Nearly everything that can be found on land eventually makes its way to a stream. This is because every bit of ground on Earth is a part of some river's watershed. Water flows downhill. Whether the water comes from rain, a hose, or a pipe, whatever doesn't sink into the ground will flow into the nearest stream. That is a simplification, of course. In reality, much of the water that soaks down into aquifers also eventually finds its way to a stream.
The things that belong in a stream are the things that have been flowing into the stream for thousands or even millions of years. They are things that the plants and creatures in the stream have evolved to use to their advantage. They include very low concentrations of chemicals like calcium, iron, phosphorus, and nitrogen. They also include things you can see, like bugs, sediment, leaves, twigs, dead animals, even tree branches and whole trees.
Before the ecology movement in the 70's, factories and sewage plants were usually the main suspects when people saw foam in streams. Today that problem has mostly been solved. There is still plenty of foam in streams, however. This foam is natural and belongs there. It is caused by diatoms, tiny one-celled algae that live in crystalline houses and creep across the rocks at the bottom of the stream. Rapids further upstream sometimes pound these diatoms to bits. The froth you see consists of fragments of their little houses, or frustules.
The things that don't belong in a stream--things that didn't show up there until human technology began to upset stream ecosystems--are called pollutants. According to theories of evolution, even a stream filled with a toxic soup of industrial chemicals may one day be able to support life. The problem is that it will take millions of years to get there.
Point-source pollution
Regulations developed since the 70's have greatly helped to cut down on certain types of point-source pollution, which is pollution that enters streams from industrial and agricultural sites. No longer do our rivers froth with piles of suds or run red and orange with dyes. That doesn't mean the pollution has gone away. Agricultural runoff from farms and ranches still contains unsavory levels of bacteria, hormones, pesticides, herbicides, excrement, and fertilizers, as well as large amounts of sediment. Industries are still responsible for such massive amounts of toxic chemical pollution that it has entered our drinking water supplies. Many of the pollutants, from both farm and factory, are endocrine disruptors. While they endanger public health, Congress and the EPA have been extremely slow to address the problem. While they pose a serious threat to human health, they endanger the animals that live in our streams as well.
Many pollutants from earlier decades remain in streambeds, building up in the bodies of fish and birds. These pollutants include heavy metals, DDT, dioxin, polychlorinated biphenyls (PCB's), and a number of recently banned pesticides. There are underground storage areas of toxic chemicals as well that have begun to leak into streams. A frightening example of this is the Hanford nuclear storage site near Richland, Washington in the U. S.. A number of the underground containers have begun to leak and radioactive materials have found their way through cracks in the rock into the Columbia River. Despite the fact that this disaster is so well known, modern countries (including the U. S.) continue to generate nuclear waste and bury it underground.
It must be remembered that rivers contain living creatures and plants, many of them microscopic but very important members of the foodweb that can be killed by pesticides and herbicides. When part of the foodweb is eradicated by these chemicals, the animals that depend on them starve or fail to reproduce. Fertilizers and animal waste cause their own problems: they enrich rivers, causing the growth of algae, removing oxygen from the water and blocking sunlight. Sediments also block sunlight as well as cover riverbeds, choking and killing off the plants and animals that live there. Hormones, which are fed to livestock to increase production and excreted in their urine, alter the reproductive processes of stream animals like fishes.
Urban Runoff
Fertilizers, pesticides, and herbicides don't all come from farms, though. Most of them these days actually come from peoples' lawns and gardens. This is part of urban runoff. Water rushes over the many hard (impermeable) surfaces that humans create, over roadways and into gutters, and from there through pipes to the nearest stream. This type of pollution contributed by whole settlements of humans is called non-point-source pollution. Urban runoff also contains a toxic soup of cancer-causing chemicals (carcinogenic hydrocarbons) that are deposited by cars, buses, and trucks on roadways. Rainwater that runs off the road collects in ditches, which run alongside the road collecting more toxins until they finally empty into a stream. The toxins include chemicals like hexane and cyclohexane, benzene, polycyclic aromatic hydrocarbons (PAH's), formaldehyde, methanol, acrolein, 1,3-butadiene, and acetaldehyde, as well as lead. If these chemicals can cause cancer, birth defects, and immune suppression in humans, it is a good bet that they will do similar damage to fishes and birds.
