Platoro Dam is on the Conejos River about 1 mile above the town of Platoro, Colorado. It is an earthfill structure consisting of a main embankment and a dike section, separated by a rock knoll in which the spillway is excavated. The maximum embankment height is 165 feet, and the dam contains 909,884 cubic yards of material. The reservoir formed by the dam has a capacity of 59,570 acre-feet, 6,060 acre-feet of which are for flood control and 53,510 acre-feet for joint use. The spillway is an open rock cut with a concrete control section having a capacity of 3,000 cubic feet per second. The outlet works, with a capacity of 1,000 cubic feet per second, is located in the right abutment.
Platoro Dam and Reservoir are located in a geologic area consisting of a complex volcanic bedrock material of, andesite, latite and related rock materials such as breccias and agglomerates. In addition, a unique basaltic material and occasional intrusions are found in the area. Land and rockslides of great magnitude dominate surficial geology. There are two large fault zones that cross the basin at right angles, one about one-half mile upstream and the other about 3 miles upstream. There are, however, three distinct shear zones within the dam and dike foundation area. The damsite bedrock is severely fractured and contains localized shear zones. Some of the shear zones contain highly stained or mineralized vein like material. Exploration indicated the fractures were healed at depth and those that were opened were generally clean and free of mud or other material. The dike and left abutment evidently lie in a huge postglacial rockslide.