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Impacts of Land Use: Impervious Surfaces |
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Studies have shown there is a connection between land use in a watershed and water quality of the river. As the amount of impervious surfaces such as roads, rooftops, and even manicured lawns increase, during a rain event the stormwater runoff causes flooding and reduced water quality in the river and its streams. Where the underlying geology is similar to the Huron River Watershed, once only 8-10% of a watershed becomes impervious, the receiving stream begins losing the ability to support aquatic life. Unplanned, spread-out residential development ("sprawl") is the number one threat to the health of the Huron River Watershed.
Links on this page Because of these processes, runoff from natural areas is negligible, except after the largest storms. Ecologically intact systems provide a number of services to the community, including; groundwater recharge, pollutant removal, temperature reduction, erosion control, air purification, flood and drought control, wildlife habitat, increased property values, and recreation.
Both the high flows and the low flows are problematic for the stream. The high flows can damage aquatic habitats and scour the banks, while low flows deprive aquatic wildlife of water and oxygen. Runoff water is also much warmer than groundwater, and carries with it soil and whatever substances it may pick up from the landscape along the way, such as fertilizer, pesticides, oils, and other pollutants. The system loses its capacity to provide the ecological services mentioned above.
The pollution in the unfiltered runoff is called "nonpoint source pollution" because it does not come from one specific source such as a factory, but rather, it results from our everyday activity on home sites, businesses, roads and farms. For tips on how you can reduce nonpoint source pollution at your home, school and place of work, check out the HRWC You Can Help In and Around the Home: Yard and Garden Tips webpage. HRWC also has developed a brochure about Stormwater Runoff titled "Runoff as Resource". Although this brochure is currently out of stock, you may access it by visiting the library at HRWC's office. Learn more about the connection between land use and water quality in the HRWC article Speaking Up for Density, pdf file: .59 megabytes and in the HRWC white paper Watershed Planning: Determining Impervious Surface Capacity to Better Manage Growth at the Rural/Urban Fringe, pdf file: 1.8 megabytes.
WHY IS INFILTRATION A GOOD THING AND HOW CAN IMPERVIOUS SURFACES BE REDUCED? Infiltration maintains the natural hydrology and biology of the watershed, especially the headwaters. When infiltration is not allowed to occur, a small stream may carry little or no water when it is not raining because there is so little stored groundwater to provide continuous flow. This means that everyone that counted on this stream for life, recreation, habitat, food, or water must find a new source. Infiltration recharges water-bearing aquifers. It may take decades and sometimes centuries for rain or snow melt to reach the deep layers of the earth from which many private and municipal wells draw water for human consumption. However, that water does come from the surface. Diversion of surface water eventually affects the quantity of groundwater. Infiltration improves the quality of the water passing through. Earth, with its wetlands and soils, is the original and still the largest water filtration system around. Infiltration protects recreational interests. As impervious areas increase, streams become "flashy", which tends to erode stream banks, water levels drop significantly in the summer, and water temperature increases thereby reducing the variety of fish. Waters will also likely have higher levels of pathogenic organisms and toxic chemicals. None of this makes for good recreation and tourism. Infiltration protects downstream areas from flooding. When headwater streams become "flashy" so do the next larger order of streams below them, and the rivers below those. This results in more severe flooding. Infiltration allows large quantities of water to be stored in the ground and released slowly - long after the storm passes. Infiltration can be maximized by reducing impervious surfaces. The goal should be to emulate as much as possible the natural stream and groundwater hydrology of your area. This means retaining stormwater runoff on site and providing vegetation to filter runoff before it reenters the watershed, thereby limiting the pollutants it has picked up. In general, the more porous the soil and the more heavily planted the landscape, the less water runs off and more infiltrates. The less porous the soil and the more hard surfaces exist on the landscape, the more water runs off. Paved surfaces are easiest to reduce when new construction is planned. When installing a new driveway or sidewalk, consider a gravel bed, bricks and flagstones, bricks or interlocking pavers, crushed shells or stone and bark chips. These porous surfaces encourage some infiltration of runoff. Consider the design and layout of roads and pathways. Roads and pathways can also be designed with small berms, terraces or holding areas to trap storm water --especially on slopes. Designs with curves and designs which reduce standard roadway widths offer water quality benefits.
PHOTOS A. Photo of Fleming Creek: the watershed of Fleming Creek has less than 10% impervious surfaces.
DIFFERENCES IN ADOPT-A-STREAM STUDY SITES A recent analysis of our monitoring results shows that stream degradation begins at when only 8% of the watershed is impervious. For comparison, the land area in Malletts creekshed, a degraded urban stream in Ann Arbor, is about 35% impervious while the land area in Hay creekshed, a high quality rural stream, is 8% impervious (using 2000 landuse data). To preserve the quality of the Huron River, we must plan for growth.
They jointly produced the report "Current Conditions, Recent Changes, and Major Threats to the Huron River: A Report Based on Eight Years of Citizen Monitoring" pdf file: 4.5 megabytes.[PHOTO at left: Don Rottiers asking Prof. Mike Wiley a question about his creek at the public presentation of the report findings] - - The poorest creeks are located in the urbanized centers and quality is best in the areas of the least suburban development. - - The study shows that, in the Huron system, streams begin to deteriorate when impervious surfaces cover more than 8% of the watershed. Impervious surfaces include roofs, streets, driveways, sidewalks, and other surfaces (including compacted turf grass lawns) that retain very little of the rain that falls on them. It would be ideal if we could keep our impervious cover under 8%. However, we have already exceeded 15% in many of our sub-watersheds. Thus it is all the more imperative that on each developed site we construct and maintain methods to retain the stormwater from the impervious surfaces. - - For every 5% of the farmland developed into residential and commercial sites, we lose about one aquatic family from the Huron stream. These aquatic families are indicators of the quality of the system, analogous to the canaries that used to warn miners when the air quality was poor. The loss of aquatic families indicates degradation of the stream and its environment. (Fish and other animals that depend upon the insects for food share our loss.) - - Malletts has the most impervious surface and the poorest aquatic insect population in the study. There is much interest in correcting the resulting problems, but restoration will be incredibly expensive. The restoration plan alone cost $300,000 and the estimated cost of the restoration is $20 million. In contrast, the cost of designing and constructing measures to protect the creek and minimize impervious surfaces is a bargain, since it is only a fraction of the restoration cost. The bottom line is that there is a challenge for planners to direct development in ways that will minimize impacts on the Huron River and its creeks.
TRUE COSTS OF URBANIZATION (comparison of the cost of protecting and preserving a watershed vs. restoring a watershed after degradation)
Last Updated: February 2006 |
In an undeveloped watershed, most rain and snow falling on the watershed either infiltrates into the ground or is taken up by vegetation. Much of the water that infiltrates into the ground eventually makes it into streams or lakes through groundwater seeps, or springs. Water taken up by vegetation cycles back into the atmosphere through evapotranspiration. 
As development within a watershed increases, it brings with it an increase in impervious surfaces. Water runs directly off these surfaces and into lakes and streams. This causes many problems for the stream and its inhabitants. In highly impervious watersheds, the stream receives a flood of runoff water just after rainfalls or large snowmelts, but is deprived of water during dry times. 
A comprehensive analysis of the data collected through the Adopt-A-Stream program from 1992-1999 was done in collaboration between Joan Martin (Adopt-A-Stream program director) and Prof. Mike Wiley (aquatic biologist).
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