What Is Surface Water Runoff

Runoff is surface water that flows from one area to another, generally from a higher point to a lower point due to gravity.  During and after rain storms (or any other event that drops water or other liquids on the ground), runoff starts from where the rain lands and flows downhill however and to where ever it can.

Runoff Before There Was a Mill Creek
Before the environment was disturbed by development, abundant trees and other vegetation helped to control this runoff.

The forests slowed down the rate that rain hit the ground, and unpaved land with plants and established root networks were able to absorb most of the runoff.  Even though the glacial soil geology in Mill Creek prevents quick absorption of water by the ground, surface water was slowed by the natural conditions enough for the soil and plant life to keep up. 

What surface water was not absorbed did flow slowly along the natural contours of the Mill Creek area to find its way generally to established streams like Penny Creek and North Creek.  Topography also generated smaller stream tributaries leading to the more established pre-development streams.  In areas where natural topography formed a depression with no or limited outflow, a forested pond or wetland could form. 

Although both erosion and the washing downstream of natural debris and sediment represent natural processes, forested conditions established a slow to moderate pace for both.

Runoff Today
Today, runoff from Mill Creek is quite different.  Whenever houses, streets, shopping centers, and businesses are built, runoff and the natural processes associated with runoff are changed.  Hardened, impervious surfaces (such as asphalt, concrete, and even building roofs) and more limited vegetation (lawn grass and reduced trees and native vegetation) all further limit amounts of rainfall that were previously either kept in place or slowed from running off.  The results are substantial increases in both the quantity of water and its speed of flow.

Development patterns also tended to constrain where surface and underground water could move.  Depressions between developments, especially ones that altered the landscape, often result in wetlands areas forming, or even ponds or lakes, if enough water accumulates.  In areas where buildings or other developments keep water from moving and spreading naturally (either above ground or underground) and no new drainage options are provided, softened yards or water intrusion into basements or crawl spaces develop instead.

Further, the presence of development introduces new or increased quantities of physical or chemical materials that were either not present, or present in much more limited amounts, before development.

Instead of broadly flowing with the topography, runoff today generally flows into either drains or the open grates of street or yard catch basins, which lead to underground pipes.  Occasionally, flow will also be guided by surface channels, especially along certain residential streets or Bothell-Everett Highway.  Eventually, much of the runoff leaves Mill Creek, typically by eventually draining into North Creek.

Just Being Mill Creek Affects Water Quality
When it rains, yard fertilizers, pet waste, garbage and similar debris, dirt, grass and vegetation clippings, paint, soaps and solvents, automobile fluids like oil or anti-freeze, dust from car tires and brakes and many other physical and chemical pollutants are washed away. Even temperature can be a pollutant.

storm waterPollutants make it into drains connected to underground pipes, open channels and street gutters, and stormwater management facilities like ponds or vaults. If these pollutants continue to travel, they will eventually discharge into our local streams. From Mill Creek, those streams lead to regional rivers, lakes,
and the ocean.

We are not flagrantly and deliberately polluting our waterways in Mill Creek, b
ut pollution still happens just by being a city. Consequently, we have to become aware that even our normal activities can introduce pollutants into our runoff water and create a real threat for our streams and other waterways.

Much of what we all do today in order to manage our surface water and stormwater runoff is intended to reduce, as much as we can, the consequences of living and working where our activities can impact our natural waterways.

Snohomish Conservation District
The Snohomish Conservation District offers a variety of education and engagement opportunities, free to all property owners. 

Check out their Urban Stormwater Program here:  https://snohomishcd.org/sound-homes
Events for education and engagement are here: https://snohomishcd.org/events