What is runoff

Water will flow along channels as it moves into larger creeks, streams, and rivers. This picture gives a graphic example of how surface runoff here flowing off a road enters a small creek. The runoff in this case is flowing over bare soil and is depositing sediment into the river not good for water quality. The runoff entering this creek is beginning its journey back to the ocean. As with all aspects of the water cycle, the interaction between precipitation and surface runoff varies according to time and geography.

Similar storms occurring in the Amazon jungle and in the desert Southwest of the United States will produce different surface-runoff effects. Surface runoff is affected by both meteorological factors and the physical geology and topography of the land. Only about a third of the precipitation that falls over land runs off into streams and rivers and is returned to the oceans. The other two-thirds is evaporated, transpired , or soaks infiltrates into groundwater. Surface runoff can also be diverted by humans for their own uses.

The small creek shown in the picture above will merge with another creek, eventually flowing into a larger river. Thus, this creek is a tributary to a river somewhere downstream, and the water in that river will eventually flow into an ocean. The concept is not that much different from the small capillaries in your body carrying blood to larger arteries, eventually finding its way to your heart, analogous to the ocean.

Severe erosion can occur when people manipulate the landscape without regard to how flowing rainfall runoff will erode exposed soil. As more and more people inhabit the Earth, and as more development and urbanization occur, more of the natural landscape is replaced by impervious surfaces , such as roads, houses, parking lots, and buildings that reduce infiltration of water into the ground and accelerate runoff to ditches and streams.

In addition to increasing imperviousness, removal of vegetation and soil, grading the land surface, and constructing drainage networks increase runoff volumes and shorten runoff time into streams from rainfall and snowmelt.

As a result, the peak discharge, volume, and frequency of floods increase in nearby streams. Urbanization can have a great effect on hydrologic processes, such as surface-runoff patterns. Imagine it this way: in a natural environment, think of the land in the watershed alongside a stream as a sponge more precisely, as layers of sponges of different porosities sloping uphill away from the stream.

When it rains some water is absorbed into the sponge infiltration and some runs off the surface of the sponge into the stream runoff.

Assume a storm lasting one hour occurs and one-half of the rainfall enters the stream and the rest is absorbed by the sponges. Now, gravity is still at play here, so the water in the sponges will start moving in a general downward direction, with most of it seeping out and into the streambanks during the next day or two.

Next, imagine that roads and buildings have replaced most of the watershed surface. When that one inch of rainfall occurs, it can't infiltrate these impervious surfaces and will runoff directly into the stream, and very quickly, too!

Garden mud, rain, and rocks were Rafiyqa Muhammad's playground growing up in a south-Harrisburg, PA neighborhood. Today, she's back in that neighborhood, empowering the community through rain gardens and green infrastructure. Despite stalling out in the Pennsylvania House, these students aren't giving up on saving and recognizing Eastern hellbenders. At Pennsylvania's Misty Mountain Farm, riparian forest buffers, no-till, and cover crop farming techniques improve soil and water quality, leading to success for business and the Bay.

At the dawn of every new year, shortly after the ball drops and the confetti is swept away, our Bay saving efforts kick into another gear. With the new year comes the start of legislative sessions in Maryland and Virginia, and its continuation in Pennsylvania. This is a critical time for bay restoration. At the halfway point between and the question is: Are we on track? See how Virginia waterfront property owner Norman Colpitts uses a cost-effective living shoreline to protect his property while benefiting the environment.

The living shoreline includes a small oyster reef to prevent erosion on his property. These oysters create wildlife habitat, filter and clean the water, and slow down waves before they erode the shore. This video captures the devastating effects of heavy rains that pummeled Pennsylvania the week of July 23, These scenes were shot on July 26, the day before the river crested.

The James River in Richmond overflowed its banks after heavy rain washed huge amounts of dirt and pollutants into the current. Runoff of course occurs during storms, and much more water flows in rivers and as runoff during storms. For example, in during a major storm at Peachtree Creek in Atlanta, Georgia, the amount of water that flowed in the river in one day was 7 percent of all the streamflow for the year. That part of the precipitation, snow melt, or irrigation water that appears in uncontrolled not regulated by a dam upstream surface streams, rivers, drains or sewers.

Runoff may be classified according to speed of appearance after rainfall or melting snow as direct runoff or base runoff, and according to source as surface runoff, storm interflow, or groundwater runoff. The depth to which a watershed drainage area would be covered if all of the runoff for a given period of time were uniformly distributed over it. A significant portion of rainfall in forested watersheds is absorbed into soils infiltration , is stored as groundwater, and is slowly discharged to streams through seeps and springs.

Flooding is less significant in these more natural conditions because some of the runoff during a storm is absorbed into the ground, thus lessening the amount of runoff into a stream during the storm. As watersheds are urbanized, much of the vegetation is replaced by impervious surfaces , thus reducing the area where infiltration to groundwater can occur.

Thus, more stormwater runoff occurs—runoff that must be collected by extensive drainage systems that combine curbs, storm sewers as shown in this picture , and ditches to carry stormwater runoff directly to streams.

More simply, in a developed watershed, much more water arrives into a stream much more quickly, resulting in an increased likelihood of more frequent and more severe flooding.

What if the street you live on had only a curb built around it, with no stormwater intake such as the one pictured here. Any low points in your street would collect water when it rained. And if your street was surrounded by houses with yards sloping uphill, then all the runoff from those yards and driveways would collect in a lake at the bottom of the street.

Credit: Robert Lawton. A storm sewer intake such as the one in this picture is a common site on almost all streets. Dunn, Margery G. For information on user permissions, please read our Terms of Service.

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Any interactives on this page can only be played while you are visiting our website. You cannot download interactives. Of that, only about 1. Most of our drinking water comes from rivers and streams. This water is the lifeline of ecosystems around the world. Erosion is the process where rocks are broken down by natural forces such as wind or water. There are two main types of erosion: chemical and physical. In physical erosion, the rock breaks down but its chemical composition remains the same, such as during a landslide or bioerosion, when plants take root and crack rocks.

Explore the process of erosion with this collection of resources. Individuals, communities, and countries depend on a variety of different resources to help them thrive: electricity, timber, oil, water, and food to name a few. Because these basic resources are such a large part of our daily lives, it is important that we manage them responsibly to ensure future generations have what they need.

Human civilization heavily impacts the environment and the rich natural resources we depend on. All communities face the challenge of managing resources responsibly, not only for themselves, but for the sake of the world around them.

Learn more about how individuals and communities can manage their resources to support themselves and the world around them. In the United Nations General Assembly adopted 17 sustainable development goals designed to transform our world by The sixth goal is to ensure the availability and sustainable management of water and sanitation for all.

According to the United Nations, one in three people live without sanitation. A lack of sanitation and sanitary waste management systems can reduce a community's access to clean water, and lack of access to clean water can allow diseases to run rampant, sometimes creating epidemics of water-borne infectious agents. Learning about how freshwater systems work in the wilderness, rural communities, and urban centers can help us better understand the challenges of providing clean water and sanitation to people around the world.