From The Pueblo Chieftain (Chris Woodka):

Called phragmites (frag-my-tees), the reeds also have been found near Lake Minnequa, said Scott Hobson, assistant city manager and head of the city’s planning department. The city has extensive plans to develop both Fountain Creek and Lake Minnequa, and the phragmites could be an impediment. “We’re monitoring it to see how much it’s spreading,” Hobson said. “It does expand very quickly, so we might have to look at controlling it somehow.”

About five years ago, the city cut down much of the tamarisk, or salt cedar, that grew in the Fountain Creek channel in an attempt to preserve the effectiveness of Pueblo’s levees. Tamarisk remains in some areas; others are a more natural mix of vegetation. A few large areas, however, have been colonized by large stands of phragmites, which look like amber waves of grain. “We’ve noticed it in the last two or three years, but it’s really taken off this year,” Hobson said.

Here’s the Wikipedia page for phragmites. Here’s an excerpt:

In North America, the status of Phragmites australis was a source of confusion and debate. It was commonly considered an exotic species and often invasive species, introduced from Europe. However now with evidence of the existence of Phragmites as a native plant in North America long before European colonization of the continent. It is now known that the North American native forms of P. a. subsp. americanus are markedly less vigorous than European forms. The recent marked expansion of Phragmites in North America may be due to the more vigorous, but otherwise almost indistinguishable European subsp. australis , best detectable by genetic analysis.

Phragmites australis subsp. australis is causing serious problems for many other North American hydrophyte wetland plants, including the native Phragmites australis subsp. americanus. Gallic acid released by Phragmites is degraded by ultraviolet light to produce mesoxalic acid, effectively hitting susceptible plants and seedlings with two harmful toxins. Phragmites so difficult to control that one of the most effective methods of eradicating the plant is to burn it over 2-3 seasons. The roots grow so deep and strong that one burn is not enough.

More Fountain Creek coverage here and here.