Sewage
Urban pollutants also enter rivers through water-treatment plants. Water-treatment plants only clean out some of the pollutants, leaving the rest in the treated water that they pipe into streams. Most of the chemicals we put in our sinks, toilets, washing machines, and bathtubs will end up in the local stream. The long list of pollutants we contribute includes caffeine as well as pharmaceutical drugs like antibiotics, antacids, digoxin, Viagra, tranquilizers, and steroids; antidepressants like Prozac and Wellbutrin; painkillers like aspirin, ibuprofen, acetaminophen, and codeine; and the medicines we use to control cholesterol, diabetes, asthma, coagulation, anxiety, chest pain, osteoporosis, and hypertension. It includes reproductive hormones like estradiol, progesterone, and testosterone. Indeed, most medicines that we put in our mouths will later be found in the local stream, to find their way eventually to bigger streams, and finally to estuaries where young marine fish mature. The other chemicals we use--antioxidants, anticorrosives, solvents, the fragrances for our laundry, and degreasers--will join them there. They will not all simply wash out to sea: much of them will remain behind to form toxic soups and sediments in the stream's floodplain.
One of the results we are beginning to see is that male fish are becoming feminized and incapable of reproducing. Sex ratios are changing. In one study downstream of the Boulder, Colorado wastewater treatment plant, 60 white suckers were collected; 50 of them were female! Upstream, the ratio was half male and half female. This is the result of endocrine disrupters, including both industrial chemicals and the chemicals that we flush down our toilets.
Of course the fish are bothered by all pollutants, not just endocrine disrupters. Besides the unknown effects of these massive amounts of pharmaceutical drugs on fishes' bodies, their food supplies such as hydras and zooplankton are killed. And it is not only fish that are at risk: fishes are eaten by higher-order carnivores such as bald eagles, herons, kingfishers, cougars, and these animals end up with our drugs in their bodies as well.
Power Plants
Nuclear, natural gas, and coal power plants inject heated water into rivers, raising temperatures above normal. This heat pollution can cause a devastating change in river ecology. Other pollutants are injected each time a plant cleans and flushes its cooling system. Coal-fired power plants pollute the air with mercury, which eventually falls on the land and runs off with rainwater, entering streams and poisoning the animals in them--as well as the fishermen who catch and eat their fish. Nuclear power plants must store their radioactive and other toxic wastes, which eventually leak into water tables and enter streams.
Mining
During the early years of gold mining, mercury was used to separate gold from ore. This mercury remains in streambeds, continuing to poison life long after the practice became illegal. Today, the main pollutant from gold mining is sulfuric acid. Acidification not only kills streams but also puts toxic metals found in the soils--such as aluminum, copper, and cadmium--into solution, enabling them to enter streams. Nearly all mining results in large quantities of acids leaching into streams as well as runoff of heavy metals like lead, cadmium, and arsenic. The government gives away lands to large mining companies, often from foreign companies, for dirt-cheap prices. The mining companies then extract their minerals and run off with the profits, leaving the public to deal with often irreversibly polluted and toxic lands. Runoff from piles of coal and coal mines carries cadmium, lead, boron, chromium, mercury, arsenic, and selenium into streams. Runoff from uranium mining operations transports heavy metals and radioactive uranium into rivers. The fishes that we most like to catch are predators near the top of the river food chain--salmon, trout, bass, sunfish, and so forth--that have the highest concentrations of heavy metals in their bodies. Mine leaching as well as coal-fired power plants and cement plants that contribute mercury to our air have ensured that fish from most of our streams--even many mountain streams--have become dangerous to eat.
Sediment
Sediments also pollute rivers. Housing construction, road construction, mines, and logging roads all contribute large amounts of sediments to rivers. Sediment can make hunting food difficult for stream residents, choke plant life, and coat gills of insect larvae and fishes, making breathing difficult or impossible. Many of the diatoms and tiny insects that form the base of the foodweb will die. This has an effect all the way up the foodchain. Finally, salmon and trout eggs, deposited safely in gravelly streambeds, are choked by the extra sediment and fail to mature.
What Can Be Done?
There are many workable solutions for those who care about the health of their streams. Citizens can monitor streams and keep a watchful eye on industry, including farms, ranches, dairies, and feedlots. Water-treatment plants can be upgraded to filter out more pollutants. Urban runoff can be minimized by providing permeable areas like gardens and rainwater collection pits. Vehicles with better fuel-efficiency pollute less per mile. Good vehicle maintenance keeps leaks and fumes in check. Lawns can be replaced with more ecologically-friendly spaces. Home-owners can decline to use pesticides and herbicides. New developments can be built using the latest in environmental practices and the advice of biologists. Construction companies can take steps to control sediment. Citizens can press for more ecologically-friendly forms of power, support organic farmers by buying their products, and help educate their communities about watersheds.
To learn more about pollution and stream ecology, check out the reading list, where you will find some excellent references listed. (You will be able to order them from here, too).
Play a Watershed Game! Be an Explorer or a Detective! 
To learn more about pollution and stream ecology, check out the reading list, where you will find some excellent references listed. (You will be able to order them from here, too).
