Increased Runoff in Natural Waterways

The porous and varied terrain of natural landscapes like forests, wetlands, and grasslands traps rainwater and snowmelt and allows them to filter slowly into the ground. In contrast, impervious (nonporous) surfaces like roads, parking lots, and rooftops prevent rain and snowmelt from infiltrating, or soaking, into the ground.

 

With areas being developed more impervious surfaces are introduced into a watershed. Most of the rainfall and snowmelt remains above the surface, where it runs off rapidly in unnaturally large amounts. Storm sewer systems or grassed ditches concentrate runoff into relatively smooth straight paths. This runoff gathers speed and erosive power as it travels over the ground. If all this extra water is diverted directly into a stream channel, several important changes will occur.

 

• More Flooding During Rainstorms: Streams that used to carry only a small portion of rain are now asked to carry most of it. Natural channels easily become overwhelmed when a big storm hits. The extra water overflows the banks and floods the surrounding land.
  
  
• Less Flow During Dry Times: Where do you think the water that infiltrates into the ground goes? Some of it moves slowly underground and ends up in stream channels months after soaking into the soil. This groundwater provides flowing water in the channel even during dry times. When impervious surfaces prevent infiltration of rainwater, there is less groundwater to move into the channel.
  
  
• Wider Channels with More Sediment in Them: Imagine rushing water moving down a channel after a rainstorm. Everything in its path that is not tied down or too heavy to move gets carried away. This includes all the loose sand, sediment, and dirt on the banks of the stream. Over time, pulses of rushing water erode the banks away and the channel gets wider and wider. Wider channels also allow large volumes of water to heat up from the sun, raising temperatures higher than what fish can tolerate.
  
  
• More Sand and Sediment on the Channel Bottom: Urban streams have more sand and sediment at the bottom of their channels than their country cousins. Part of this material comes from eroding stream banks. A large fraction, however, is carried by runoff water washing the pavement and land clean. When runoff reaches the stream it drops its load of sediment on the bottom of the channel.