Colorado Division of Wildlife officials are considering closing parts of the South Platte River to fishing, or mandating wading restrictions on these famous trout waters, after the discovery of New Zealand mud snails below Elevenmile Canyon Reservoir.

The snails aren't a new threat. They've been documented in American rivers for nearly 20 years. Scientific opinions on their potential impact vary from tempered concern to "if you think whirling disease is a problem, you haven't seen anything yet." The invasive species eats the same organic materials that feed aquatic insects. By outcompeting native bugs, the snails can devastate the trout's food chain.

What really alarms Colorado officials such as aquatic wildlife manager Robin Knox is the high volume that appeared seemingly overnight in the South Platte: "We have to assume they already have spread to other rivers." —KIRK DEETER