The state’s nine mines produced 23.8 million tons of coal, according to the federal Energy Information Agency.
This marked a 17% decrease from 2012 and down a massive 41% from its peak in 2004.
Mine closures, a mild winter and losing key contracts were all attributed to the states production downturn.
Colorado levels slipped more than the national average, but decline was seen nationally.
The clean air act has also played a role in the states poor production rates.
Xcel Energy, Colorado’s largest electricity provider closed six Front Range coal-fired power units and replaced them with gas-fired generation.
The move will also reduce demand by another 4 million tons when all the conversion is completed in 2017.
The aim of the act is to cut power-plant pollution to help comply with Colorado’s federal Clean Air Act requirements.