I found a wild guppy population of what appeared to be Trinidad guppies in the South Platte River Colorado last summer--I was shocked find them are there US?

It is unlikely that you found a wild population of Trinidad guppies in the South Platte River in Colorado. Trinidad guppies are native to the Northern Range Mountains in Trinidad and Tobago, and they are not known to be established in any natural waterways in the United States. While it is possible that someone may have released a small number of Trinidad guppies into the South Platte River, it is unlikely that they would have established a viable population given the environmental conditions in the river.

The South Platte River is a large river that flows through Colorado, Nebraska, and Wyoming. It is a cold-water river, with water temperatures that can range from freezing to around 20°C (68°F). Trinidad guppies are tropical fish that prefer warm water temperatures of around 26-28°C (79-82°F). They are also very sensitive to water quality, and they require clean, well-oxygenated water. The South Platte River is not always able to provide these ideal conditions, as it can be subject to pollution and runoff from agricultural activities.

In addition, the South Platte River is home to a number of predators that could eat Trinidad guppies, including bass, trout, and pike. These predators would make it difficult for Trinidad guppies to establish a viable population in the river.

Overall, it is very unlikely that you found a wild population of Trinidad guppies in the South Platte River in Colorado. It is more likely that you saw a different species of fish that resembled Trinidad guppies.