Known by early Spanish explorers as El Río de las Animas Perdidas del Purgatorio (The River of Lost Souls of Purgatory), the Animas River slices through 126 miles of diverse terrain from its source in alpine tundra north of Silverton to mixed conifer forests at the New Mexico state line.
In its trajectory, the Animas drops 7,000 feet, roaring through pinched mountain gorges or lolling along in valley oxbows until it empties into the San Juan River.
The caldera northeast of Silverton that gathers the snow and rain to give rise to the Animas River was created by a volcano about 27 million years ago.
Animas Forks, located in the caldera about five miles from Silverton, is the nominal headwaters.
The caldera is relatively new compared with the Precambrian geological formations through which it flows on its journey southward, said David Gonzales, a professor and chairman of the geosciences department at Fort Lewis College.
The Precambrian formations, the most ancient strata of the Earths crust, date to 1.8 billion years ago when blocks of earth created mostly by volcanic activity were still adding to the continent, he said.
These formations continue from about a mile south of Silverton to Bakers Bridge in the Animas Valley, Gonzales said.
The river canyon, carved by later glaciers, is narrow, with steep walls and a sharp descent that sends water churning through the channel.
The fury of the torrent, at times, is such that only the most intrepid kayakers attempt to run that stretch of the river, which at its narrowest point is 20 feet wide. Not all of them have come out alive.
At Bakers Bridge, the river escapes the confines of the mountains, and the terrain flattens.
The hue of the Red Cliffs on the east side of the river is from oxidation of iron-bearing minerals, Gonzales said.
The river from Bakers Bridge to Trimble Lane becomes a braided stream that meanders across the fertile valley floor forming U-shaped oxbows.
At one time, Gonzales said, gravel impelled by a glacier created a dam to form a lake in the Animas Valley.
Later erosion of the debris drained the lake but caused the relatively flat and wide channel.
The farthest reaching glacier, which receded about 12,000 years ago, carried gravel as far as 32nd Street, Gonzales said.
As it flows behind Walmart and Home Depot, the Animas passes the Purple Cliffs, which get their color from iron and manganese in the igneous rock that eroded from the La Plata Mountains, Gonzales said.
Near Aztec, the Animas joins the San Juan River.