The Ten Percent.
Scientists and planners say: if ten percent of the surface area of a watershed is impermeable, its’ stream channels are destabilized and fish habitat degrades. The number of fish species as well as the abundance of their eggs and larvae declines sharply at this level. Plant and amphibian density also declineswith higher streamflows.
Even without urban land cover – roofs, sidewalks, lawns, patios – 
roads within a watershed also have profound impacts on water quality downstream. The National Oceanic and Atmospheric Administration Fisheries agency uses the ratio of fewer than two miles of road per square mile to define a ‘properly functioning watershed’ for salmon, steelhead, and many other aquatic and marine species important in the Gorge reach of the Columbia (although this figure has been sharply criticized as far too high).
A Klickitat County watershed assessment found the road density in the Klickitat River watershed – a relatively sparsely populated area – to be 2.6 road miles per square mile. The assessment further notes that when the human population exceeds four people per acre, aquatic insect diversity in urban streams plummets, and when insects go, so do fish. Coho salmon are particularly sensitive.
Contaminant Ride-Along
The Columbia River drains 259,000 square miles and is 1,200 miles long. It receives a huge amount of material from human sources. Along with the sediment comes everything else that gets entrained in the flow: fertilizer, Roundup, cockroach killer, soap, paint, concrete sealant, grit from roof shingles. So it’s both the amount of water and what’s in the water that upsets the natural pattern.
Most agricultural input enters upstream in the Columbia Basin. A recent U.S. Geological Survey study reports an average of 182 chemicals–adding up to about 46,000 metric tons of active ingredients a year poured into the Columbia between 1999 and 2004. The amount contributed by urban areas is likely much smaller but significant, because people use chemicals in their houses and yards and cities release treated municipal sewage, which also contains numerous chemicals anging from prescription drugs to hair dye.
The Gorge has a low population and relatively low levels of impervious surfaces, but within its urbanized areas there are zones of very high imperviousness. For urban areas, an emerging contaminant issue is the chemicals used to seal concrete and road surfaces. The goal is to prevent staining, to control degradation from freeze-thaw cycles, and to preserve decorative colors. But the sealants enter stormwater through normal daily abrasion of the concrete surface, and in the process they release polycyclic aromatic hydrocarbons (PAHs), a class of compounds known for their persistence in the environment.
Human exposure to PAHs is associated with cancers and various developmental and cognitive deficits in children exposed prenatally and there is evidence that PAHs have a range of harmful effects on downstream invertebrates. There are a few greener alternatives available, such as acrylic and soy-based concrete sealants, but the toxicity of the former and the long-term effectiveness of the latter are still largely unknown.
And then there are ‘domestic’ pesticides. For example, pyrethroids insecticides are constituents in more than 3,500 products, many used in households, in pet products including fog ‘bombs’ and flea collars, and in agriculture. A recent study of several pyrethroid pesticides’ behavior on concrete found that the chemicals washed off rapidly for about three days, after which runoff slowed, but because the pesticides were retained in the pores of the concrete, there continued to be measurable runoff for more than a month. Pyrethroids are extremely toxic to aquatic organisms.
Soil Survival
Covering soil with impervious materials basically destroys it, but much less attention is paid to this than to the consequences of stormwater runoff. Although the Oregon DEQ monitors and regulates runoff from roads, “We don’t have any regulations associated with road building that protects worms under the pavement,” says Mason.
This is regrettable, because soils harbor something like a third of all the life on the planet, and only about one percent of its organisms have even been identified. According to a European Commission brochure, “A single teaspoon of garden soil may contain thousands of species, millions of individuals and 100 metres of fungal networks.” The EC has also found that “two and a half acres of ?fully functioning soil”can store up to 4,134 metric tons of water, the equivalent of 16 inches of precipitation.
