In natural systems, precipitation falling on land make take several paths. It may infiltrate the ground, perhaps eventually recharging groundwater aquifers. It may evaporate. It may enter the soil and be used by plants. And in cases of heavy rainfall or quick snowmelt, it may travel overland as runoff to the nearest lake, stream, river or ocean.
Urban land use changes the hydrologic cycle by covering the ground with impermeable surfaces such as roofs, roads, driveways and sidewalks. Precipitation cannot infiltrate impermeable surfaces, and less area covered with plants means less precipitation is transpired (taken from the soil and lost to the air through plant leaves). Increasing the area covered by impermeable surfaces thus has two effects: it increases the amount of runoff and it increases the speed of this runoff since water flows quickly over roads and other such surfaces.
In urban areas, this runoff is called stormwater. A stormwater system, which may or may not be distinct from a sewage system, collects this water. In Saint Peter, the stormwater system is separate from the sewage system and the stormwater drains directly to the Minnesota River or to holding ponds where some impurities settle out before the water enters the Minnesota River.
An additional concern with urban runoff is the pollutants and sediment it carries. Anything on our lawns, sidewalks and roads can eventually reach the river. This includes trash, oil, lawn fertilizer, pesticides, paint, salt, and pet waste. (In case you were wondering, there are an estimated 710 dogs who call Saint Peter home and they produce about 355 pounds of solid waste each day.)
Gustavus students Tyler Melhorn and Jason Quiram were interested in quantifying the effects of urban development in northern Saint Peter between 1991 and 2008 on the hydrologic cycle. They used aerial photographs from 1991 and 2008 (see above) to measure the area converted from vegetation to impermeable surfaces between these two dates. Given that southern Minnesota receives on average 34 inches of rain per year, they calculated that the 30 percent increase in impermeable surfaces in northern Saint Peter could be responsible for over 65,600,000 additional gallons of runoff per year.
Individuals can decrease the amount and rate of urban runoff by using rain gardens to collect rainwater in the yard, rain barrels to collect rainwater from roofs, and special ditches to help water infiltrate the ground. You can even create a “green roof” where plants grow on top of your house!
For more information on stormwater in Saint Peter, see http://www.ci.st-peter.mn.us/publicworks/stormwater/swaterfaq.php3.