I'll give you a hint:
Figured it out yet?
Yep, it's sediment. Dirt. Earth. Soil. Mud. Whatever you want to call it, it is carried by runoff and pulled from banks by erosion and our streams are FULL of it.
But, you say, dirt is part of nature, that's no pollutant! Well, you know the saying, "all things in moderation?" Well, that applies to ecology too.
Because of a variety of factors that have to do with channel shape, current speed, and stream path, a running body of water has a limit to the amount of sediment it can carry and move without becoming overloaded. Each stream, therefore, does have the ability to clear sediment out by carrying it and depositing it either along the bottom, on a bank or bar, or at its mouth. Likewise, the animals that live in the stream, mussels, fish, bugs, etc, each have their own tolerance to limited amounts of sediment for short periods of time.
It's when those tolerances are exceeded that this sediment becomes, in fact, a pollutant. When streams are frequently running muddy, when the bottom is nothing more than shifting mud or unconsolidated sand, when the banks are cut by erosion, dumping their clay into the water itself, problems abound.
Sediment smothers mussels so they cannot feed or breathe. Fish gills get clogged so the fish can no longer absorb oxygen. Many fish also rely on their excellent eyesight to find food; in muddy water, they are hungry and helpless. By blocking light, sediment also suffocates plant life beneath the surface that may be vital to fish and insect life. When all of this happens, it is an ecosystem in chaos and it also means the water supply is in trouble.
I mentioned that this sediment can come from bank erosion -- this erosion is often caused by a stream trying to channel more water than it is built for. This is usually due to our compulsion to pave and cover everything we can. Every square inch of ground we pave or harden with a rooftop is one less square inch that can absorb water. Thus, every drop of water that hits there will become runoff and we all have seen what a powerful force runoff can be.
In developed areas, most of the time, runoff is channeled into storm drains. Which usually then lead directly to a stream or other body of water, they ARE NOT filtered or cleaned in any way. This water also usually enters the stream at a single point -- it's like turning a fire hose on and blasting sediment right off of the banks, that's what kind of power this water has, especially during rain events. I remind you again, streams are your water supply, so anything you dump down that storm drain, you are effectively pouring into your glass of water. Sediment often enters the water supply here, running off from construction sites and yards. Most counties do have regulations about sediment control on construction sites, but the problem is that, even if these regulations are enforced (the enforcers are often far too few), the conventional technologies they require (silt fences, stormwater ponds) are very ineffective.
If you see a stream choked with sediment, especially if it is not raining, you can and should notify your county and notify your state water quality agency. A simple Google search can direct you right to them. Notice silt fences around a construction site are collapsed or leaking mud everywhere? Call and report it to your county, contractors and developers ARE required by law to maintain those. You have a right to clean water, wanderers, and it is up to each of us to stand up for that!
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