since weeds are usually discouraged and inter-row cover crops are not always used, there’s plenty of bare soil. Not only heavy rainfall, but imprecise irrigation using overhead sprinkler systems like pivots or spray guns can produce runoff when the soil can no longer absorb the incoming water. Runoff that travels over bare soil picks up particles of dirt and carries it with it, ultimately depositing it into lakes and streams. This kind of sedimentation can carry excess nutrients and make the water cloudy, which contributes to the process of eutrophication, algae blooms, and eventually dead zones. High levels of sediment can also directly harm fish and aquatic plants.
