At Great Falls, which uses the Missouri as its major source of drinking water, the arsenic level averages about 10 ppb, says Melstad. As measured by the U.S. Geological Survey, the Madison also has a relatively high phosphorus level. Though not dangerous to people, phosphorus contributes to algae growth, and decomposing algae reduces the water's oxygen that is available to fish and other aquatic life. Sediment can silt-up rivers making them poor spawning grounds for fish. It also carries minerals and organic matter that cause other problems.
Some sediment comes from natural erosion.
For example, when melting snow and rain run into the Madison River near the Bear Trap south of Three Forks, it makes the water muddy even though there is no human-caused land disturbance upstream. Distinguishing between natural sediment levels and those due to human activity is a challenge for water regulators. Agriculture puts 78 percent of the water back immediately, says Bauder, but 22 percent either is used by plants or evaporates. Most Montana rivers begin in the western mountains. As they flow from their headwaters, they steadily pick up more dissolved minerals and sediments.