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Stormwater Runoff Basics

What is stormwater runoff?

Historically, when it rained on the prairie, the majority of rainfall soaked into the organic rich soils.  Our streams were fed mainly through groundwater recharge.  As soil surfaces in our communities are paved over or built upon, the rainfall has few areas to infiltrate into the soil and instead moves as runoff to local streams.  This runoff is called stormwater runoff, which is rainfall and snowmelt that drains off impervious surfaces such as streets, parking lots, driveways, roof tops and compacted soils.  It eventually drains into a storm drain in the street that enters an underground storm sewer system that discharges in most communities directly into local waterways such as rivers, streams, creeks and lakes.  The concern with stormwater, is how it is managed locally for water quality and quantity or flood control.  Most stormwater is released directly into local bodies without any treatment.  Some communities use detention (dry basins) and retention (wet basins) to temporarily hold back stormwater during major rainfall events and release it more slowly to a local waterbody to minimize flooding impacts.

Did You Know? There are 27,150 gallons of water in one inch of rain falling over an acre of ground – that is .62 gallons of rain for every square foot of rootop, lawn, sidewalk and driveway.

stormwater runoff basics

What are the challenges with stormwater?

Once 10% of a given watershed (drainage basin) has been converted to impervious surfaces, significant ecological damage occurs as an increase in stormwater runoff from urban development is quickly collected and discharged into the storm sewer drainage system. The following impacts occur:

  1. Groundwater is no longer recharged
  2. Surface water becomes polluted
  3. Streambanks and channels are degraded
  4. Flooding occurs

The slow infiltration of rainfall into the surface of the soil and downward percolation through the soil profile is essential for replenishing groundwater that is water stored underground.   When urban development occurs it creates impervious surfaces such as streets, buildings, and driveways that prevents rain water from infiltrating into the soil and decreases groundwater recharge rates.  Most Iowan’s depend on groundwater as a source of drinking water, so it’s important to replenish groundwater supplies when possible.

Surface Water Pollution

Stormwater can carry many pollutants that have negative impacts on water quality. The following are some common pollutants in Iowa’s waterways.

Bacteria. Too much of the wrong bacteria is not good for the environment. Sources of bacteria, like e.coli and fecal coliform come from pet & wildlife waste as well as failing septic systems. This bacteria gets flushed into stormwater runoff to local water bodies and groundwater and can result in drinking water boil orders and closed beaches throughout Iowa. Bad bacteria can make humans and animals sick.

Sediment. Too much sediment (dirt that ends up in water) is not good for our environment. Sources of sediment include dirt on impervious surfaces that are washed away with rain, as well as eroded soil from unprotected construction sites and streambanks. Muddy water in streams and lakes that is full of sediment impacts fish and other aquatic life as well as changes the water chemistry.

Fertilizers. Too much nitrogen and phosphorus in water is not good for the environment. Sources of nitrogen and phosphorus come from fertilized urban lawns and landscapes. Excess phosphorus in streams and lakes promotes algae growth, that depletes oxygen in the water. When this happens it not only makes for an unpleasant environment in which to swim or kayak, but fish and other aquatic life are impacted as well.

Pesticides. A modest number of currently used and recently banned insecticides and herbicides have been detected in urban streamflow at concentrations that approach or exceed toxicity thresholds for aquatic life.

Gas & Oil (hydrocarbons). Sources of hydrocarbons include oil and grease that leak from vehicles, gas station hot spots, parking lots and illegal dumping directly into storm drains.

Heavy Metals. Cadmium, copper, lead and zinc are commonly found in urban stormwater runoff. Sources include vehicles (brake linings), junk yards, metal rooftops, etc. While the quantities are not usually harmful to humans, these metals can be toxic to aquatic life and accumulate in sediment (dirt) found in streams, ponds, lakes and rivers.

Salt (Chlorides). Salts applied to sidewalks, roads and parking lots in winter appear in stormwater runoff and meltwater in concentrations that are starting to impact many freshwater organisms.

Trash (Litter & Debris). Considerable quantities of trash wash through the storm sewer systems and end up in our local streams, ruining the beauty of our natural environments

Thermal Pollution. Rain falling on concrete and asphalt on a summer’s day that runs off into the storm swere system and then to local streams and lakes increases the temperature that can harm fish and other aquatic life that require cold and cool water conditions (e.g., trout).

Streambank Degradation

Stormwater runoff empties into our streams fast and furious causing flashy flows during heavy rains.  That exposes the bed (bottom) and bank (sides) of a stream to highly erosive flows more often and for longer periods of time due to urban development.  Streams typically respond to this change by increasing their cross-sectional area to handle the more frequent and erosive flows either by getting wider or deeper, or both.  Streambank erosion adds to the amount of undesirable sediment in streams.  Streams are then constantly trying to find a state of equilibrium so they keep changing shape.  This causes undesireable changes in the movement of the stream and impacts fish and wildlife.


Urban development increases the amount of water draining to the closest stream, lake or river due to impervious surfaces (rooftops, sidewalks, streets & parking lots) generating greater runoff volumes and storm drains delivering it more rapidly to a stream. Streams are historically sized to handle only so much water – when more water is generated than a stream channel can handle the water spills out into the adjacent floodplain, the areas bordering streams and rivers.  This is called an “overbank” flood, and can damage property and downstream drainage structures.  Urban areas are especially prone to “flash” floods, as the water comes and goes quickly, and can be as destructive to structures and aquatic life in streams. 

Operationally, the floodplain is usually defined as the land area within the limits of the 100-year storm flow water elevation. The 100-year storm has a 1% chance of occurring in any given year. In Iowa, a 100-year flood occurs after seven inches of rainfall in a 24-hour period (which is a storm that has a 10% chance of occurring in any given year – not one that occurs every 100 years, i.e. the 100-year storm).   As a consequence of water draining faster and in larger quantities off the land surface than before development, the elevation of a stream’s 100-year floodplain becomes higher and the boundaries of its floodplain expand.  In some instances, property and structures that had not previously been subject to flooding are now at risk.