The USGS Water Science School
These pictures show a very common item around our growing cities -- sediment fences at new construction sites. There are probably miles of these fences through every town. They actually serve a very useful purpose -- to keep the exposed dirt and mud from construction sites out of the streets and streams.
The picture on the right shows the job being done somewhat correctly. As the left-side picture shows, a big mess can occur when the job is done wrong. Dirt and mud wash into streets (where my third grader would just love to stomp through it and then come running into the house) and eventually into creeks. Often, mud from construction sites flows along the street during a rain, into a storm sewer and directly into a creek or river.
Excess sediment can harm river quality. This pictures shows sediment-laden water from a tributary where construction was taking place entering the clearer water of the Chattahoochee River in Atlanta, Georgia. Sediment can carry unwanted chemicals with it, it can lower the amount of light hitting the streambed, which can be bad for biological life in the stream, and it can make problems for industrial equipment an systems that supply people with drinking water.