This research focuses on documenting and analyzing the factors that affect mountain climbing in the state of Colorado and assessing the potential environmental impacts caused by the growing number of climbers visiting the Fourteeners—the 58 mountain peaks located within the Rocky Mountains exceeding an elevation of 14,000 feet. Key objectives were to: 1. identify factors that have a significant effect on mountain climbing frequency; 2. collect information from physical trail and route evaluation to develop an interim classification index; 3. combine relevant variables to formulate a composite Fourteeners Environmental Degradation Index (FEDI) and use it to evaluate, rank, and compare the 58 fourteeners within the six major mountain ranges; and 4. examine sensitivity of this composite index based on changing relative importance of the input variables. Results from the first phase, based on statistical tests, indicated that annual climbing frequency has a significant positive association with (a) distance from Denver; (b) direct distance from the summit to the nearest paved road; (c) length of the trail/route; and (d) climbing route difficulty. Elevation of a peak’s trailhead, however, is the only variable with a significantly negative relationship with climbing frequency of the fourteeners. The second phase of the study involved the assessment of adverse impacts (trail erosion and trail status) through extensive fieldwork. The data was used to develop an interim FEDI. Peaks in the Front Range (e.g., Mount Evans, Longs Peak, and Pikes Peak) indicate the most adverse human-environmental impacts. Variables from both previous phases were combined to develop the final FEDI. Analyses indicated that Mount Evans (Front Range) was ranked highest (highest level of adverse impacts), while Culebra (Sangre de Cristo Range) ranked lowest. The mountain ranges closest to Denver (Front Range and Tenmile/Mosquito) yielded the highest average ranks, while ranges farthest from Denver showed lowest ranks. Sensitivity analyses of the FEDI suggested that rankings were not drastically altered by adjusting relative importance of input factors. The findings provide important insights on identifying preservation needs within heavily visited mountain environments and can be used to guide future protection efforts, trail construction, and maintenance for existing trails and routes.